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

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Date:	Sat, 20 Jun 2009 16:29:31 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"Alan.Brunelle@...com" <Alan.Brunelle@...com>,
	"hifumi.hisashi@....ntt.co.jp" <hifumi.hisashi@....ntt.co.jp>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"randy.dunlap@...cle.com" <randy.dunlap@...cle.com>,
	Beheer InterCommIT <intercommit@...il.com>
Subject: Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev


Wu Fengguang, on 06/20/2009 07:55 AM wrote:
> On Fri, Jun 19, 2009 at 03:04:36AM +0800, Andrew Morton wrote:
>> 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.
> 
> Agreed. The expected (and interesting) test on a properly configured
> HW RAID has not happened yet, hence the theory remains unsupported.

Hmm, do you see anything improper in the Ronald's setup (see 
http://sourceforge.net/mailarchive/forum.php?thread_name=a0272b440906030714g67eabc5k8f847fb1e538cc62%40mail.gmail.com&forum_name=scst-devel)? 
It is HW RAID based.

As I already wrote, we can ask Ronald to perform any needed tests.

> Thanks,
> Fengguang

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