Re: [PATCH] readahead:add blk_run_backing_dev

[Wu Fengguang]

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?

Thanks,
Fengguang
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