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

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Date:	Thu, 18 Jun 2009 12:04:36 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	kosaki.motohiro@...fujitsu.com, Alan.Brunelle@...com,
	hifumi.hisashi@....ntt.co.jp, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, jens.axboe@...cle.com,
	randy.dunlap@...cle.com
Subject: Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev

On Sun, 7 Jun 2009 06:45:38 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> > > Do you have a place where the raw blktrace data can be retrieved for
> > > more in-depth analysis?
> >
> > I think your comment is really adequate. In another thread, Wu Fengguang pointed
> > out the same issue.
> > I and Wu also wait his analysis.
> 
> And do it with a large readahead size :)
> 
> Alan, this was my analysis:
> 
> : Hifumi, can you help retest with some large readahead size?
> :
> : Your readahead size (128K) is smaller than your max_sectors_kb (256K),
> : so two readahead IO requests get merged into one real IO, that means
> : half of the readahead requests are delayed.
> 
> ie. two readahead requests get merged and complete together, thus the effective
> IO size is doubled but at the same time it becomes completely synchronous IO.
> 
> :
> : The IO completion size goes down from 512 to 256 sectors:
> :
> : before patch:
> :   8,0    3   177955    50.050313976     0  C   R 8724991 + 512 [0]
> :   8,0    3   177966    50.053380250     0  C   R 8725503 + 512 [0]
> :   8,0    3   177977    50.056970395     0  C   R 8726015 + 512 [0]
> :   8,0    3   177988    50.060326743     0  C   R 8726527 + 512 [0]
> :   8,0    3   177999    50.063922341     0  C   R 8727039 + 512 [0]
> :
> : after patch:
> :   8,0    3   257297    50.000760847     0  C   R 9480703 + 256 [0]
> :   8,0    3   257306    50.003034240     0  C   R 9480959 + 256 [0]
> :   8,0    3   257307    50.003076338     0  C   R 9481215 + 256 [0]
> :   8,0    3   257323    50.004774693     0  C   R 9481471 + 256 [0]
> :   8,0    3   257332    50.006865854     0  C   R 9481727 + 256 [0]
> 

I haven't sent readahead-add-blk_run_backing_dev.patch in to Linus yet
and it's looking like 2.6.32 material, if ever.

If it turns out to be wonderful, we could always ask the -stable
maintainers to put it in 2.6.x.y I guess.


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