Re: [PATCH] readahead:add blk_run_backing_dev

Hisashi Hifumi
2009-05-21T06:06:18Z

At 11:51 09/05/20, Wu Fengguang wrote:
>On Mon, May 18, 2009 at 07:53:00PM +0200, Jens Axboe wrote:
>> On Mon, May 18 2009, Hisashi Hifumi wrote:
>> > Hi.
>> >
>> > I wrote a patch that adds blk_run_backing_dev on page_cache_async_readahead
>> > so readahead I/O is unpluged to improve throughput.
>> >
>> > Following is the test result with dd.
>> >
>> > #dd if=testdir/testfile of=/dev/null bs=16384
>> >
>> > -2.6.30-rc6
>> > 1048576+0 records in
>> > 1048576+0 records out
>> > 17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s
>> >
>> > -2.6.30-rc6-patched
>> > 1048576+0 records in
>> > 1048576+0 records out
>> > 17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s
>> >
>> > Sequential read performance on a big file was improved.
>> > Please merge my patch.
>> >
>> > Thanks.
>> >
>> > Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>
>> >
>> > diff -Nrup linux-2.6.30-rc6.org/mm/readahead.c
>linux-2.6.30-rc6.unplug/mm/readahead.c
>> > --- linux-2.6.30-rc6.org/mm/readahead.c 2009-05-18 10:46:15.000000000 +0900
>> > +++ linux-2.6.30-rc6.unplug/mm/readahead.c 2009-05-18
>13:00:42.000000000 +0900
>> > @@ -490,5 +490,7 @@ page_cache_async_readahead(struct addres
>> >
>> > /* do read-ahead */
>> > ondemand_readahead(mapping, ra, filp, true, offset, req_size);
>> > +
>> > + blk_run_backing_dev(mapping->backing_dev_info, NULL);
>> > }
>> > EXPORT_SYMBOL_GPL(page_cache_async_readahead);
>>
>> I'm surprised this makes much of a difference. It seems correct to me to
>> NOT unplug the device, since it will get unplugged when someone ends up
>> actually waiting for a page. And that will then kick off the remaining
>> IO as well. For this dd case, you'll be hitting lock_page() for the
>> readahead page really soon, definitely not long enough to warrant such a
>> big difference in speed.
>
>The possible timing change of this patch is (assuming readahead size=100):
>
>T0 read(100), which triggers readahead(200, 100)
>T1 read(101)
>T2 read(102)
>...
>T100 read(200), find_get_page(200) => readahead(300, 100)
> lock_page(200) => implicit unplug
>
>The readahead(200, 100) submitted at time T0 *might* be delayed to the
>unplug time of T100.
>
>But that is only a possibility. In normal cases, the read(200) would
>be blocking and there will be a lock_page(200) that will immediately
>unplug device for readahead(300, 100).

Hi Andrew.
Following patch improves sequential read performance and does not harm
other performance.
Please merge my patch.
Comments?
Thanks.

#dd if=testdir/testfile of=/dev/null bs=16384
-2.6.30-rc6
1048576+0 records in
1048576+0 records out
17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s

-2.6.30-rc6-patched
1048576+0 records in
1048576+0 records out
17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s

Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>

diff -Nrup linux-2.6.30-rc6.org/mm/readahead.c linux-2.6.30-rc6.unplug/mm/readahead.c
--- linux-2.6.30-rc6.org/mm/readahead.c 2009-05-18 10:46:15.000000000 +0900
+++ linux-2.6.30-rc6.unplug/mm/readahead.c 2009-05-18 13:00:42.000000000 +0900
@@ -490,5 +490,7 @@ page_cache_async_readahead(struct addres

/* do read-ahead */
ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+
+ blk_run_backing_dev(mapping->backing_dev_info, NULL);
}
EXPORT_SYMBOL_GPL(page_cache_async_readahead);