Re: [PATCH] readahead:add blk_run_backing_dev

From: Hisashi Hifumi
Date: Tue May 26 2009 - 22:26:45 EST

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@xxxxxxxxx> 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?

My testing environment is as follows:
Hardware: HP DL580
CPU:Xeon 3.2GHz *4 HT enabled
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.


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