Re: [PATCH] readahead:add blk_run_backing_dev

Hisashi Hifumi
2009-05-27T00:29:20Z

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.

Thanks.