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

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Date:	Tue, 19 May 2009 09:44:34 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] readahead:add blk_run_backing_dev


At 02:53 09/05/19, 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.
>
>So, are these numbers 100% reproducible? Could you capture blktrace data
>for both with and without the patch, so we can take a closer look at the
>generated IO for each case?

Thank you for your comment. This number is 100% reproducible but the number 
depends on hardware environment.
I will try to get blktrace data and post this.

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