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

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Date:	Tue, 26 May 2009 16:42:52 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	hifumi.hisashi@....ntt.co.jp, 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

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
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?

Thanks.
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