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

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Date:	Wed, 27 May 2009 11:21:53 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"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 11:09 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.

Thanks.




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