From: Wu Fengguang <fengguang.wu@intel.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>, Wu Fengguang <fengguang.wu@intel.com>
Cc: Linux Memory Management List <linux-mm@kvack.org>,
<linux-fsdevel@vger.kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>
Subject: [PATCH 02/10] readahead: make context readahead more conservative
Date: Mon, 19 Dec 2011 18:23:10 +0800 [thread overview]
Message-ID: <20111219102356.754591774@intel.com> (raw)
In-Reply-To: 20111219102308.488847921@intel.com
[-- Attachment #1: readahead-context-tt --]
[-- Type: text/plain, Size: 2301 bytes --]
Try to prevent negatively impact moderately dense random reads on SSD.
Transaction-Per-Second numbers provided by Taobao:
QPS case
-------------------------------------------------------
7536 disable context readahead totally
w/ patch: 7129 slower size rampup and start RA on the 3rd read
6717 slower size rampup
w/o patch: 5581 unmodified context readahead
Before, readahead will be started whenever reading page N+1 when it
happen to read N recently. After patch, we'll only start readahead
when *three* random reads happen to access pages N, N+1, N+2. The
probability of this happening is extremely low for pure random reads,
unless they are very dense, which actually deserves some readahead.
Also start with a smaller readahead window. The impact to interleaved
sequential reads should be small, because for a long run stream, the
the small readahead window rampup phase is negletable.
The context readahead actually benefits clustered random reads on HDD
whose seek cost is pretty high. However as SSD is increasingly used for
random read workloads it's better for the context readahead to
concentrate on interleaved sequential reads.
Tested-by: Tao Ma <tm@tao.ma>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/readahead.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
--- linux-next.orig/mm/readahead.c 2011-12-19 18:12:35.000000000 +0800
+++ linux-next/mm/readahead.c 2011-12-19 18:12:51.000000000 +0800
@@ -369,10 +369,10 @@ static int try_context_readahead(struct
size = count_history_pages(mapping, ra, offset, max);
/*
- * no history pages:
+ * not enough history pages:
* it could be a random read
*/
- if (!size)
+ if (size <= req_size)
return 0;
/*
@@ -383,8 +383,8 @@ static int try_context_readahead(struct
size *= 2;
ra->start = offset;
- ra->size = get_init_ra_size(size + req_size, max);
- ra->async_size = ra->size;
+ ra->size = min(size + req_size, max);
+ ra->async_size = 1;
return 1;
}
--
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next prev parent reply other threads:[~2011-12-19 10:23 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-19 10:23 [PATCH 00/10] readahead stats/tracing, backwards prefetching and more (v3) Wu Fengguang
2011-12-19 10:23 ` [PATCH 01/10] block: limit default readahead size for small devices Wu Fengguang
2011-12-19 10:23 ` Wu Fengguang [this message]
2011-12-19 10:23 ` [PATCH 03/10] readahead: record readahead patterns Wu Fengguang
2011-12-19 10:23 ` [PATCH 04/10] readahead: tag mmap page fault call sites Wu Fengguang
2011-12-19 10:23 ` [PATCH 05/10] readahead: tag metadata " Wu Fengguang
2011-12-19 10:23 ` [PATCH 06/10] readahead: add vfs/readahead tracing event Wu Fengguang
2011-12-19 10:23 ` [PATCH 07/10] readahead: add /debug/readahead/stats Wu Fengguang
2011-12-23 12:59 ` [PATCH 07/10 v2] " Wu Fengguang
2011-12-23 13:48 ` Jan Kara
2011-12-19 10:23 ` [PATCH 08/10] readahead: basic support for backwards prefetching Wu Fengguang
2011-12-19 10:23 ` [PATCH 09/10] readahead: dont do start-of-file readahead after lseek() Wu Fengguang
2011-12-19 10:23 ` [PATCH 10/10] readahead: snap readahead request to EOF Wu Fengguang
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