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

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Date:	Wed, 27 May 2009 09:25:04 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	jens.axboe@...cle.com
Subject: Re: [PATCH] readahead:add blk_run_backing_dev


At 08:42 09/05/27, Andrew Morton wrote:
>On Fri, 22 May 2009 10:33:23 +0800
>Wu Fengguang <fengguang.wu@...el.com> wrote:
>
>> > I tested above patch, and I got same performance number.
>> > I wonder why if (PageUptodate(page)) check is there...
>> 
>> Thanks!  This is an interesting micro timing behavior that
>> demands some research work.  The above check is to confirm if it's
>> the PageUptodate() case that makes the difference. So why that case
>> happens so frequently so as to impact the performance? Will it also
>> happen in NFS?
>> 
>> The problem is readahead IO pipeline is not running smoothly, which is
>> undesirable and not well understood for now.
>
>The patch causes a remarkably large performance increase.  A 9%
>reduction in time for a linear read? I'd be surprised if the workload

Hi Andrew.
Yes, I tested this with dd.

>even consumed 9% of a CPU, so where on earth has the kernel gone to?
>
>Have you been able to reproduce this in your testing?

Yes, this test on my environment is reproducible.

Thanks.

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