Re: [PATCH] readahead:add blk_run_backing_dev

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 27 May 2009 11:21:53 +0900 Hisashi Hifumi <hifumi.hisashi@oss.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@intel.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?

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