* [PATCH 01/11] readahead: limit readahead size for small devices
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 02/11] readahead: retain inactive lru pages to be accessed soon Wu Fengguang
` (9 subsequent siblings)
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Li Shaohua, Clemens Ladisch, Wu Fengguang,
Chris Mason, Peter Zijlstra, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
[-- Attachment #1: readahead-size-for-tiny-device.patch --]
[-- Type: text/plain, Size: 7181 bytes --]
Linus reports a _really_ small & slow (505kB, 15kB/s) USB device,
on which blkid runs unpleasantly slow. He manages to optimize the blkid
reads down to 1kB+16kB, but still kernel read-ahead turns it into 48kB.
lseek 0, read 1024 => readahead 4 pages (start of file)
lseek 1536, read 16384 => readahead 8 pages (page contiguous)
The readahead heuristics involved here are reasonable ones in general.
So it's good to fix blkid with fadvise(RANDOM), as Linus already did.
For the kernel part, Linus suggests:
So maybe we could be less aggressive about read-ahead when the size of
the device is small? Turning a 16kB read into a 64kB one is a big deal,
when it's about 15% of the whole device!
This looks reasonable: smaller device tend to be slower (USB sticks as
well as micro/mobile/old hard disks).
Given that the non-rotational attribute is not always reported, we can
take disk size as a max readahead size hint. This patch uses a formula
that generates the following concrete limits:
disk size readahead size
(scale by 4) (scale by 2)
1M 8k
4M 16k
16M 32k
64M 64k
256M 128k
1G 256k
--------------------------- (*)
4G 512k
16G 1024k
64G 2048k
256G 4096k
(*) Since the default readahead size is 512k, this limit only takes
effect for devices whose size is less than 4G.
The formula is determined on the following data, collected by script:
#!/bin/sh
# please make sure BDEV is not mounted or opened by others
BDEV=sdb
for rasize in 4 16 32 64 128 256 512 1024 2048 4096 8192
do
echo $rasize > /sys/block/$BDEV/queue/read_ahead_kb
time dd if=/dev/$BDEV of=/dev/null bs=4k count=102400
done
The principle is, the formula shall not limit readahead size to such a
degree that will impact some device's sequential read performance.
The Intel SSD is special in that its throughput increases steadily with
larger readahead size. However it may take years for Linux to increase
its default readahead size to 2MB, so we don't take it seriously in the
formula.
SSD 80G Intel x25-M SSDSA2M080 (reported by Li Shaohua)
rasize 1st run 2nd run
----------------------------------
4k 123 MB/s 122 MB/s
16k 153 MB/s 153 MB/s
32k 161 MB/s 162 MB/s
64k 167 MB/s 168 MB/s
128k 197 MB/s 197 MB/s
256k 217 MB/s 217 MB/s
512k 238 MB/s 234 MB/s
1M 251 MB/s 248 MB/s
2M 259 MB/s 257 MB/s
==> 4M 269 MB/s 264 MB/s
8M 266 MB/s 266 MB/s
Note that ==> points to the readahead size that yields plateau throughput.
SSD 22G MARVELL SD88SA02 MP1F (reported by Jens Axboe)
rasize 1st 2nd
--------------------------------
4k 41 MB/s 41 MB/s
16k 85 MB/s 81 MB/s
32k 102 MB/s 109 MB/s
64k 125 MB/s 144 MB/s
128k 183 MB/s 185 MB/s
256k 216 MB/s 216 MB/s
512k 216 MB/s 236 MB/s
1024k 251 MB/s 252 MB/s
2M 258 MB/s 258 MB/s
==> 4M 266 MB/s 266 MB/s
8M 266 MB/s 266 MB/s
SSD 30G SanDisk SATA 5000
4k 29.6 MB/s 29.6 MB/s 29.6 MB/s
16k 52.1 MB/s 52.1 MB/s 52.1 MB/s
32k 61.5 MB/s 61.5 MB/s 61.5 MB/s
64k 67.2 MB/s 67.2 MB/s 67.1 MB/s
128k 71.4 MB/s 71.3 MB/s 71.4 MB/s
256k 73.4 MB/s 73.4 MB/s 73.3 MB/s
==> 512k 74.6 MB/s 74.6 MB/s 74.6 MB/s
1M 74.7 MB/s 74.6 MB/s 74.7 MB/s
2M 76.1 MB/s 74.6 MB/s 74.6 MB/s
USB stick 32G Teclast CoolFlash idVendor=1307, idProduct=0165
4k 7.9 MB/s 7.9 MB/s 7.9 MB/s
16k 17.9 MB/s 17.9 MB/s 17.9 MB/s
32k 24.5 MB/s 24.5 MB/s 24.5 MB/s
64k 28.7 MB/s 28.7 MB/s 28.7 MB/s
128k 28.8 MB/s 28.9 MB/s 28.9 MB/s
==> 256k 30.5 MB/s 30.5 MB/s 30.5 MB/s
512k 30.9 MB/s 31.0 MB/s 30.9 MB/s
1M 31.0 MB/s 30.9 MB/s 30.9 MB/s
2M 30.9 MB/s 30.9 MB/s 30.9 MB/s
USB stick 4G SanDisk Cruzer idVendor=0781, idProduct=5151
4k 6.4 MB/s 6.4 MB/s 6.4 MB/s
16k 13.4 MB/s 13.4 MB/s 13.2 MB/s
32k 17.8 MB/s 17.9 MB/s 17.8 MB/s
64k 21.3 MB/s 21.3 MB/s 21.2 MB/s
128k 21.4 MB/s 21.4 MB/s 21.4 MB/s
==> 256k 23.3 MB/s 23.2 MB/s 23.2 MB/s
512k 23.3 MB/s 23.8 MB/s 23.4 MB/s
1M 23.8 MB/s 23.4 MB/s 23.3 MB/s
2M 23.4 MB/s 23.2 MB/s 23.4 MB/s
USB stick 2G idVendor=0204, idProduct=6025 SerialNumber: 08082005000113
4k 6.7 MB/s 6.9 MB/s 6.7 MB/s
16k 11.7 MB/s 11.7 MB/s 11.7 MB/s
32k 12.4 MB/s 12.4 MB/s 12.4 MB/s
64k 13.4 MB/s 13.4 MB/s 13.4 MB/s
128k 13.4 MB/s 13.4 MB/s 13.4 MB/s
==> 256k 13.6 MB/s 13.6 MB/s 13.6 MB/s
512k 13.7 MB/s 13.7 MB/s 13.7 MB/s
1M 13.7 MB/s 13.7 MB/s 13.7 MB/s
2M 13.7 MB/s 13.7 MB/s 13.7 MB/s
64 MB, USB full speed (collected by Clemens Ladisch)
Bus 003 Device 003: ID 08ec:0011 M-Systems Flash Disk Pioneers DiskOnKey
4KB: 139.339 s, 376 kB/s
16KB: 81.0427 s, 647 kB/s
32KB: 71.8513 s, 730 kB/s
==> 64KB: 67.3872 s, 778 kB/s
128KB: 67.5434 s, 776 kB/s
256KB: 65.9019 s, 796 kB/s
512KB: 66.2282 s, 792 kB/s
1024KB: 67.4632 s, 777 kB/s
2048KB: 69.9759 s, 749 kB/s
CC: Li Shaohua <shaohua.li@intel.com>
CC: Clemens Ladisch <clemens@ladisch.de>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Tested-by: Vivek Goyal <vgoyal@redhat.com>
Tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
block/genhd.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
--- linux.orig/block/genhd.c 2010-02-03 20:40:37.000000000 +0800
+++ linux/block/genhd.c 2010-02-04 21:19:07.000000000 +0800
@@ -518,6 +518,7 @@ void add_disk(struct gendisk *disk)
struct backing_dev_info *bdi;
dev_t devt;
int retval;
+ unsigned long size;
/* minors == 0 indicates to use ext devt from part0 and should
* be accompanied with EXT_DEVT flag. Make sure all
@@ -551,6 +552,29 @@ void add_disk(struct gendisk *disk)
retval = sysfs_create_link(&disk_to_dev(disk)->kobj, &bdi->dev->kobj,
"bdi");
WARN_ON(retval);
+
+ /*
+ * Limit default readahead size for small devices.
+ * disk size readahead size
+ * 1M 8k
+ * 4M 16k
+ * 16M 32k
+ * 64M 64k
+ * 256M 128k
+ * 1G 256k
+ * ---------------------------
+ * 4G 512k
+ * 16G 1024k
+ * 64G 2048k
+ * 256G 4096k
+ * Since the default readahead size is 512k, this limit
+ * only takes effect for devices whose size is less than 4G.
+ */
+ if (get_capacity(disk)) {
+ size = get_capacity(disk) >> 9;
+ size = 1UL << (ilog2(size) / 2);
+ bdi->ra_pages = min(bdi->ra_pages, size);
+ }
}
EXPORT_SYMBOL(add_disk);
--
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^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 02/11] readahead: retain inactive lru pages to be accessed soon
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
2010-02-07 4:10 ` [PATCH 01/11] readahead: limit readahead size for small devices Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 03/11] readahead: bump up the default readahead size Wu Fengguang
` (8 subsequent siblings)
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Chris Frost, Steve VanDeBogart, KAMEZAWA Hiroyuki,
Wu Fengguang, Chris Mason, Peter Zijlstra, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List, linux-fsdevel,
LKML
[-- Attachment #1: readahead-retain-pages-find_get_page.patch --]
[-- Type: text/plain, Size: 3571 bytes --]
From: Chris Frost <frost@cs.ucla.edu>
Ensure that cached pages in the inactive list are not prematurely evicted;
move such pages to lru head when they are covered by
- in-kernel heuristic readahead
- an posix_fadvise(POSIX_FADV_WILLNEED) hint from an application
Before this patch, pages already in core may be evicted before the
pages covered by the same prefetch scan but that were not yet in core.
Many small read requests may be forced on the disk because of this
behavior.
In particular, posix_fadvise(... POSIX_FADV_WILLNEED) on an in-core page
has no effect on the page's location in the LRU list, even if it is the
next victim on the inactive list.
This change helps address the performance problems we encountered
while modifying SQLite and the GIMP to use large file prefetching.
Overall these prefetching techniques improved the runtime of large
benchmarks by 10-17x for these applications. More in the publication
_Reducing Seek Overhead with Application-Directed Prefetching_ in
USENIX ATC 2009 and at http://libprefetch.cs.ucla.edu/.
Signed-off-by: Chris Frost <frost@cs.ucla.edu>
Signed-off-by: Steve VanDeBogart <vandebo@cs.ucla.edu>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/readahead.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 44 insertions(+)
--- linux.orig/mm/readahead.c 2010-02-01 10:18:57.000000000 +0800
+++ linux/mm/readahead.c 2010-02-01 10:20:51.000000000 +0800
@@ -9,7 +9,9 @@
#include <linux/kernel.h>
#include <linux/fs.h>
+#include <linux/memcontrol.h>
#include <linux/mm.h>
+#include <linux/mm_inline.h>
#include <linux/module.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
@@ -133,6 +135,40 @@ out:
}
/*
+ * The file range is expected to be accessed in near future. Move pages
+ * (possibly in inactive lru tail) to lru head, so that they are retained
+ * in memory for some reasonable time.
+ */
+static void retain_inactive_pages(struct address_space *mapping,
+ pgoff_t index, int len)
+{
+ int i;
+ struct page *page;
+ struct zone *zone;
+
+ for (i = 0; i < len; i++) {
+ page = find_get_page(mapping, index + i);
+ if (!page)
+ continue;
+
+ zone = page_zone(page);
+ spin_lock_irq(&zone->lru_lock);
+
+ if (PageLRU(page) &&
+ !PageActive(page) &&
+ !PageUnevictable(page)) {
+ int lru = page_lru_base_type(page);
+
+ del_page_from_lru_list(zone, page, lru);
+ add_page_to_lru_list(zone, page, lru);
+ }
+
+ spin_unlock_irq(&zone->lru_lock);
+ put_page(page);
+ }
+}
+
+/*
* __do_page_cache_readahead() actually reads a chunk of disk. It allocates all
* the pages first, then submits them all for I/O. This avoids the very bad
* behaviour which would occur if page allocations are causing VM writeback.
@@ -184,6 +220,14 @@ __do_page_cache_readahead(struct address
}
/*
+ * Normally readahead will auto stop on cached segments, so we won't
+ * hit many cached pages. If it does happen, bring the inactive pages
+ * adjecent to the newly prefetched ones(if any).
+ */
+ if (ret < nr_to_read)
+ retain_inactive_pages(mapping, offset, page_idx);
+
+ /*
* Now start the IO. We ignore I/O errors - if the page is not
* uptodate then the caller will launch readpage again, and
* will then handle the error.
--
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^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 03/11] readahead: bump up the default readahead size
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
2010-02-07 4:10 ` [PATCH 01/11] readahead: limit readahead size for small devices Wu Fengguang
2010-02-07 4:10 ` [PATCH 02/11] readahead: retain inactive lru pages to be accessed soon Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-08 7:20 ` Christian Ehrhardt
2010-02-07 4:10 ` [PATCH 04/11] readahead: introduce {MAX|MIN}_READAHEAD_PAGES macros for ease of use Wu Fengguang
` (7 subsequent siblings)
10 siblings, 1 reply; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Chris Mason, Peter Zijlstra, Martin Schwidefsky,
Christian Ehrhardt, Wu Fengguang, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List, linux-fsdevel,
LKML
[-- Attachment #1: readahead-enlarge-default-size.patch --]
[-- Type: text/plain, Size: 5841 bytes --]
Use 512kb max readahead size, and 32kb min readahead size.
The former helps io performance for common workloads.
The latter will be used in the thrashing safe context readahead.
-- Rationals on the 512kb size --
I believe it yields more I/O throughput without noticeably increasing
I/O latency for today's HDD.
For example, for a 100MB/s and 8ms access time HDD, its random IO or
highly concurrent sequential IO would in theory be:
io_size KB access_time transfer_time io_latency util% throughput KB/s
4 8 0.04 8.04 0.49% 497.57
8 8 0.08 8.08 0.97% 990.33
16 8 0.16 8.16 1.92% 1961.69
32 8 0.31 8.31 3.76% 3849.62
64 8 0.62 8.62 7.25% 7420.29
128 8 1.25 9.25 13.51% 13837.84
256 8 2.50 10.50 23.81% 24380.95
512 8 5.00 13.00 38.46% 39384.62
1024 8 10.00 18.00 55.56% 56888.89
2048 8 20.00 28.00 71.43% 73142.86
4096 8 40.00 48.00 83.33% 85333.33
The 128KB => 512KB readahead size boosts IO throughput from ~13MB/s to
~39MB/s, while merely increases (minimal) IO latency from 9.25ms to 13ms.
As for SSD, I find that Intel X25-M SSD desires large readahead size
even for sequential reads:
rasize 1st run 2nd run
----------------------------------
4k 123 MB/s 122 MB/s
16k 153 MB/s 153 MB/s
32k 161 MB/s 162 MB/s
64k 167 MB/s 168 MB/s
128k 197 MB/s 197 MB/s
256k 217 MB/s 217 MB/s
512k 238 MB/s 234 MB/s
1M 251 MB/s 248 MB/s
2M 259 MB/s 257 MB/s
4M 269 MB/s 264 MB/s
8M 266 MB/s 266 MB/s
The two other impacts of an enlarged readahead size are
- memory footprint (caused by readahead miss)
Sequential readahead hit ratio is pretty high regardless of max
readahead size; the extra memory footprint is mainly caused by
enlarged mmap read-around.
I measured my desktop:
- under Xwindow:
128KB readahead hit ratio = 143MB/230MB = 62%
512KB readahead hit ratio = 138MB/248MB = 55%
1MB readahead hit ratio = 130MB/253MB = 51%
- under console: (seems more stable than the Xwindow data)
128KB readahead hit ratio = 30MB/56MB = 53%
1MB readahead hit ratio = 30MB/59MB = 51%
So the impact to memory footprint looks acceptable.
- readahead thrashing
It will now cost 1MB readahead buffer per stream. Memory tight
systems typically do not run multiple streams; but if they do
so, it should help I/O performance as long as we can avoid
thrashing, which can be achieved with the following patches.
-- Benchmarks by Vivek Goyal --
I have got two paths to the HP EVA and got multipath device setup(dm-3).
I run increasing number of sequential readers. File system is ext3 and
filesize is 1G.
I have run the tests 3 times (3sets) and taken the average of it.
Workload=bsr iosched=cfq Filesz=1G bs=32K
======================================================================
2.6.33-rc5 2.6.33-rc5-readahead
job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s) MaxClat(us)
--- --- -- ------------ ----------- ------------ -----------
bsr 3 1 141768 130965 190302 97937.3
bsr 3 2 131979 135402 185636 223286
bsr 3 4 132351 420733 185986 363658
bsr 3 8 133152 455434 184352 428478
bsr 3 16 130316 674499 185646 594311
I ran same test on a different piece of hardware. There are few SATA disks
(5-6) in striped configuration behind a hardware RAID controller.
Workload=bsr iosched=cfq Filesz=1G bs=32K
======================================================================
2.6.33-rc5 2.6.33-rc5-readahead
job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s) MaxClat(us)
--- --- -- ------------ ----------- ------------ -----------
bsr 3 1 147569 14369.7 160191 22752
bsr 3 2 124716 243932 149343 184698
bsr 3 4 123451 327665 147183 430875
bsr 3 8 122486 455102 144568 484045
bsr 3 16 117645 1.03957e+06 137485 1.06257e+06
Tested-by: Vivek Goyal <vgoyal@redhat.com>
CC: Jens Axboe <jens.axboe@oracle.com>
CC: Chris Mason <chris.mason@oracle.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Martin Schwidefsky <schwidefsky@de.ibm.com>
CC: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/mm.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- linux.orig/include/linux/mm.h 2010-01-30 17:38:49.000000000 +0800
+++ linux/include/linux/mm.h 2010-01-30 18:09:58.000000000 +0800
@@ -1184,8 +1184,8 @@ int write_one_page(struct page *page, in
void task_dirty_inc(struct task_struct *tsk);
/* readahead.c */
-#define VM_MAX_READAHEAD 128 /* kbytes */
-#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
+#define VM_MAX_READAHEAD 512 /* kbytes */
+#define VM_MIN_READAHEAD 32 /* kbytes (includes current page) */
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read);
--
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^ permalink raw reply [flat|nested] 22+ messages in thread* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-07 4:10 ` [PATCH 03/11] readahead: bump up the default readahead size Wu Fengguang
@ 2010-02-08 7:20 ` Christian Ehrhardt
2010-02-08 13:46 ` Wu Fengguang
0 siblings, 1 reply; 22+ messages in thread
From: Christian Ehrhardt @ 2010-02-08 7:20 UTC (permalink / raw)
To: Wu Fengguang
Cc: Andrew Morton, Jens Axboe, Chris Mason, Peter Zijlstra,
Martin Schwidefsky, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
This is related to our discussion from October 09 e.g.
http://lkml.indiana.edu/hypermail/linux/kernel/0910.1/01468.html
I work for s390 where - as mainframe - we only have environments that
benefit from 512k readahead, but I still expect some embedded devices won't.
While my idea of making it configurable was not liked in the past, it
may be still useful when introducing this default change to let some
small devices choose without patching the src (a number field defaulting
to 512 and explaining the past of that value would be really nice).
For the discussion of 512 vs. 128 I can add from my measurements that I
have seen the following:
- 512 is by far superior to 128 for sequential reads
- improvements with iozone sequential read scaling from 1 to 64 parallel
processes up to +35%
- readahead sizes larger than 512 reevealed to not be "more useful" but
increasing the chance of trashing in low mem systems
So I appreciate this change with a little note that I would prefer a
config option.
-> tested & acked-by Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Wu Fengguang wrote:
>
> Use 512kb max readahead size, and 32kb min readahead size.
>
> The former helps io performance for common workloads.
> The latter will be used in the thrashing safe context readahead.
>
> -- Rationals on the 512kb size --
>
> I believe it yields more I/O throughput without noticeably increasing
> I/O latency for today's HDD.
>
> For example, for a 100MB/s and 8ms access time HDD, its random IO or
> highly concurrent sequential IO would in theory be:
>
> io_size KB access_time transfer_time io_latency util%
throughput KB/s
> 4 8 0.04 8.04 0.49% 497.57
> 8 8 0.08 8.08 0.97% 990.33
> 16 8 0.16 8.16 1.92% 1961.69
> 32 8 0.31 8.31 3.76% 3849.62
> 64 8 0.62 8.62 7.25% 7420.29
> 128 8 1.25 9.25 13.51% 13837.84
> 256 8 2.50 10.50 23.81% 24380.95
> 512 8 5.00 13.00 38.46% 39384.62
> 1024 8 10.00 18.00 55.56% 56888.89
> 2048 8 20.00 28.00 71.43% 73142.86
> 4096 8 40.00 48.00 83.33% 85333.33
>
> The 128KB => 512KB readahead size boosts IO throughput from ~13MB/s to
> ~39MB/s, while merely increases (minimal) IO latency from 9.25ms to 13ms.
>
> As for SSD, I find that Intel X25-M SSD desires large readahead size
> even for sequential reads:
>
> rasize 1st run 2nd run
> ----------------------------------
> 4k 123 MB/s 122 MB/s
> 16k 153 MB/s 153 MB/s
> 32k 161 MB/s 162 MB/s
> 64k 167 MB/s 168 MB/s
> 128k 197 MB/s 197 MB/s
> 256k 217 MB/s 217 MB/s
> 512k 238 MB/s 234 MB/s
> 1M 251 MB/s 248 MB/s
> 2M 259 MB/s 257 MB/s
> 4M 269 MB/s 264 MB/s
> 8M 266 MB/s 266 MB/s
>
> The two other impacts of an enlarged readahead size are
>
> - memory footprint (caused by readahead miss)
> Sequential readahead hit ratio is pretty high regardless of max
> readahead size; the extra memory footprint is mainly caused by
> enlarged mmap read-around.
> I measured my desktop:
> - under Xwindow:
> 128KB readahead hit ratio = 143MB/230MB = 62%
> 512KB readahead hit ratio = 138MB/248MB = 55%
> 1MB readahead hit ratio = 130MB/253MB = 51%
> - under console: (seems more stable than the Xwindow data)
> 128KB readahead hit ratio = 30MB/56MB = 53%
> 1MB readahead hit ratio = 30MB/59MB = 51%
> So the impact to memory footprint looks acceptable.
>
> - readahead thrashing
> It will now cost 1MB readahead buffer per stream. Memory tight
> systems typically do not run multiple streams; but if they do
> so, it should help I/O performance as long as we can avoid
> thrashing, which can be achieved with the following patches.
>
> -- Benchmarks by Vivek Goyal --
>
> I have got two paths to the HP EVA and got multipath device setup(dm-3).
> I run increasing number of sequential readers. File system is ext3 and
> filesize is 1G.
> I have run the tests 3 times (3sets) and taken the average of it.
>
> Workload=bsr iosched=cfq Filesz=1G bs=32K
> ======================================================================
> 2.6.33-rc5 2.6.33-rc5-readahead
> job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s) MaxClat(us)
> --- --- -- ------------ ----------- ------------ -----------
> bsr 3 1 141768 130965 190302 97937.3
> bsr 3 2 131979 135402 185636 223286
> bsr 3 4 132351 420733 185986 363658
> bsr 3 8 133152 455434 184352 428478
> bsr 3 16 130316 674499 185646 594311
>
> I ran same test on a different piece of hardware. There are few SATA
disks
> (5-6) in striped configuration behind a hardware RAID controller.
>
> Workload=bsr iosched=cfq Filesz=1G bs=32K
> ======================================================================
> 2.6.33-rc5 2.6.33-rc5-readahead
> job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s)
MaxClat(us)
> --- --- -- ------------ ----------- ------------
-----------
> bsr 3 1 147569 14369.7 160191
22752
> bsr 3 2 124716 243932 149343
184698
> bsr 3 4 123451 327665 147183
430875
> bsr 3 8 122486 455102 144568
484045
> bsr 3 16 117645 1.03957e+06 137485
1.06257e+06
>
> Tested-by: Vivek Goyal <vgoyal@redhat.com>
> CC: Jens Axboe <jens.axboe@oracle.com>
> CC: Chris Mason <chris.mason@oracle.com>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> CC: Martin Schwidefsky <schwidefsky@de.ibm.com>
> CC: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> include/linux/mm.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> --- linux.orig/include/linux/mm.h 2010-01-30 17:38:49.000000000 +0800
> +++ linux/include/linux/mm.h 2010-01-30 18:09:58.000000000 +0800
> @@ -1184,8 +1184,8 @@ int write_one_page(struct page *page, in
> void task_dirty_inc(struct task_struct *tsk);
>
> /* readahead.c */
> -#define VM_MAX_READAHEAD 128 /* kbytes */
> -#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
> +#define VM_MAX_READAHEAD 512 /* kbytes */
> +#define VM_MIN_READAHEAD 32 /* kbytes (includes current page) */
>
> int force_page_cache_readahead(struct address_space *mapping, struct
file *filp,
> pgoff_t offset, unsigned long nr_to_read);
>
>
--
Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, Open Virtualization
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^ permalink raw reply [flat|nested] 22+ messages in thread
* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-08 7:20 ` Christian Ehrhardt
@ 2010-02-08 13:46 ` Wu Fengguang
2010-02-11 21:37 ` Matt Mackall
0 siblings, 1 reply; 22+ messages in thread
From: Wu Fengguang @ 2010-02-08 13:46 UTC (permalink / raw)
To: Christian Ehrhardt
Cc: Andrew Morton, Jens Axboe, Chris Mason, Peter Zijlstra,
Martin Schwidefsky, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel@vger.kernel.org, LKML,
Paul Gortmaker, Matt Mackall, David Woodhouse, linux-embedded
Chris,
Firstly inform the linux-embedded maintainers :)
I think it's a good suggestion to add a config option
(CONFIG_READAHEAD_SIZE). Will update the patch..
Thanks,
Fengguang
On Mon, Feb 08, 2010 at 03:20:31PM +0800, Christian Ehrhardt wrote:
> This is related to our discussion from October 09 e.g.
> http://lkml.indiana.edu/hypermail/linux/kernel/0910.1/01468.html
>
> I work for s390 where - as mainframe - we only have environments that
> benefit from 512k readahead, but I still expect some embedded devices won't.
> While my idea of making it configurable was not liked in the past, it
> may be still useful when introducing this default change to let some
> small devices choose without patching the src (a number field defaulting
> to 512 and explaining the past of that value would be really nice).
>
> For the discussion of 512 vs. 128 I can add from my measurements that I
> have seen the following:
> - 512 is by far superior to 128 for sequential reads
> - improvements with iozone sequential read scaling from 1 to 64 parallel
> processes up to +35%
> - readahead sizes larger than 512 reevealed to not be "more useful" but
> increasing the chance of trashing in low mem systems
>
> So I appreciate this change with a little note that I would prefer a
> config option.
> -> tested & acked-by Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
>
> Wu Fengguang wrote:
> >
> > Use 512kb max readahead size, and 32kb min readahead size.
> >
> > The former helps io performance for common workloads.
> > The latter will be used in the thrashing safe context readahead.
> >
> > -- Rationals on the 512kb size --
> >
> > I believe it yields more I/O throughput without noticeably increasing
> > I/O latency for today's HDD.
> >
> > For example, for a 100MB/s and 8ms access time HDD, its random IO or
> > highly concurrent sequential IO would in theory be:
> >
> > io_size KB access_time transfer_time io_latency util%
> throughput KB/s
> > 4 8 0.04 8.04 0.49% 497.57
> > 8 8 0.08 8.08 0.97% 990.33
> > 16 8 0.16 8.16 1.92% 1961.69
> > 32 8 0.31 8.31 3.76% 3849.62
> > 64 8 0.62 8.62 7.25% 7420.29
> > 128 8 1.25 9.25 13.51% 13837.84
> > 256 8 2.50 10.50 23.81% 24380.95
> > 512 8 5.00 13.00 38.46% 39384.62
> > 1024 8 10.00 18.00 55.56% 56888.89
> > 2048 8 20.00 28.00 71.43% 73142.86
> > 4096 8 40.00 48.00 83.33% 85333.33
> >
> > The 128KB => 512KB readahead size boosts IO throughput from ~13MB/s to
> > ~39MB/s, while merely increases (minimal) IO latency from 9.25ms to 13ms.
> >
> > As for SSD, I find that Intel X25-M SSD desires large readahead size
> > even for sequential reads:
> >
> > rasize 1st run 2nd run
> > ----------------------------------
> > 4k 123 MB/s 122 MB/s
> > 16k 153 MB/s 153 MB/s
> > 32k 161 MB/s 162 MB/s
> > 64k 167 MB/s 168 MB/s
> > 128k 197 MB/s 197 MB/s
> > 256k 217 MB/s 217 MB/s
> > 512k 238 MB/s 234 MB/s
> > 1M 251 MB/s 248 MB/s
> > 2M 259 MB/s 257 MB/s
> > 4M 269 MB/s 264 MB/s
> > 8M 266 MB/s 266 MB/s
> >
> > The two other impacts of an enlarged readahead size are
> >
> > - memory footprint (caused by readahead miss)
> > Sequential readahead hit ratio is pretty high regardless of max
> > readahead size; the extra memory footprint is mainly caused by
> > enlarged mmap read-around.
> > I measured my desktop:
> > - under Xwindow:
> > 128KB readahead hit ratio = 143MB/230MB = 62%
> > 512KB readahead hit ratio = 138MB/248MB = 55%
> > 1MB readahead hit ratio = 130MB/253MB = 51%
> > - under console: (seems more stable than the Xwindow data)
> > 128KB readahead hit ratio = 30MB/56MB = 53%
> > 1MB readahead hit ratio = 30MB/59MB = 51%
> > So the impact to memory footprint looks acceptable.
> >
> > - readahead thrashing
> > It will now cost 1MB readahead buffer per stream. Memory tight
> > systems typically do not run multiple streams; but if they do
> > so, it should help I/O performance as long as we can avoid
> > thrashing, which can be achieved with the following patches.
> >
> > -- Benchmarks by Vivek Goyal --
> >
> > I have got two paths to the HP EVA and got multipath device setup(dm-3).
> > I run increasing number of sequential readers. File system is ext3 and
> > filesize is 1G.
> > I have run the tests 3 times (3sets) and taken the average of it.
> >
> > Workload=bsr iosched=cfq Filesz=1G bs=32K
> > ======================================================================
> > 2.6.33-rc5 2.6.33-rc5-readahead
> > job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s) MaxClat(us)
> > --- --- -- ------------ ----------- ------------ -----------
> > bsr 3 1 141768 130965 190302 97937.3
> > bsr 3 2 131979 135402 185636 223286
> > bsr 3 4 132351 420733 185986 363658
> > bsr 3 8 133152 455434 184352 428478
> > bsr 3 16 130316 674499 185646 594311
> >
> > I ran same test on a different piece of hardware. There are few SATA
> disks
> > (5-6) in striped configuration behind a hardware RAID controller.
> >
> > Workload=bsr iosched=cfq Filesz=1G bs=32K
> > ======================================================================
> > 2.6.33-rc5 2.6.33-rc5-readahead
> > job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s)
> MaxClat(us)
> > --- --- -- ------------ ----------- ------------
> -----------
> > bsr 3 1 147569 14369.7 160191
> 22752
> > bsr 3 2 124716 243932 149343
> 184698
> > bsr 3 4 123451 327665 147183
> 430875
> > bsr 3 8 122486 455102 144568
> 484045
> > bsr 3 16 117645 1.03957e+06 137485
> 1.06257e+06
> >
> > Tested-by: Vivek Goyal <vgoyal@redhat.com>
> > CC: Jens Axboe <jens.axboe@oracle.com>
> > CC: Chris Mason <chris.mason@oracle.com>
> > CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > CC: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > CC: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> > include/linux/mm.h | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > --- linux.orig/include/linux/mm.h 2010-01-30 17:38:49.000000000 +0800
> > +++ linux/include/linux/mm.h 2010-01-30 18:09:58.000000000 +0800
> > @@ -1184,8 +1184,8 @@ int write_one_page(struct page *page, in
> > void task_dirty_inc(struct task_struct *tsk);
> >
> > /* readahead.c */
> > -#define VM_MAX_READAHEAD 128 /* kbytes */
> > -#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
> > +#define VM_MAX_READAHEAD 512 /* kbytes */
> > +#define VM_MIN_READAHEAD 32 /* kbytes (includes current page) */
> >
> > int force_page_cache_readahead(struct address_space *mapping, struct
> file *filp,
> > pgoff_t offset, unsigned long nr_to_read);
> >
> >
>
> --
>
> Grüsse / regards, Christian Ehrhardt
> IBM Linux Technology Center, Open Virtualization
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^ permalink raw reply [flat|nested] 22+ messages in thread
* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-08 13:46 ` Wu Fengguang
@ 2010-02-11 21:37 ` Matt Mackall
2010-02-11 23:42 ` Jamie Lokier
0 siblings, 1 reply; 22+ messages in thread
From: Matt Mackall @ 2010-02-11 21:37 UTC (permalink / raw)
To: Wu Fengguang
Cc: Christian Ehrhardt, Andrew Morton, Jens Axboe, Chris Mason,
Peter Zijlstra, Martin Schwidefsky, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List,
linux-fsdevel@vger.kernel.org, LKML, Paul Gortmaker,
David Woodhouse, linux-embedded
On Mon, 2010-02-08 at 21:46 +0800, Wu Fengguang wrote:
> Chris,
>
> Firstly inform the linux-embedded maintainers :)
>
> I think it's a good suggestion to add a config option
> (CONFIG_READAHEAD_SIZE). Will update the patch..
I don't have a strong opinion here beyond the nagging feeling that we
should be using a per-bdev scaling window scheme rather than something
static.
--
http://selenic.com : development and support for Mercurial and Linux
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^ permalink raw reply [flat|nested] 22+ messages in thread
* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-11 21:37 ` Matt Mackall
@ 2010-02-11 23:42 ` Jamie Lokier
2010-02-12 0:04 ` Matt Mackall
2010-02-12 13:59 ` Wu Fengguang
0 siblings, 2 replies; 22+ messages in thread
From: Jamie Lokier @ 2010-02-11 23:42 UTC (permalink / raw)
To: Matt Mackall
Cc: Wu Fengguang, Christian Ehrhardt, Andrew Morton, Jens Axboe,
Chris Mason, Peter Zijlstra, Martin Schwidefsky, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List,
linux-fsdevel@vger.kernel.org, LKML, Paul Gortmaker,
David Woodhouse, linux-embedded
Matt Mackall wrote:
> On Mon, 2010-02-08 at 21:46 +0800, Wu Fengguang wrote:
> > Chris,
> >
> > Firstly inform the linux-embedded maintainers :)
> >
> > I think it's a good suggestion to add a config option
> > (CONFIG_READAHEAD_SIZE). Will update the patch..
>
> I don't have a strong opinion here beyond the nagging feeling that we
> should be using a per-bdev scaling window scheme rather than something
> static.
I agree with both. 100Mb/s isn't typical on little devices, even if a
fast ATA disk is attached. I've got something here where the ATA
interface itself (on a SoC) gets about 10MB/s max when doing nothing
else, or 4MB/s when talking to the network at the same time.
It's not a modern design, but you know, it's junk we try to use :-)
It sounds like a calculation based on throughput and seek time or IOP
rate, and maybe clamped if memory is small, would be good.
Is the window size something that could be meaningfully adjusted
according to live measurements?
-- Jamie
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^ permalink raw reply [flat|nested] 22+ messages in thread
* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-11 23:42 ` Jamie Lokier
@ 2010-02-12 0:04 ` Matt Mackall
2010-02-12 13:59 ` Wu Fengguang
1 sibling, 0 replies; 22+ messages in thread
From: Matt Mackall @ 2010-02-12 0:04 UTC (permalink / raw)
To: Jamie Lokier
Cc: Wu Fengguang, Christian Ehrhardt, Andrew Morton, Jens Axboe,
Chris Mason, Peter Zijlstra, Martin Schwidefsky, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List,
linux-fsdevel@vger.kernel.org, LKML, Paul Gortmaker,
David Woodhouse, linux-embedded
On Thu, 2010-02-11 at 23:42 +0000, Jamie Lokier wrote:
> Matt Mackall wrote:
> > On Mon, 2010-02-08 at 21:46 +0800, Wu Fengguang wrote:
> > > Chris,
> > >
> > > Firstly inform the linux-embedded maintainers :)
> > >
> > > I think it's a good suggestion to add a config option
> > > (CONFIG_READAHEAD_SIZE). Will update the patch..
> >
> > I don't have a strong opinion here beyond the nagging feeling that we
> > should be using a per-bdev scaling window scheme rather than something
> > static.
>
> I agree with both. 100Mb/s isn't typical on little devices, even if a
> fast ATA disk is attached. I've got something here where the ATA
> interface itself (on a SoC) gets about 10MB/s max when doing nothing
> else, or 4MB/s when talking to the network at the same time.
> It's not a modern design, but you know, it's junk we try to use :-)
>
> It sounds like a calculation based on throughput and seek time or IOP
> rate, and maybe clamped if memory is small, would be good.
>
> Is the window size something that could be meaningfully adjusted
> according to live measurements?
I think so. You've basically got a few different things you want to
balance: throughput, latency, and memory pressure. Successful readaheads
expand the window, as do empty request queues, while long request queues
and memory reclaim events collapse it. With any luck, we'll then
automatically do the right thing with fast/slow devices on big/small
boxes with varying load. And, like TCP, we don't need to 'know' anything
about the hardware, except to watch what happens when we use it.
--
http://selenic.com : development and support for Mercurial and Linux
^ permalink raw reply [flat|nested] 22+ messages in thread
* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-11 23:42 ` Jamie Lokier
2010-02-12 0:04 ` Matt Mackall
@ 2010-02-12 13:59 ` Wu Fengguang
2010-02-12 20:20 ` Matt Mackall
1 sibling, 1 reply; 22+ messages in thread
From: Wu Fengguang @ 2010-02-12 13:59 UTC (permalink / raw)
To: Jamie Lokier
Cc: Matt Mackall, Christian Ehrhardt, Andrew Morton, Jens Axboe,
Chris Mason, Peter Zijlstra, Martin Schwidefsky, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List,
linux-fsdevel@vger.kernel.org, LKML, Paul Gortmaker,
David Woodhouse, linux-embedded@vger.kernel.org
On Fri, Feb 12, 2010 at 07:42:49AM +0800, Jamie Lokier wrote:
> Matt Mackall wrote:
> > On Mon, 2010-02-08 at 21:46 +0800, Wu Fengguang wrote:
> > > Chris,
> > >
> > > Firstly inform the linux-embedded maintainers :)
> > >
> > > I think it's a good suggestion to add a config option
> > > (CONFIG_READAHEAD_SIZE). Will update the patch..
> >
> > I don't have a strong opinion here beyond the nagging feeling that we
> > should be using a per-bdev scaling window scheme rather than something
> > static.
It's good to do dynamic scaling -- in fact this patchset has code to do
- scale down readahead size (per-bdev) for small devices
- scale down readahead size (per-stream) to thrashing threshold
At the same time, I'd prefer
- to _only_ do scale down (below the default size) for low end
- and have a uniform default readahead size for the mainstream
IMHO scaling up automatically
- would be risky
- hurts to build one common expectation on Linux behavior
(not only developers, but also admins will run into the question:
"what on earth is the readahead size?")
- and still not likely to please the high end guys ;)
> I agree with both. 100Mb/s isn't typical on little devices, even if a
> fast ATA disk is attached. I've got something here where the ATA
> interface itself (on a SoC) gets about 10MB/s max when doing nothing
> else, or 4MB/s when talking to the network at the same time.
> It's not a modern design, but you know, it's junk we try to use :-)
Good to know this. I guess the same situation for some USB-capable
wireless routers -- they typically don't have powerful hardware to
exert the full 100MB/s disk speed.
> It sounds like a calculation based on throughput and seek time or IOP
> rate, and maybe clamped if memory is small, would be good.
>
> Is the window size something that could be meaningfully adjusted
> according to live measurements?
We currently have live adjustment for
- small devices
- thrashed read streams
We could add new adjustments based on throughput (estimation is the
problem) and memory size.
Note that it does not really hurt to have big _readahead_ size on low
throughput or small memory conditions, because it's merely _max_
readahead size, the actual readahead size scales up step-by-step, and
scales down if thrashed, and the sequential readahead hit ratio is
pretty high (so no memory/bandwidth is wasted).
What may hurt is to have big mmap _readaround_ size. The larger
readaround size, the more readaround miss ratio (but still not
disastrous), hence more memory pages and bandwidth wasted. It's not a
big problem for mainstream, however embedded systems may be more
sensitive.
I would guess most embedded systems put executables on MTD devices
(anyone to confirm this?). And I wonder if MTDs have general
characteristics that are suitable for smaller readahead/readaround
size (the two sizes are bundled for simplicity)?
Thanks,
Fengguang
^ permalink raw reply [flat|nested] 22+ messages in thread
* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-12 13:59 ` Wu Fengguang
@ 2010-02-12 20:20 ` Matt Mackall
2010-02-21 2:25 ` Wu Fengguang
0 siblings, 1 reply; 22+ messages in thread
From: Matt Mackall @ 2010-02-12 20:20 UTC (permalink / raw)
To: Wu Fengguang
Cc: Jamie Lokier, Christian Ehrhardt, Andrew Morton, Jens Axboe,
Chris Mason, Peter Zijlstra, Martin Schwidefsky, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List,
linux-fsdevel@vger.kernel.org, LKML, Paul Gortmaker,
David Woodhouse, linux-embedded@vger.kernel.org
On Fri, 2010-02-12 at 21:59 +0800, Wu Fengguang wrote:
> On Fri, Feb 12, 2010 at 07:42:49AM +0800, Jamie Lokier wrote:
> > Matt Mackall wrote:
> > > On Mon, 2010-02-08 at 21:46 +0800, Wu Fengguang wrote:
> > > > Chris,
> > > >
> > > > Firstly inform the linux-embedded maintainers :)
> > > >
> > > > I think it's a good suggestion to add a config option
> > > > (CONFIG_READAHEAD_SIZE). Will update the patch..
> > >
> > > I don't have a strong opinion here beyond the nagging feeling that we
> > > should be using a per-bdev scaling window scheme rather than something
> > > static.
>
> It's good to do dynamic scaling -- in fact this patchset has code to do
> - scale down readahead size (per-bdev) for small devices
I'm not sure device size is a great metric. It's only weakly correlated
with the things we actually care about: memory pressure (small devices
are often attached to systems with small and therefore full memory) and
latency (small devices are often old and slow and attached to slow
CPUs). I think we should instead use hints about latency (large request
queues) and memory pressure (reclaim passes) directly.
> - scale down readahead size (per-stream) to thrashing threshold
Yeah, I'm happy to call that part orthogonal to this discussion.
> At the same time, I'd prefer
> - to _only_ do scale down (below the default size) for low end
> - and have a uniform default readahead size for the mainstream
I don't think that's important, given that we're dynamically fiddling
with related things.
> IMHO scaling up automatically
> - would be risky
What, explicitly, are the risks? If we bound the window with memory
pressure and latency, I don't think it can get too far out of hand.
There are also some other bounds in here: we have other limits on how
big I/O requests can be.
I'm happy to worry about only scaling down for now, but it's only a
matter of time before we have to bump the number up again.
We've got an IOPS range from < 1 (mp3 player with power-saving
spin-down) to > 1M (high-end SSD). And the one that needs the most
readahead is the former!
> I would guess most embedded systems put executables on MTD devices
> (anyone to confirm this?).
It's hard to generalize here. Even on flash devices, interleaving with
writes can result in high latencies that make it behave more like
spinning media, but there's no way to generalize about what the write
mix is going to be.
> And I wonder if MTDs have general
> characteristics that are suitable for smaller readahead/readaround
> size (the two sizes are bundled for simplicity)?
Perhaps, but the trend is definitely towards larger blocks here.
> We could add new adjustments based on throughput (estimation is the
> problem) and memory size.
Note that throughput is not enough information here. More interesting is
the "bandwidth delay product" of the I/O path. If latency (of the whole
I/O stack) is zero, it's basically always better to read on demand. But
if every request takes 100ms whether it's for 4k or 4M (see optical
media), then you might want to consider reading 4M every time. And
latency is of course generally not independent of usage pattern. Which
is why I think TCP-like feedback scaling is the right approach.
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^ permalink raw reply [flat|nested] 22+ messages in thread
* Re: [PATCH 03/11] readahead: bump up the default readahead size
2010-02-12 20:20 ` Matt Mackall
@ 2010-02-21 2:25 ` Wu Fengguang
0 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-21 2:25 UTC (permalink / raw)
To: Matt Mackall
Cc: Jamie Lokier, Christian Ehrhardt, Andrew Morton, Jens Axboe,
Chris Mason, Peter Zijlstra, Martin Schwidefsky, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List,
linux-fsdevel@vger.kernel.org, LKML, Paul Gortmaker,
David Woodhouse, linux-embedded@vger.kernel.org
Hi Matt,
On Sat, Feb 13, 2010 at 04:20:23AM +0800, Matt Mackall wrote:
> On Fri, 2010-02-12 at 21:59 +0800, Wu Fengguang wrote:
> > On Fri, Feb 12, 2010 at 07:42:49AM +0800, Jamie Lokier wrote:
> > > Matt Mackall wrote:
> > > > On Mon, 2010-02-08 at 21:46 +0800, Wu Fengguang wrote:
> > > > > Chris,
> > > > >
> > > > > Firstly inform the linux-embedded maintainers :)
> > > > >
> > > > > I think it's a good suggestion to add a config option
> > > > > (CONFIG_READAHEAD_SIZE). Will update the patch..
> > > >
> > > > I don't have a strong opinion here beyond the nagging feeling that we
> > > > should be using a per-bdev scaling window scheme rather than something
> > > > static.
> >
> > It's good to do dynamic scaling -- in fact this patchset has code to do
> > - scale down readahead size (per-bdev) for small devices
>
> I'm not sure device size is a great metric. It's only weakly correlated
Yes, it's only weakly correlated. However device size is a good metric
in itself -- when it's small, ie. Linus' 500KB sized USB device.
> with the things we actually care about: memory pressure (small devices
> are often attached to systems with small and therefore full memory) and
> latency (small devices are often old and slow and attached to slow
> CPUs). I think we should instead use hints about latency (large request
> queues) and memory pressure (reclaim passes) directly.
In principle I think it's OK to use memory pressure and IO latency as hints.
1) memory pressure
For read-ahead, the memory pressure is mainly readahead buffers
consumed by too many concurrent streams. The context readahead in this
patchset can adapt readahead size to thrashing threshold well. So in
principle we don't need to adapt the default _max_ read-ahead size to
memory pressure.
For read-around, the memory pressure is mainly read-around misses on
executables/libraries. Which could be reduced by scaling down
read-around size on fast "reclaim passes".
The more straightforward solution could be to limit default
read-around size proportional to available system memory, ie.
512MB mem => 512KB read-around size
128MB mem => 128KB read-around size
32MB mem => 32KB read-around size (minimal)
2) IO latency
We might estimate the average service time and throughput for IOs of
different size, and choose the default readahead size based on
- good throughput
- low service time
- reasonable size bounds
IMHO the estimation should reflect the nature of the device, and do
not depend on specific workloads. Some points:
- in most cases, reducing readahead size on large request queues
(which is typical in large file servers) only hurts performance
- we don't know whether the application is latency-sensitive (and to
what degree), hence no need to be over-zealous to optimize for latency
- a dynamic changing readahead size is nightmare to benchmarks
That means to avoid estimation when there are any concurrent
reads/writes. It also means that the estimation can be turned off for
this boot after enough data have been collected and the averages go
stable.
> > - scale down readahead size (per-stream) to thrashing threshold
>
> Yeah, I'm happy to call that part orthogonal to this discussion.
>
> > At the same time, I'd prefer
> > - to _only_ do scale down (below the default size) for low end
> > - and have a uniform default readahead size for the mainstream
>
> I don't think that's important, given that we're dynamically fiddling
> with related things.
Before we can dynamically tune things and do it smart enough, it would
be good to have clear rules :)
> > IMHO scaling up automatically
> > - would be risky
>
> What, explicitly, are the risks? If we bound the window with memory
Risks could be readahead misses and higher latency.
Generally the risk:perf_gain ratio goes up for larger readahead size.
> pressure and latency, I don't think it can get too far out of hand.
> There are also some other bounds in here: we have other limits on how
> big I/O requests can be.
OK, if we do some bounds based mainly on foreseeable single device
performance needs.. 16MB?
> I'm happy to worry about only scaling down for now, but it's only a
> matter of time before we have to bump the number up again.
Agreed.
> We've got an IOPS range from < 1 (mp3 player with power-saving
> spin-down) to > 1M (high-end SSD). And the one that needs the most
> readahead is the former!
We have laptop mode for the former, which will elevate readahead size
and (legitimately) disregard IO performance impacts.
> > I would guess most embedded systems put executables on MTD devices
> > (anyone to confirm this?).
>
> It's hard to generalize here. Even on flash devices, interleaving with
> writes can result in high latencies that make it behave more like
> spinning media, but there's no way to generalize about what the write
> mix is going to be.
I'd prefer to not consider impact of writes when choosing default
readahead size.
> > And I wonder if MTDs have general
> > characteristics that are suitable for smaller readahead/readaround
> > size (the two sizes are bundled for simplicity)?
>
> Perhaps, but the trend is definitely towards larger blocks here.
OK.
> > We could add new adjustments based on throughput (estimation is the
> > problem) and memory size.
>
> Note that throughput is not enough information here. More interesting is
> the "bandwidth delay product" of the I/O path. If latency (of the whole
> I/O stack) is zero, it's basically always better to read on demand. But
> if every request takes 100ms whether it's for 4k or 4M (see optical
> media), then you might want to consider reading 4M every time. And
> latency is of course generally not independent of usage pattern. Which
> is why I think TCP-like feedback scaling is the right approach.
OK.
Thanks,
Fengguang
^ permalink raw reply [flat|nested] 22+ messages in thread
* [PATCH 04/11] readahead: introduce {MAX|MIN}_READAHEAD_PAGES macros for ease of use
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (2 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 03/11] readahead: bump up the default readahead size Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 05/11] readahead: replace ra->mmap_miss with ra->ra_flags Wu Fengguang
` (6 subsequent siblings)
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Wu Fengguang, Chris Mason, Peter Zijlstra,
Clemens Ladisch, Olivier Galibert, Linux Memory Management List,
linux-fsdevel, LKML
[-- Attachment #1: readahead-min-max-pages.patch --]
[-- Type: text/plain, Size: 2534 bytes --]
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
block/blk-core.c | 3 +--
fs/fuse/inode.c | 2 +-
include/linux/mm.h | 3 +++
mm/backing-dev.c | 2 +-
4 files changed, 6 insertions(+), 4 deletions(-)
--- linux.orig/block/blk-core.c 2010-01-30 17:38:48.000000000 +0800
+++ linux/block/blk-core.c 2010-01-30 18:10:01.000000000 +0800
@@ -498,8 +498,7 @@ struct request_queue *blk_alloc_queue_no
q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
q->backing_dev_info.unplug_io_data = q;
- q->backing_dev_info.ra_pages =
- (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+ q->backing_dev_info.ra_pages = MAX_READAHEAD_PAGES;
q->backing_dev_info.state = 0;
q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
q->backing_dev_info.name = "block";
--- linux.orig/fs/fuse/inode.c 2010-01-30 17:38:48.000000000 +0800
+++ linux/fs/fuse/inode.c 2010-01-30 18:10:01.000000000 +0800
@@ -870,7 +870,7 @@ static int fuse_bdi_init(struct fuse_con
int err;
fc->bdi.name = "fuse";
- fc->bdi.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+ fc->bdi.ra_pages = MAX_READAHEAD_PAGES;
fc->bdi.unplug_io_fn = default_unplug_io_fn;
/* fuse does it's own writeback accounting */
fc->bdi.capabilities = BDI_CAP_NO_ACCT_WB;
--- linux.orig/include/linux/mm.h 2010-01-30 18:09:58.000000000 +0800
+++ linux/include/linux/mm.h 2010-01-30 18:10:01.000000000 +0800
@@ -1187,6 +1187,9 @@ void task_dirty_inc(struct task_struct *
#define VM_MAX_READAHEAD 512 /* kbytes */
#define VM_MIN_READAHEAD 32 /* kbytes (includes current page) */
+#define MAX_READAHEAD_PAGES (VM_MAX_READAHEAD*1024 / PAGE_CACHE_SIZE)
+#define MIN_READAHEAD_PAGES DIV_ROUND_UP(VM_MIN_READAHEAD*1024, PAGE_CACHE_SIZE)
+
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read);
--- linux.orig/mm/backing-dev.c 2010-01-30 17:38:48.000000000 +0800
+++ linux/mm/backing-dev.c 2010-01-30 18:10:01.000000000 +0800
@@ -18,7 +18,7 @@ EXPORT_SYMBOL(default_unplug_io_fn);
struct backing_dev_info default_backing_dev_info = {
.name = "default",
- .ra_pages = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
+ .ra_pages = MAX_READAHEAD_PAGES,
.state = 0,
.capabilities = BDI_CAP_MAP_COPY,
.unplug_io_fn = default_unplug_io_fn,
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
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^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 05/11] readahead: replace ra->mmap_miss with ra->ra_flags
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (3 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 04/11] readahead: introduce {MAX|MIN}_READAHEAD_PAGES macros for ease of use Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-08 8:19 ` Nick Piggin
2010-02-07 4:10 ` [PATCH 06/11] readahead: thrashing safe context readahead Wu Fengguang
` (5 subsequent siblings)
10 siblings, 1 reply; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Nick Piggin, Andi Kleen, Steven Whitehouse,
Wu Fengguang, Chris Mason, Peter Zijlstra, Clemens Ladisch,
Olivier Galibert, Linux Memory Management List, linux-fsdevel,
LKML
[-- Attachment #1: readahead-flags.patch --]
[-- Type: text/plain, Size: 2546 bytes --]
Introduce a readahead flags field and embed the existing mmap_miss in it
(to save space).
It will be possible to lose the flags in race conditions, however the
impact should be limited.
CC: Nick Piggin <npiggin@suse.de>
CC: Andi Kleen <andi@firstfloor.org>
CC: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/fs.h | 30 +++++++++++++++++++++++++++++-
mm/filemap.c | 7 ++-----
2 files changed, 31 insertions(+), 6 deletions(-)
--- linux.orig/include/linux/fs.h 2010-02-07 11:46:35.000000000 +0800
+++ linux/include/linux/fs.h 2010-02-07 11:46:37.000000000 +0800
@@ -892,10 +892,38 @@ struct file_ra_state {
there are only # of pages ahead */
unsigned int ra_pages; /* Maximum readahead window */
- unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
+ unsigned int ra_flags;
loff_t prev_pos; /* Cache last read() position */
};
+/* ra_flags bits */
+#define READAHEAD_MMAP_MISS 0x0000ffff /* cache misses for mmap access */
+
+/*
+ * Don't do ra_flags++ directly to avoid possible overflow:
+ * the ra fields can be accessed concurrently in a racy way.
+ */
+static inline unsigned int ra_mmap_miss_inc(struct file_ra_state *ra)
+{
+ unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
+
+ if (miss < READAHEAD_MMAP_MISS) {
+ miss++;
+ ra->ra_flags = miss | (ra->ra_flags &~ READAHEAD_MMAP_MISS);
+ }
+ return miss;
+}
+
+static inline void ra_mmap_miss_dec(struct file_ra_state *ra)
+{
+ unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
+
+ if (miss) {
+ miss--;
+ ra->ra_flags = miss | (ra->ra_flags &~ READAHEAD_MMAP_MISS);
+ }
+}
+
/*
* Check if @index falls in the readahead windows.
*/
--- linux.orig/mm/filemap.c 2010-02-07 11:46:35.000000000 +0800
+++ linux/mm/filemap.c 2010-02-07 11:46:37.000000000 +0800
@@ -1418,14 +1418,12 @@ static void do_sync_mmap_readahead(struc
return;
}
- if (ra->mmap_miss < INT_MAX)
- ra->mmap_miss++;
/*
* Do we miss much more than hit in this file? If so,
* stop bothering with read-ahead. It will only hurt.
*/
- if (ra->mmap_miss > MMAP_LOTSAMISS)
+ if (ra_mmap_miss_inc(ra) > MMAP_LOTSAMISS)
return;
/*
@@ -1455,8 +1453,7 @@ static void do_async_mmap_readahead(stru
/* If we don't want any read-ahead, don't bother */
if (VM_RandomReadHint(vma))
return;
- if (ra->mmap_miss > 0)
- ra->mmap_miss--;
+ ra_mmap_miss_dec(ra);
if (PageReadahead(page))
page_cache_async_readahead(mapping, ra, file,
page, offset, ra->ra_pages);
^ permalink raw reply [flat|nested] 22+ messages in thread* Re: [PATCH 05/11] readahead: replace ra->mmap_miss with ra->ra_flags
2010-02-07 4:10 ` [PATCH 05/11] readahead: replace ra->mmap_miss with ra->ra_flags Wu Fengguang
@ 2010-02-08 8:19 ` Nick Piggin
2010-02-08 13:43 ` Wu Fengguang
0 siblings, 1 reply; 22+ messages in thread
From: Nick Piggin @ 2010-02-08 8:19 UTC (permalink / raw)
To: Wu Fengguang
Cc: Andrew Morton, Jens Axboe, Andi Kleen, Steven Whitehouse,
Chris Mason, Peter Zijlstra, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
On Sun, Feb 07, 2010 at 12:10:18PM +0800, Wu Fengguang wrote:
> Introduce a readahead flags field and embed the existing mmap_miss in it
> (to save space).
Is that the only reason?
> It will be possible to lose the flags in race conditions, however the
> impact should be limited.
Is this really a good tradeoff? Randomly readahead behaviour can
change.
I never liked this mmap_miss counter, though. It doesn't seem like
it can adapt properly for changing mmap access patterns.
Is there any reason why the normal readahead algorithms can't
detect this kind of behaviour (in much fewer than 100 misses) and
also adapt much faster if the access changes?
>
> CC: Nick Piggin <npiggin@suse.de>
> CC: Andi Kleen <andi@firstfloor.org>
> CC: Steven Whitehouse <swhiteho@redhat.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> include/linux/fs.h | 30 +++++++++++++++++++++++++++++-
> mm/filemap.c | 7 ++-----
> 2 files changed, 31 insertions(+), 6 deletions(-)
>
> --- linux.orig/include/linux/fs.h 2010-02-07 11:46:35.000000000 +0800
> +++ linux/include/linux/fs.h 2010-02-07 11:46:37.000000000 +0800
> @@ -892,10 +892,38 @@ struct file_ra_state {
> there are only # of pages ahead */
>
> unsigned int ra_pages; /* Maximum readahead window */
> - unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
> + unsigned int ra_flags;
> loff_t prev_pos; /* Cache last read() position */
> };
>
> +/* ra_flags bits */
> +#define READAHEAD_MMAP_MISS 0x0000ffff /* cache misses for mmap access */
> +
> +/*
> + * Don't do ra_flags++ directly to avoid possible overflow:
> + * the ra fields can be accessed concurrently in a racy way.
> + */
> +static inline unsigned int ra_mmap_miss_inc(struct file_ra_state *ra)
> +{
> + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
> +
> + if (miss < READAHEAD_MMAP_MISS) {
> + miss++;
> + ra->ra_flags = miss | (ra->ra_flags &~ READAHEAD_MMAP_MISS);
> + }
> + return miss;
> +}
> +
> +static inline void ra_mmap_miss_dec(struct file_ra_state *ra)
> +{
> + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
> +
> + if (miss) {
> + miss--;
> + ra->ra_flags = miss | (ra->ra_flags &~ READAHEAD_MMAP_MISS);
> + }
> +}
> +
> /*
> * Check if @index falls in the readahead windows.
> */
> --- linux.orig/mm/filemap.c 2010-02-07 11:46:35.000000000 +0800
> +++ linux/mm/filemap.c 2010-02-07 11:46:37.000000000 +0800
> @@ -1418,14 +1418,12 @@ static void do_sync_mmap_readahead(struc
> return;
> }
>
> - if (ra->mmap_miss < INT_MAX)
> - ra->mmap_miss++;
>
> /*
> * Do we miss much more than hit in this file? If so,
> * stop bothering with read-ahead. It will only hurt.
> */
> - if (ra->mmap_miss > MMAP_LOTSAMISS)
> + if (ra_mmap_miss_inc(ra) > MMAP_LOTSAMISS)
> return;
>
> /*
> @@ -1455,8 +1453,7 @@ static void do_async_mmap_readahead(stru
> /* If we don't want any read-ahead, don't bother */
> if (VM_RandomReadHint(vma))
> return;
> - if (ra->mmap_miss > 0)
> - ra->mmap_miss--;
> + ra_mmap_miss_dec(ra);
> if (PageReadahead(page))
> page_cache_async_readahead(mapping, ra, file,
> page, offset, ra->ra_pages);
>
^ permalink raw reply [flat|nested] 22+ messages in thread* Re: [PATCH 05/11] readahead: replace ra->mmap_miss with ra->ra_flags
2010-02-08 8:19 ` Nick Piggin
@ 2010-02-08 13:43 ` Wu Fengguang
0 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-08 13:43 UTC (permalink / raw)
To: Nick Piggin
Cc: Andrew Morton, Jens Axboe, Andi Kleen, Steven Whitehouse,
Chris Mason, Peter Zijlstra, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel@vger.kernel.org, LKML
On Mon, Feb 08, 2010 at 04:19:18PM +0800, Nick Piggin wrote:
> On Sun, Feb 07, 2010 at 12:10:18PM +0800, Wu Fengguang wrote:
> > Introduce a readahead flags field and embed the existing mmap_miss in it
> > (to save space).
>
> Is that the only reason?
Several readahead flags/states will be introduced in the next patches.
> > It will be possible to lose the flags in race conditions, however the
> > impact should be limited.
>
> Is this really a good tradeoff? Randomly readahead behaviour can
> change.
It's OK. The readahead behavior won't change in "big" way.
For the race to happen, there must be two threads sharing the same
file descriptor to be in page fault or readahead at the same time.
Note that it has always been racy for "page faults" at the same time.
And if ever the race happen, we'll lose one mmap_miss++ or
mmap_miss--. Which may change some concrete readahead behavior, but
won't really impact overall I/O performance.
> I never liked this mmap_miss counter, though. It doesn't seem like
> it can adapt properly for changing mmap access patterns.
The mmap_miss aims to avoid excessive pointless read-around for sparse
random reads. As long as mmap_miss does not exceed MMAP_LOTSAMISS=100,
the read-around is expected to help the common clustered random reads
(aka. strong locality of references).
> Is there any reason why the normal readahead algorithms can't
> detect this kind of behaviour (in much fewer than 100 misses) and
> also adapt much faster if the access changes?
Assuming there's only two page fault patterns:
- those with strong locality of references
- those sparse random reads
Then:
1) MMAP_LOTSAMISS may be reduced to 10 when we increase the default
readahead size to 512K. The "10" is big enough to not hurt the
typical executable/lib page faults.
2) the mmap_miss ceiling value (reduced to 0xffff in this patch) may be
further reduced to 0xff to adapt faster to access changes? I'm not
sure though - I don't have any real world workload in mind.. Nor
have we heard of complaints on the mmap_miss magic.
For a better (2), it's possible to have a page cache context based
heuristic to determine if we need to do read-around immediately
(instead of slowly counting down mmap_miss):
when page fault at @index:
if (mmap_miss > MMAP_LOTSAMISS) {
- don't do read-around;
+ struct radix_tree_node *node = radix_tree_lookup_node(index);
+ if (node && node->count) /* any nearby page cached? */
+ do read-around;
}
Thanks,
Fengguang
> >
> > CC: Nick Piggin <npiggin@suse.de>
> > CC: Andi Kleen <andi@firstfloor.org>
> > CC: Steven Whitehouse <swhiteho@redhat.com>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> > include/linux/fs.h | 30 +++++++++++++++++++++++++++++-
> > mm/filemap.c | 7 ++-----
> > 2 files changed, 31 insertions(+), 6 deletions(-)
> >
> > --- linux.orig/include/linux/fs.h 2010-02-07 11:46:35.000000000 +0800
> > +++ linux/include/linux/fs.h 2010-02-07 11:46:37.000000000 +0800
> > @@ -892,10 +892,38 @@ struct file_ra_state {
> > there are only # of pages ahead */
> >
> > unsigned int ra_pages; /* Maximum readahead window */
> > - unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
> > + unsigned int ra_flags;
> > loff_t prev_pos; /* Cache last read() position */
> > };
> >
> > +/* ra_flags bits */
> > +#define READAHEAD_MMAP_MISS 0x0000ffff /* cache misses for mmap access */
> > +
> > +/*
> > + * Don't do ra_flags++ directly to avoid possible overflow:
> > + * the ra fields can be accessed concurrently in a racy way.
> > + */
> > +static inline unsigned int ra_mmap_miss_inc(struct file_ra_state *ra)
> > +{
> > + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
> > +
> > + if (miss < READAHEAD_MMAP_MISS) {
> > + miss++;
> > + ra->ra_flags = miss | (ra->ra_flags &~ READAHEAD_MMAP_MISS);
> > + }
> > + return miss;
> > +}
> > +
> > +static inline void ra_mmap_miss_dec(struct file_ra_state *ra)
> > +{
> > + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS;
> > +
> > + if (miss) {
> > + miss--;
> > + ra->ra_flags = miss | (ra->ra_flags &~ READAHEAD_MMAP_MISS);
> > + }
> > +}
> > +
> > /*
> > * Check if @index falls in the readahead windows.
> > */
> > --- linux.orig/mm/filemap.c 2010-02-07 11:46:35.000000000 +0800
> > +++ linux/mm/filemap.c 2010-02-07 11:46:37.000000000 +0800
> > @@ -1418,14 +1418,12 @@ static void do_sync_mmap_readahead(struc
> > return;
> > }
> >
> > - if (ra->mmap_miss < INT_MAX)
> > - ra->mmap_miss++;
> >
> > /*
> > * Do we miss much more than hit in this file? If so,
> > * stop bothering with read-ahead. It will only hurt.
> > */
> > - if (ra->mmap_miss > MMAP_LOTSAMISS)
> > + if (ra_mmap_miss_inc(ra) > MMAP_LOTSAMISS)
> > return;
> >
> > /*
> > @@ -1455,8 +1453,7 @@ static void do_async_mmap_readahead(stru
> > /* If we don't want any read-ahead, don't bother */
> > if (VM_RandomReadHint(vma))
> > return;
> > - if (ra->mmap_miss > 0)
> > - ra->mmap_miss--;
> > + ra_mmap_miss_dec(ra);
> > if (PageReadahead(page))
> > page_cache_async_readahead(mapping, ra, file,
> > page, offset, ra->ra_pages);
> >
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
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^ permalink raw reply [flat|nested] 22+ messages in thread
* [PATCH 06/11] readahead: thrashing safe context readahead
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (4 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 05/11] readahead: replace ra->mmap_miss with ra->ra_flags Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 07/11] readahead: record readahead patterns Wu Fengguang
` (4 subsequent siblings)
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Wu Fengguang, Chris Mason, Peter Zijlstra,
Clemens Ladisch, Olivier Galibert, Linux Memory Management List,
linux-fsdevel, LKML
[-- Attachment #1: readahead-thrashing-safe-mode.patch --]
[-- Type: text/plain, Size: 13951 bytes --]
Introduce a more complete version of context readahead, which is a
full-fledged readahead algorithm by itself. It replaces some of the
existing cases.
- oversize read
no behavior change; except in thrashed mode, async_size will be 0
- random read
no behavior change; implies some different internal handling
The random read will now be recorded in file_ra_state, which means in
an intermixed sequential+random pattern, the sequential part's state
will be flushed by random ones, and hence will be serviced by the
context readahead instead of the stateful one. Also means that the
first readahead for a sequential read in the middle of file will be
started by the stateful one, instead of the sequential cache miss.
- sequential cache miss
better
When walking out of a cached page segment, the readahead size will
be fully restored immediately instead of ramping up from initial size.
- hit readahead marker without valid state
better in rare cases; costs more radix tree lookups, but won't be a
problem with optimized radix_tree_prev_hole(). The added radix tree
scan for history pages is to calculate the thrashing safe readahead
size and adaptive async size.
The algorithm first looks ahead to find the start point of next
read-ahead, then looks backward in the page cache to get an estimation
of the thrashing-threshold.
It is able to automatically adapt to the thrashing threshold in a smooth
workload. The estimation theory can be illustrated with figure:
chunk A chunk B chunk C head
l01 l11 l12 l21 l22
| |-->|-->| |------>|-->| |------>|
| +-------+ +-----------+ +-------------+ |
| | # | | # | | # | |
| +-------+ +-----------+ +-------------+ |
| |<==============|<===========================|<============================|
L0 L1 L2
Let f(l) = L be a map from
l: the number of pages read by the stream
to
L: the number of pages pushed into inactive_list in the mean time
then
f(l01) <= L0
f(l11 + l12) = L1
f(l21 + l22) = L2
...
f(l01 + l11 + ...) <= Sum(L0 + L1 + ...)
<= Length(inactive_list) = f(thrashing-threshold)
So the count of continuous history pages left in inactive_list is always a
lower estimation of the true thrashing-threshold. Given a stable workload,
the readahead size will keep ramping up and then stabilize in range
(thrashing_threshold/2, thrashing_threshold)
This is good because, it's in fact bad to always reach thrashing_threshold.
That would not only be more susceptible to fluctuations, but also impose
eviction pressure to the cached pages.
To demo the thrashing safety, I run 300 200KB/s streams with mem=128M.
Only 2031/61325=3.3% readahead windows are thrashed (due to workload
fluctuation):
# cat /debug/readahead/stats
pattern readahead eof_hit cache_hit io sync_io mmap_io size async_size io_size
initial 20 9 4 20 20 12 73 37 35
subsequent 3 3 0 1 0 1 8 8 1
context 61325 1 5479 61325 6788 5 14 2 13
thrash 2031 0 1222 2031 2031 0 9 0 6
around 235 90 142 235 235 235 60 0 19
fadvise 0 0 0 0 0 0 0 0 0
random 223 133 0 91 91 1 1 0 1
all 63837 236 6847 63703 9165 0 14 2 13
And the readahead inside a single stream is working as expected:
# grep streams-3162 /debug/tracing/trace
streams-3162 [000] 8602.455953: readahead: readahead-context(dev=0:2, ino=0, req=287352+1, ra=287354+10-2, async=1) = 10
streams-3162 [000] 8602.907873: readahead: readahead-context(dev=0:2, ino=0, req=287362+1, ra=287364+20-3, async=1) = 20
streams-3162 [000] 8604.027879: readahead: readahead-context(dev=0:2, ino=0, req=287381+1, ra=287384+14-2, async=1) = 14
streams-3162 [000] 8604.754722: readahead: readahead-context(dev=0:2, ino=0, req=287396+1, ra=287398+10-2, async=1) = 10
streams-3162 [000] 8605.191228: readahead: readahead-context(dev=0:2, ino=0, req=287406+1, ra=287408+18-3, async=1) = 18
streams-3162 [000] 8606.831895: readahead: readahead-context(dev=0:2, ino=0, req=287423+1, ra=287426+12-2, async=1) = 12
streams-3162 [000] 8606.919614: readahead: readahead-thrash(dev=0:2, ino=0, req=287425+1, ra=287425+8-0, async=0) = 1
streams-3162 [000] 8607.545016: readahead: readahead-context(dev=0:2, ino=0, req=287436+1, ra=287438+9-2, async=1) = 9
streams-3162 [000] 8607.960039: readahead: readahead-context(dev=0:2, ino=0, req=287445+1, ra=287447+18-3, async=1) = 18
streams-3162 [000] 8608.790973: readahead: readahead-context(dev=0:2, ino=0, req=287462+1, ra=287465+21-3, async=1) = 21
streams-3162 [000] 8609.763138: readahead: readahead-context(dev=0:2, ino=0, req=287483+1, ra=287486+15-2, async=1) = 15
streams-3162 [000] 8611.467401: readahead: readahead-context(dev=0:2, ino=0, req=287499+1, ra=287501+11-2, async=1) = 11
streams-3162 [000] 8642.512413: readahead: readahead-context(dev=0:2, ino=0, req=288053+1, ra=288056+10-2, async=1) = 10
streams-3162 [000] 8643.246618: readahead: readahead-context(dev=0:2, ino=0, req=288064+1, ra=288066+22-3, async=1) = 22
streams-3162 [000] 8644.278613: readahead: readahead-context(dev=0:2, ino=0, req=288085+1, ra=288088+16-3, async=1) = 16
streams-3162 [000] 8644.395782: readahead: readahead-context(dev=0:2, ino=0, req=288087+1, ra=288087+21-3, async=0) = 5
streams-3162 [000] 8645.109918: readahead: readahead-context(dev=0:2, ino=0, req=288101+1, ra=288108+8-1, async=1) = 8
streams-3162 [000] 8645.285078: readahead: readahead-context(dev=0:2, ino=0, req=288105+1, ra=288116+8-1, async=1) = 8
streams-3162 [000] 8645.731794: readahead: readahead-context(dev=0:2, ino=0, req=288115+1, ra=288122+14-2, async=1) = 13
streams-3162 [000] 8646.114250: readahead: readahead-context(dev=0:2, ino=0, req=288123+1, ra=288136+8-1, async=1) = 8
streams-3162 [000] 8646.626320: readahead: readahead-context(dev=0:2, ino=0, req=288134+1, ra=288144+16-3, async=1) = 16
streams-3162 [000] 8647.035721: readahead: readahead-context(dev=0:2, ino=0, req=288143+1, ra=288160+10-2, async=1) = 10
streams-3162 [000] 8647.693082: readahead: readahead-context(dev=0:2, ino=0, req=288157+1, ra=288165+12-2, async=1) = 8
streams-3162 [000] 8648.221368: readahead: readahead-context(dev=0:2, ino=0, req=288168+1, ra=288177+15-2, async=1) = 15
streams-3162 [000] 8649.280800: readahead: readahead-context(dev=0:2, ino=0, req=288190+1, ra=288192+23-3, async=1) = 23
[...]
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/fs.h | 1
mm/readahead.c | 155 ++++++++++++++++++++++++-------------------
2 files changed, 88 insertions(+), 68 deletions(-)
--- linux.orig/mm/readahead.c 2010-02-07 11:46:35.000000000 +0800
+++ linux/mm/readahead.c 2010-02-07 11:46:39.000000000 +0800
@@ -20,6 +20,11 @@
#include <linux/pagemap.h>
/*
+ * Set async size to 1/# of the thrashing threshold.
+ */
+#define READAHEAD_ASYNC_RATIO 8
+
+/*
* Initialise a struct file's readahead state. Assumes that the caller has
* memset *ra to zero.
*/
@@ -393,39 +398,16 @@ static pgoff_t count_history_pages(struc
}
/*
- * page cache context based read-ahead
+ * Is @index recently readahead but not yet read by application?
+ * The low boundary is permissively estimated.
*/
-static int try_context_readahead(struct address_space *mapping,
- struct file_ra_state *ra,
- pgoff_t offset,
- unsigned long req_size,
- unsigned long max)
+static bool ra_thrashed(struct file_ra_state *ra, pgoff_t index)
{
- pgoff_t size;
-
- size = count_history_pages(mapping, ra, offset, max);
-
- /*
- * no history pages:
- * it could be a random read
- */
- if (!size)
- return 0;
-
- /*
- * starts from beginning of file:
- * it is a strong indication of long-run stream (or whole-file-read)
- */
- if (size >= offset)
- size *= 2;
-
- ra->start = offset;
- ra->size = get_init_ra_size(size + req_size, max);
- ra->async_size = ra->size;
-
- return 1;
+ return (index >= ra->start - ra->size &&
+ index < ra->start + ra->size);
}
+
/*
* A minimal readahead algorithm for trivial sequential/random reads.
*/
@@ -436,12 +418,26 @@ ondemand_readahead(struct address_space
unsigned long req_size)
{
unsigned long max = max_sane_readahead(ra->ra_pages);
+ unsigned int size;
+ pgoff_t start;
/*
* start of file
*/
- if (!offset)
- goto initial_readahead;
+ if (!offset) {
+ ra->start = offset;
+ ra->size = get_init_ra_size(req_size, max);
+ ra->async_size = ra->size > req_size ?
+ ra->size - req_size : ra->size;
+ goto readit;
+ }
+
+ /*
+ * Context readahead is thrashing safe, and can adapt to near the
+ * thrashing threshold given a stable workload.
+ */
+ if (ra->ra_flags & READAHEAD_THRASHED)
+ goto context_readahead;
/*
* It's the expected callback offset, assume sequential access.
@@ -456,58 +452,81 @@ ondemand_readahead(struct address_space
}
/*
- * Hit a marked page without valid readahead state.
- * E.g. interleaved reads.
- * Query the pagecache for async_size, which normally equals to
- * readahead size. Ramp it up and use it as the new readahead size.
+ * oversize read, no need to query page cache
*/
- if (hit_readahead_marker) {
- pgoff_t start;
+ if (req_size > max && !hit_readahead_marker) {
+ ra->start = offset;
+ ra->size = max;
+ ra->async_size = max;
+ goto readit;
+ }
+ /*
+ * page cache context based read-ahead
+ *
+ * ==========================_____________..............
+ * [ current window ]
+ * ^offset
+ * 1) |---- A ---->[start
+ * 2) |<----------- H -----------|
+ * 3) |----------- H ----------->]end
+ * [ new window ]
+ * [=] cached,visited [_] cached,to-be-visited [.] not cached
+ *
+ * 1) A = pages ahead = previous async_size
+ * 2) H = history pages = thrashing safe size
+ * 3) H - A = new readahead size
+ */
+context_readahead:
+ if (hit_readahead_marker) {
rcu_read_lock();
- start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
+ start = radix_tree_next_hole(&mapping->page_tree,
+ offset + 1, max);
rcu_read_unlock();
-
+ /*
+ * there are enough pages ahead: no readahead
+ */
if (!start || start - offset > max)
return 0;
+ } else
+ start = offset;
+ size = count_history_pages(mapping, ra, offset,
+ READAHEAD_ASYNC_RATIO * max);
+ /*
+ * no history pages cached, could be
+ * - a random read
+ * - a thrashed sequential read
+ */
+ if (!size && !hit_readahead_marker) {
+ if (!ra_thrashed(ra, offset)) {
+ ra->size = min(req_size, max);
+ } else {
+ retain_inactive_pages(mapping, offset, min(2 * max,
+ ra->start + ra->size - offset));
+ ra->size = max_t(int, ra->size/2, MIN_READAHEAD_PAGES);
+ ra->ra_flags |= READAHEAD_THRASHED;
+ }
+ ra->async_size = 0;
ra->start = start;
- ra->size = start - offset; /* old async_size */
- ra->size += req_size;
- ra->size = get_next_ra_size(ra, max);
- ra->async_size = ra->size;
goto readit;
}
-
/*
- * oversize read
+ * history pages start from beginning of file:
+ * it is a strong indication of long-run stream (or whole-file reads)
*/
- if (req_size > max)
- goto initial_readahead;
-
- /*
- * sequential cache miss
- */
- if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
- goto initial_readahead;
-
- /*
- * Query the page cache and look for the traces(cached history pages)
- * that a sequential stream would leave behind.
- */
- if (try_context_readahead(mapping, ra, offset, req_size, max))
- goto readit;
-
+ if (size >= offset)
+ size *= 2;
/*
- * standalone, small random read
- * Read as is, and do not pollute the readahead state.
+ * pages to readahead are already cached
*/
- return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
+ if (size <= start - offset)
+ return 0;
-initial_readahead:
- ra->start = offset;
- ra->size = get_init_ra_size(req_size, max);
- ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
+ size -= start - offset;
+ ra->start = start;
+ ra->size = clamp_t(unsigned int, size, MIN_READAHEAD_PAGES, max);
+ ra->async_size = min(ra->size, 1 + size / READAHEAD_ASYNC_RATIO);
readit:
/*
--- linux.orig/include/linux/fs.h 2010-02-07 11:46:37.000000000 +0800
+++ linux/include/linux/fs.h 2010-02-07 11:46:39.000000000 +0800
@@ -898,6 +898,7 @@ struct file_ra_state {
/* ra_flags bits */
#define READAHEAD_MMAP_MISS 0x0000ffff /* cache misses for mmap access */
+#define READAHEAD_THRASHED 0x10000000
/*
* Don't do ra_flags++ directly to avoid possible overflow:
--
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^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 07/11] readahead: record readahead patterns
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (5 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 06/11] readahead: thrashing safe context readahead Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 08/11] readahead: add tracing event Wu Fengguang
` (3 subsequent siblings)
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Ingo Molnar, Peter Zijlstra, Wu Fengguang,
Chris Mason, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
[-- Attachment #1: readahead-tracepoints.patch --]
[-- Type: text/plain, Size: 6387 bytes --]
Record the readahead pattern in ra_flags. This info can be examined by
users via the readahead tracing/stats interfaces.
Currently 7 patterns are defined:
pattern readahead for
-----------------------------------------------------------
RA_PATTERN_INITIAL start-of-file/oversize read
RA_PATTERN_SUBSEQUENT trivial sequential read
RA_PATTERN_CONTEXT interleaved sequential read
RA_PATTERN_THRASH thrashed sequential read
RA_PATTERN_MMAP_AROUND mmap fault
RA_PATTERN_FADVISE posix_fadvise()
RA_PATTERN_RANDOM random read
CC: Ingo Molnar <mingo@elte.hu>
CC: Jens Axboe <jens.axboe@oracle.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/linux/fs.h | 32 ++++++++++++++++++++++++++++++++
include/linux/mm.h | 4 +++-
mm/filemap.c | 9 +++++++--
mm/readahead.c | 17 +++++++++++++----
4 files changed, 55 insertions(+), 7 deletions(-)
--- linux.orig/include/linux/fs.h 2010-02-07 11:46:39.000000000 +0800
+++ linux/include/linux/fs.h 2010-02-07 11:46:40.000000000 +0800
@@ -897,8 +897,40 @@ struct file_ra_state {
};
/* ra_flags bits */
+#define READAHEAD_PATTERN_SHIFT 20
+#define READAHEAD_PATTERN 0x00f00000
#define READAHEAD_MMAP_MISS 0x0000ffff /* cache misses for mmap access */
#define READAHEAD_THRASHED 0x10000000
+#define READAHEAD_MMAP 0x20000000
+
+/*
+ * Which policy makes decision to do the current read-ahead IO?
+ */
+enum readahead_pattern {
+ RA_PATTERN_INITIAL,
+ RA_PATTERN_SUBSEQUENT,
+ RA_PATTERN_CONTEXT,
+ RA_PATTERN_THRASH,
+ RA_PATTERN_MMAP_AROUND,
+ RA_PATTERN_FADVISE,
+ RA_PATTERN_RANDOM,
+ RA_PATTERN_ALL, /* for summary stats */
+ RA_PATTERN_MAX
+};
+
+static inline int ra_pattern(int ra_flags)
+{
+ int pattern = (ra_flags & READAHEAD_PATTERN)
+ >> READAHEAD_PATTERN_SHIFT;
+
+ return min(pattern, RA_PATTERN_ALL);
+}
+
+static inline void ra_set_pattern(struct file_ra_state *ra, int pattern)
+{
+ ra->ra_flags = (ra->ra_flags & ~READAHEAD_PATTERN) |
+ (pattern << READAHEAD_PATTERN_SHIFT);
+}
/*
* Don't do ra_flags++ directly to avoid possible overflow:
--- linux.orig/mm/readahead.c 2010-02-07 11:46:39.000000000 +0800
+++ linux/mm/readahead.c 2010-02-07 11:46:40.000000000 +0800
@@ -291,7 +291,10 @@ unsigned long max_sane_readahead(unsigne
* Submit IO for the read-ahead request in file_ra_state.
*/
unsigned long ra_submit(struct file_ra_state *ra,
- struct address_space *mapping, struct file *filp)
+ struct address_space *mapping,
+ struct file *filp,
+ pgoff_t offset,
+ unsigned long req_size)
{
int actual;
@@ -425,6 +428,7 @@ ondemand_readahead(struct address_space
* start of file
*/
if (!offset) {
+ ra_set_pattern(ra, RA_PATTERN_INITIAL);
ra->start = offset;
ra->size = get_init_ra_size(req_size, max);
ra->async_size = ra->size > req_size ?
@@ -445,6 +449,7 @@ ondemand_readahead(struct address_space
*/
if ((offset == (ra->start + ra->size - ra->async_size) ||
offset == (ra->start + ra->size))) {
+ ra_set_pattern(ra, RA_PATTERN_SUBSEQUENT);
ra->start += ra->size;
ra->size = get_next_ra_size(ra, max);
ra->async_size = ra->size;
@@ -455,6 +460,7 @@ ondemand_readahead(struct address_space
* oversize read, no need to query page cache
*/
if (req_size > max && !hit_readahead_marker) {
+ ra_set_pattern(ra, RA_PATTERN_INITIAL);
ra->start = offset;
ra->size = max;
ra->async_size = max;
@@ -500,8 +506,10 @@ context_readahead:
*/
if (!size && !hit_readahead_marker) {
if (!ra_thrashed(ra, offset)) {
+ ra_set_pattern(ra, RA_PATTERN_RANDOM);
ra->size = min(req_size, max);
} else {
+ ra_set_pattern(ra, RA_PATTERN_THRASH);
retain_inactive_pages(mapping, offset, min(2 * max,
ra->start + ra->size - offset));
ra->size = max_t(int, ra->size/2, MIN_READAHEAD_PAGES);
@@ -518,12 +526,13 @@ context_readahead:
if (size >= offset)
size *= 2;
/*
- * pages to readahead are already cached
+ * Pages to readahead are already cached?
*/
if (size <= start - offset)
return 0;
-
size -= start - offset;
+
+ ra_set_pattern(ra, RA_PATTERN_CONTEXT);
ra->start = start;
ra->size = clamp_t(unsigned int, size, MIN_READAHEAD_PAGES, max);
ra->async_size = min(ra->size, 1 + size / READAHEAD_ASYNC_RATIO);
@@ -539,7 +548,7 @@ readit:
ra->size += ra->async_size;
}
- return ra_submit(ra, mapping, filp);
+ return ra_submit(ra, mapping, filp, offset, req_size);
}
/**
--- linux.orig/include/linux/mm.h 2010-02-07 11:46:35.000000000 +0800
+++ linux/include/linux/mm.h 2010-02-07 11:46:40.000000000 +0800
@@ -1209,7 +1209,9 @@ void page_cache_async_readahead(struct a
unsigned long max_sane_readahead(unsigned long nr);
unsigned long ra_submit(struct file_ra_state *ra,
struct address_space *mapping,
- struct file *filp);
+ struct file *filp,
+ pgoff_t offset,
+ unsigned long req_size);
/* Do stack extension */
extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
--- linux.orig/mm/filemap.c 2010-02-07 11:46:37.000000000 +0800
+++ linux/mm/filemap.c 2010-02-07 11:46:40.000000000 +0800
@@ -1413,6 +1413,7 @@ static void do_sync_mmap_readahead(struc
if (VM_SequentialReadHint(vma) ||
offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
+ ra->ra_flags |= READAHEAD_MMAP;
page_cache_sync_readahead(mapping, ra, file, offset,
ra->ra_pages);
return;
@@ -1431,10 +1432,12 @@ static void do_sync_mmap_readahead(struc
*/
ra_pages = max_sane_readahead(ra->ra_pages);
if (ra_pages) {
+ ra->ra_flags |= READAHEAD_MMAP;
+ ra_set_pattern(ra, RA_PATTERN_MMAP_AROUND);
ra->start = max_t(long, 0, offset - ra_pages/2);
ra->size = ra_pages;
ra->async_size = 0;
- ra_submit(ra, mapping, file);
+ ra_submit(ra, mapping, file, offset, 1);
}
}
@@ -1454,9 +1457,11 @@ static void do_async_mmap_readahead(stru
if (VM_RandomReadHint(vma))
return;
ra_mmap_miss_dec(ra);
- if (PageReadahead(page))
+ if (PageReadahead(page)) {
+ ra->ra_flags |= READAHEAD_MMAP;
page_cache_async_readahead(mapping, ra, file,
page, offset, ra->ra_pages);
+ }
}
/**
--
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^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 08/11] readahead: add tracing event
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (6 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 07/11] readahead: record readahead patterns Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 09/11] readahead: add /debug/readahead/stats Wu Fengguang
` (2 subsequent siblings)
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Ingo Molnar, Peter Zijlstra, Wu Fengguang,
Chris Mason, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
[-- Attachment #1: readahead-tracer.patch --]
[-- Type: text/plain, Size: 4460 bytes --]
Example output:
# echo 1 > /debug/tracing/events/readahead/enable
# cp test-file /dev/null
# cat /debug/tracing/trace # trimmed output
readahead-initial(dev=0:15, ino=100177, req=0+2, ra=0+4-2, async=0) = 4
readahead-subsequent(dev=0:15, ino=100177, req=2+2, ra=4+8-8, async=1) = 8
readahead-subsequent(dev=0:15, ino=100177, req=4+2, ra=12+16-16, async=1) = 16
readahead-subsequent(dev=0:15, ino=100177, req=12+2, ra=28+32-32, async=1) = 32
readahead-subsequent(dev=0:15, ino=100177, req=28+2, ra=60+60-60, async=1) = 24
readahead-subsequent(dev=0:15, ino=100177, req=60+2, ra=120+60-60, async=1) = 0
CC: Ingo Molnar <mingo@elte.hu>
CC: Jens Axboe <jens.axboe@oracle.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
include/trace/events/readahead.h | 69 +++++++++++++++++++++++++++++
mm/readahead.c | 22 +++++++++
2 files changed, 91 insertions(+)
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux/include/trace/events/readahead.h 2010-02-07 11:46:42.000000000 +0800
@@ -0,0 +1,69 @@
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM readahead
+
+#if !defined(_TRACE_READAHEAD_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_READAHEAD_H
+
+#include <linux/tracepoint.h>
+
+extern const char * const ra_pattern_names[];
+
+/*
+ * Tracepoint for guest mode entry.
+ */
+TRACE_EVENT(readahead,
+ TP_PROTO(struct address_space *mapping,
+ pgoff_t offset,
+ unsigned long req_size,
+ unsigned int ra_flags,
+ pgoff_t start,
+ unsigned int size,
+ unsigned int async_size,
+ unsigned int actual),
+
+ TP_ARGS(mapping, offset, req_size,
+ ra_flags, start, size, async_size, actual),
+
+ TP_STRUCT__entry(
+ __field( dev_t, dev )
+ __field( ino_t, ino )
+ __field( pgoff_t, offset )
+ __field( unsigned long, req_size )
+ __field( unsigned int, pattern )
+ __field( pgoff_t, start )
+ __field( unsigned int, size )
+ __field( unsigned int, async_size )
+ __field( unsigned int, actual )
+ ),
+
+ TP_fast_assign(
+ __entry->dev = mapping->host->i_sb->s_dev;
+ __entry->ino = mapping->host->i_ino;
+ __entry->pattern = ra_pattern(ra_flags);
+ __entry->offset = offset;
+ __entry->req_size = req_size;
+ __entry->start = start;
+ __entry->size = size;
+ __entry->async_size = async_size;
+ __entry->actual = actual;
+ ),
+
+ TP_printk("readahead-%s(dev=%d:%d, ino=%lu, "
+ "req=%lu+%lu, ra=%lu+%d-%d, async=%d) = %d",
+ ra_pattern_names[__entry->pattern],
+ MAJOR(__entry->dev),
+ MINOR(__entry->dev),
+ __entry->ino,
+ __entry->offset,
+ __entry->req_size,
+ __entry->start,
+ __entry->size,
+ __entry->async_size,
+ __entry->start > __entry->offset,
+ __entry->actual)
+);
+
+#endif /* _TRACE_READAHEAD_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>
--- linux.orig/mm/readahead.c 2010-02-07 11:46:40.000000000 +0800
+++ linux/mm/readahead.c 2010-02-07 11:46:42.000000000 +0800
@@ -19,11 +19,25 @@
#include <linux/pagevec.h>
#include <linux/pagemap.h>
+#define CREATE_TRACE_POINTS
+#include <trace/events/readahead.h>
+
/*
* Set async size to 1/# of the thrashing threshold.
*/
#define READAHEAD_ASYNC_RATIO 8
+const char * const ra_pattern_names[] = {
+ [RA_PATTERN_INITIAL] = "initial",
+ [RA_PATTERN_SUBSEQUENT] = "subsequent",
+ [RA_PATTERN_CONTEXT] = "context",
+ [RA_PATTERN_THRASH] = "thrash",
+ [RA_PATTERN_MMAP_AROUND] = "around",
+ [RA_PATTERN_FADVISE] = "fadvise",
+ [RA_PATTERN_RANDOM] = "random",
+ [RA_PATTERN_ALL] = "all",
+};
+
/*
* Initialise a struct file's readahead state. Assumes that the caller has
* memset *ra to zero.
@@ -274,6 +288,11 @@ int force_page_cache_readahead(struct ad
offset += this_chunk;
nr_to_read -= this_chunk;
}
+
+ trace_readahead(mapping, offset, nr_to_read,
+ RA_PATTERN_FADVISE << READAHEAD_PATTERN_SHIFT,
+ offset, nr_to_read, 0, ret);
+
return ret;
}
@@ -301,6 +320,9 @@ unsigned long ra_submit(struct file_ra_s
actual = __do_page_cache_readahead(mapping, filp,
ra->start, ra->size, ra->async_size);
+ trace_readahead(mapping, offset, req_size, ra->ra_flags,
+ ra->start, ra->size, ra->async_size, actual);
+
return actual;
}
--
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^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 09/11] readahead: add /debug/readahead/stats
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (7 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 08/11] readahead: add tracing event Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 10/11] readahead: dont do start-of-file readahead after lseek() Wu Fengguang
2010-02-07 4:10 ` [PATCH 11/11] radixtree: speed up next/prev hole search Wu Fengguang
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Ingo Molnar, Peter Zijlstra, Wu Fengguang,
Chris Mason, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
[-- Attachment #1: readahead-stats.patch --]
[-- Type: text/plain, Size: 7889 bytes --]
Collect readahead stats when CONFIG_READAHEAD_STATS=y.
This is enabled by default because the added overheads are trivial:
two readahead_stats() calls per readahead.
Example output:
(taken from a fresh booted NFS-ROOT box with rsize=16k)
$ cat /debug/readahead/stats
pattern readahead eof_hit cache_hit io sync_io mmap_io size async_size io_size
initial 524 216 26 498 498 18 7 4 4
subsequent 181 80 1 130 13 60 25 25 24
context 94 28 3 85 64 8 7 2 5
thrash 0 0 0 0 0 0 0 0 0
around 162 121 33 162 162 162 60 0 21
fadvise 0 0 0 0 0 0 0 0 0
random 137 0 0 137 137 0 1 0 1
all 1098 445 63 1012 874 0 17 6 9
The two most important columns are
- io number of readahead IO
- io_size average readahead IO size
CC: Ingo Molnar <mingo@elte.hu>
CC: Jens Axboe <jens.axboe@oracle.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/Kconfig | 13 +++
mm/readahead.c | 177 ++++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 188 insertions(+), 2 deletions(-)
--- linux.orig/mm/readahead.c 2010-02-01 21:55:46.000000000 +0800
+++ linux/mm/readahead.c 2010-02-01 21:57:07.000000000 +0800
@@ -38,6 +38,179 @@ const char * const ra_pattern_names[] =
[RA_PATTERN_ALL] = "all",
};
+#ifdef CONFIG_READAHEAD_STATS
+#include <linux/seq_file.h>
+#include <linux/debugfs.h>
+enum ra_account {
+ /* number of readaheads */
+ RA_ACCOUNT_COUNT, /* readahead request */
+ RA_ACCOUNT_EOF, /* readahead request contains/beyond EOF page */
+ RA_ACCOUNT_CHIT, /* readahead request covers some cached pages */
+ RA_ACCOUNT_IOCOUNT, /* readahead IO */
+ RA_ACCOUNT_SYNC, /* readahead IO that is synchronous */
+ RA_ACCOUNT_MMAP, /* readahead IO by mmap accesses */
+ /* number of readahead pages */
+ RA_ACCOUNT_SIZE, /* readahead size */
+ RA_ACCOUNT_ASIZE, /* readahead async size */
+ RA_ACCOUNT_ACTUAL, /* readahead actual IO size */
+ /* end mark */
+ RA_ACCOUNT_MAX,
+};
+
+static unsigned long ra_stats[RA_PATTERN_MAX][RA_ACCOUNT_MAX];
+
+static void readahead_stats(struct address_space *mapping,
+ pgoff_t offset,
+ unsigned long req_size,
+ unsigned int ra_flags,
+ pgoff_t start,
+ unsigned int size,
+ unsigned int async_size,
+ int actual)
+{
+ unsigned int pattern = ra_pattern(ra_flags);
+
+ ra_stats[pattern][RA_ACCOUNT_COUNT]++;
+ ra_stats[pattern][RA_ACCOUNT_SIZE] += size;
+ ra_stats[pattern][RA_ACCOUNT_ASIZE] += async_size;
+ ra_stats[pattern][RA_ACCOUNT_ACTUAL] += actual;
+
+ if (actual < size) {
+ if (start + size >
+ (i_size_read(mapping->host) - 1) >> PAGE_CACHE_SHIFT)
+ ra_stats[pattern][RA_ACCOUNT_EOF]++;
+ else
+ ra_stats[pattern][RA_ACCOUNT_CHIT]++;
+ }
+
+ if (!actual)
+ return;
+
+ ra_stats[pattern][RA_ACCOUNT_IOCOUNT]++;
+
+ if (start <= offset && start + size > offset)
+ ra_stats[pattern][RA_ACCOUNT_SYNC]++;
+
+ if (ra_flags & READAHEAD_MMAP)
+ ra_stats[pattern][RA_ACCOUNT_MMAP]++;
+}
+
+static int readahead_stats_show(struct seq_file *s, void *_)
+{
+ unsigned long i;
+ unsigned long count, iocount;
+
+ seq_printf(s, "%-10s %10s %10s %10s %10s %10s %10s %10s %10s %10s\n",
+ "pattern",
+ "readahead", "eof_hit", "cache_hit",
+ "io", "sync_io", "mmap_io",
+ "size", "async_size", "io_size");
+
+ for (i = 0; i < RA_PATTERN_MAX; i++) {
+ count = ra_stats[i][RA_ACCOUNT_COUNT];
+ iocount = ra_stats[i][RA_ACCOUNT_IOCOUNT];
+ /*
+ * avoid division-by-zero
+ */
+ if (count == 0)
+ count = 1;
+ if (iocount == 0)
+ iocount = 1;
+
+ seq_printf(s, "%-10s %10lu %10lu %10lu %10lu %10lu %10lu "
+ "%10lu %10lu %10lu\n",
+ ra_pattern_names[i],
+ ra_stats[i][RA_ACCOUNT_COUNT],
+ ra_stats[i][RA_ACCOUNT_EOF],
+ ra_stats[i][RA_ACCOUNT_CHIT],
+ ra_stats[i][RA_ACCOUNT_IOCOUNT],
+ ra_stats[i][RA_ACCOUNT_SYNC],
+ ra_stats[i][RA_ACCOUNT_MMAP],
+ ra_stats[i][RA_ACCOUNT_SIZE] / count,
+ ra_stats[i][RA_ACCOUNT_ASIZE] / count,
+ ra_stats[i][RA_ACCOUNT_ACTUAL] / iocount);
+ }
+
+ return 0;
+}
+
+static int readahead_stats_open(struct inode *inode, struct file *file)
+{
+ return single_open(file, readahead_stats_show, NULL);
+}
+
+static ssize_t readahead_stats_write(struct file *file, const char __user *buf,
+ size_t size, loff_t *offset)
+{
+ memset(ra_stats, 0, sizeof(ra_stats));
+ return size;
+}
+
+static struct file_operations readahead_stats_fops = {
+ .owner = THIS_MODULE,
+ .open = readahead_stats_open,
+ .write = readahead_stats_write,
+ .read = seq_read,
+ .llseek = seq_lseek,
+ .release = single_release,
+};
+
+static struct dentry *ra_debug_root;
+
+static int debugfs_create_readahead(void)
+{
+ struct dentry *debugfs_stats;
+
+ ra_debug_root = debugfs_create_dir("readahead", NULL);
+ if (!ra_debug_root)
+ goto out;
+
+ debugfs_stats = debugfs_create_file("stats", 0644, ra_debug_root,
+ NULL, &readahead_stats_fops);
+ if (!debugfs_stats)
+ goto out;
+
+ return 0;
+out:
+ printk(KERN_ERR "readahead: failed to create debugfs entries\n");
+ return -ENOMEM;
+}
+
+static int __init readahead_init(void)
+{
+ debugfs_create_readahead();
+ return 0;
+}
+
+static void __exit readahead_exit(void)
+{
+ debugfs_remove_recursive(ra_debug_root);
+}
+
+module_init(readahead_init);
+module_exit(readahead_exit);
+#endif
+
+static void readahead_event(struct address_space *mapping,
+ pgoff_t offset,
+ unsigned long req_size,
+ unsigned int ra_flags,
+ pgoff_t start,
+ unsigned int size,
+ unsigned int async_size,
+ unsigned int actual)
+{
+#ifdef CONFIG_READAHEAD_STATS
+ readahead_stats(mapping, offset, req_size, ra_flags,
+ start, size, async_size, actual);
+ readahead_stats(mapping, offset, req_size,
+ RA_PATTERN_ALL << READAHEAD_PATTERN_SHIFT,
+ start, size, async_size, actual);
+#endif
+ trace_readahead(mapping, offset, req_size, ra_flags,
+ start, size, async_size, actual);
+}
+
/*
* Initialise a struct file's readahead state. Assumes that the caller has
* memset *ra to zero.
@@ -289,7 +462,7 @@ int force_page_cache_readahead(struct ad
nr_to_read -= this_chunk;
}
- trace_readahead(mapping, offset, nr_to_read,
+ readahead_event(mapping, offset, nr_to_read,
RA_PATTERN_FADVISE << READAHEAD_PATTERN_SHIFT,
offset, nr_to_read, 0, ret);
@@ -320,7 +493,7 @@ unsigned long ra_submit(struct file_ra_s
actual = __do_page_cache_readahead(mapping, filp,
ra->start, ra->size, ra->async_size);
- trace_readahead(mapping, offset, req_size, ra->ra_flags,
+ readahead_event(mapping, offset, req_size, ra->ra_flags,
ra->start, ra->size, ra->async_size, actual);
return actual;
--- linux.orig/mm/Kconfig 2010-02-01 21:55:28.000000000 +0800
+++ linux/mm/Kconfig 2010-02-01 21:55:49.000000000 +0800
@@ -283,3 +283,16 @@ config NOMMU_INITIAL_TRIM_EXCESS
of 1 says that all excess pages should be trimmed.
See Documentation/nommu-mmap.txt for more information.
+
+config READAHEAD_STATS
+ bool "Collect page-cache readahead stats"
+ depends on DEBUG_FS
+ default y
+ help
+ Enable readahead events accounting. Usage:
+
+ # mount -t debugfs none /debug
+
+ # echo > /debug/readahead/stats # reset counters
+ # do benchmarks
+ # cat /debug/readahead/stats # check counters
^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 10/11] readahead: dont do start-of-file readahead after lseek()
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (8 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 09/11] readahead: add /debug/readahead/stats Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
2010-02-07 4:10 ` [PATCH 11/11] radixtree: speed up next/prev hole search Wu Fengguang
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Linus Torvalds, Wu Fengguang, Chris Mason,
Peter Zijlstra, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
[-- Attachment #1: readahead-lseek.patch --]
[-- Type: text/plain, Size: 2266 bytes --]
Some applications (eg. blkid, id3tool etc.) seek around the file
to get information. For example, blkid does
seek to 0
read 1024
seek to 1536
read 16384
The start-of-file readahead heuristic is wrong for them, whose
access pattern can be identified by lseek() calls.
So test-and-set a READAHEAD_LSEEK flag on lseek() and don't
do start-of-file readahead on seeing it. Proposed by Linus.
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/read_write.c | 3 +++
include/linux/fs.h | 1 +
mm/readahead.c | 5 +++++
3 files changed, 9 insertions(+)
--- linux.orig/mm/readahead.c 2010-02-07 11:46:44.000000000 +0800
+++ linux/mm/readahead.c 2010-02-07 11:46:45.000000000 +0800
@@ -625,6 +625,11 @@ ondemand_readahead(struct address_space
if (!offset) {
ra_set_pattern(ra, RA_PATTERN_INITIAL);
ra->start = offset;
+ if ((ra->ra_flags & READAHEAD_LSEEK) && req_size <= max) {
+ ra->size = req_size;
+ ra->async_size = 0;
+ goto readit;
+ }
ra->size = get_init_ra_size(req_size, max);
ra->async_size = ra->size > req_size ?
ra->size - req_size : ra->size;
--- linux.orig/fs/read_write.c 2010-02-07 11:46:29.000000000 +0800
+++ linux/fs/read_write.c 2010-02-07 11:46:45.000000000 +0800
@@ -71,6 +71,9 @@ generic_file_llseek_unlocked(struct file
file->f_version = 0;
}
+ if (!(file->f_ra.ra_flags & READAHEAD_LSEEK))
+ file->f_ra.ra_flags |= READAHEAD_LSEEK;
+
return offset;
}
EXPORT_SYMBOL(generic_file_llseek_unlocked);
--- linux.orig/include/linux/fs.h 2010-02-07 11:46:40.000000000 +0800
+++ linux/include/linux/fs.h 2010-02-07 11:46:45.000000000 +0800
@@ -902,6 +902,7 @@ struct file_ra_state {
#define READAHEAD_MMAP_MISS 0x0000ffff /* cache misses for mmap access */
#define READAHEAD_THRASHED 0x10000000
#define READAHEAD_MMAP 0x20000000
+#define READAHEAD_LSEEK 0x40000000 /* be conservative after lseek() */
/*
* Which policy makes decision to do the current read-ahead IO?
--
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^ permalink raw reply [flat|nested] 22+ messages in thread* [PATCH 11/11] radixtree: speed up next/prev hole search
2010-02-07 4:10 [PATCH 00/11] 512K readahead size with thrashing safe readahead Wu Fengguang
` (9 preceding siblings ...)
2010-02-07 4:10 ` [PATCH 10/11] readahead: dont do start-of-file readahead after lseek() Wu Fengguang
@ 2010-02-07 4:10 ` Wu Fengguang
10 siblings, 0 replies; 22+ messages in thread
From: Wu Fengguang @ 2010-02-07 4:10 UTC (permalink / raw)
To: Andrew Morton
Cc: Jens Axboe, Nick Piggin, Wu Fengguang, Chris Mason,
Peter Zijlstra, Clemens Ladisch, Olivier Galibert,
Linux Memory Management List, linux-fsdevel, LKML
[-- Attachment #1: radixtree-scan-hole-fast.patch --]
[-- Type: text/plain, Size: 3629 bytes --]
Replace the hole scan functions with more fast versions:
- radix_tree_next_hole(root, index, max_scan)
- radix_tree_prev_hole(root, index, max_scan)
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
lib/radix-tree.c | 85 +++++++++++++++++++++++++++++++++++++++------
1 file changed, 74 insertions(+), 11 deletions(-)
--- linux.orig/lib/radix-tree.c 2010-01-09 21:45:16.000000000 +0800
+++ linux/lib/radix-tree.c 2010-01-21 22:04:22.000000000 +0800
@@ -609,6 +609,24 @@ int radix_tree_tag_get(struct radix_tree
}
EXPORT_SYMBOL(radix_tree_tag_get);
+/*
+ * Find the bottom radix tree node that contains @index.
+ * Return NULL if @index is hole, or is the special root node.
+ */
+static struct radix_tree_node *
+radix_tree_lookup_node(struct radix_tree_root *root, unsigned long index)
+{
+ void *slot;
+
+ slot = radix_tree_lookup_element(root, index, 1);
+ if (!slot || slot == &root->rnode)
+ return NULL;
+
+ slot -= (index & RADIX_TREE_MAP_MASK) * sizeof(void *);
+
+ return container_of(slot, struct radix_tree_node, slots);
+}
+
/**
* radix_tree_next_hole - find the next hole (not-present entry)
* @root: tree root
@@ -630,18 +648,41 @@ EXPORT_SYMBOL(radix_tree_tag_get);
* under rcu_read_lock.
*/
unsigned long radix_tree_next_hole(struct radix_tree_root *root,
- unsigned long index, unsigned long max_scan)
+ unsigned long index, unsigned long max_scan)
{
- unsigned long i;
+ struct radix_tree_node *node;
+ unsigned long origin = index;
+ int i;
+
+ node = rcu_dereference(root->rnode);
+ if (node == NULL)
+ return index;
+
+ if (!radix_tree_is_indirect_ptr(node))
+ return index ? index : 1;
- for (i = 0; i < max_scan; i++) {
- if (!radix_tree_lookup(root, index))
+ while (index - origin < max_scan) {
+ node = radix_tree_lookup_node(root, index);
+ if (!node)
break;
- index++;
- if (index == 0)
+
+ if (node->count == RADIX_TREE_MAP_SIZE) {
+ index = (index | RADIX_TREE_MAP_MASK) + 1;
+ goto check_overflow;
+ }
+
+ for (i = index & RADIX_TREE_MAP_MASK;
+ i < RADIX_TREE_MAP_SIZE;
+ i++, index++)
+ if (rcu_dereference(node->slots[i]) == NULL)
+ goto out;
+
+check_overflow:
+ if (unlikely(index == 0))
break;
}
+out:
return index;
}
EXPORT_SYMBOL(radix_tree_next_hole);
@@ -669,16 +710,38 @@ EXPORT_SYMBOL(radix_tree_next_hole);
unsigned long radix_tree_prev_hole(struct radix_tree_root *root,
unsigned long index, unsigned long max_scan)
{
- unsigned long i;
+ struct radix_tree_node *node;
+ unsigned long origin = index;
+ int i;
+
+ node = rcu_dereference(root->rnode);
+ if (node == NULL)
+ return index;
+
+ if (!radix_tree_is_indirect_ptr(node))
+ return index ? index : ULONG_MAX;
- for (i = 0; i < max_scan; i++) {
- if (!radix_tree_lookup(root, index))
+ while (origin - index < max_scan) {
+ node = radix_tree_lookup_node(root, index);
+ if (!node)
break;
- index--;
- if (index == LONG_MAX)
+
+ if (node->count == RADIX_TREE_MAP_SIZE) {
+ index = (index - RADIX_TREE_MAP_SIZE) |
+ RADIX_TREE_MAP_MASK;
+ goto check_underflow;
+ }
+
+ for (i = index & RADIX_TREE_MAP_MASK; i >= 0; i--, index--)
+ if (rcu_dereference(node->slots[i]) == NULL)
+ goto out;
+
+check_underflow:
+ if (unlikely(index == ULONG_MAX))
break;
}
+out:
return index;
}
EXPORT_SYMBOL(radix_tree_prev_hole);
--
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^ permalink raw reply [flat|nested] 22+ messages in thread