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

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Date:	Wed, 27 May 2009 11:38:08 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>
Subject: Re: [PATCH] readahead:add blk_run_backing_dev


At 11:36 09/05/27, Andrew Morton wrote:
>On Wed, 27 May 2009 11:21:53 +0900 Hisashi Hifumi 
><hifumi.hisashi@....ntt.co.jp> wrote:
>
>> 
>> At 11:09 09/05/27, Wu Fengguang wrote:
>> >On Wed, May 27, 2009 at 08:25:04AM +0800, Hisashi Hifumi wrote:
>> >> 
>> >> 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.
>> >
>> >Hisashi, does your environment have some special configurations?
>> 
>> Hi.
>> My testing environment is as follows:
>> Hardware: HP DL580 
>> CPU:Xeon 3.2GHz *4 HT enabled
>> Memory:8GB
>> Storage: Dothill SANNet2 FC (7Disks RAID-0 Array)
>> 
>> I did dd to this disk-array and got improved performance number.
>> 
>> I noticed that when a disk is just one HDD, performance improvement
>> is very small.
>> 
>
>Ah.  So it's likely to be some strange interaction with the RAID setup.
>
>I assume that you're using the SANNet 2's "hardware raid"?  Or is the
>array set up as jbod and you're using kernel raid0?

I used SANNet 2's "hardware raid".

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