linux-kernel - Re: [PATCH] readahead:add blk_run_backing

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Date:	Thu, 21 May 2009 15:01:47 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, Wu Fengguang <fengguang.wu@...el.com>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH] readahead:add blk_run_backing_dev


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@....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@....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);



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