Re: [PATCH] readahead:add blk_run_backing_dev

Hisashi Hifumi
2009-05-27T02:51:40Z

At 11:36 09/05/27, Wu Fengguang wrote:
>On Wed, May 27, 2009 at 10:21:53AM +0800, Hisashi Hifumi 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)
>
>This is a big hardware RAID. What's the readahead size?
>
>The numbers look too small for a 7 disk RAID:
>
> > #dd if=testdir/testfile of=/dev/null bs=16384
> >
> > -2.6.30-rc6
> > 1048576+0 records in
> > 1048576+0 records out
> > 17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s
> >
> > -2.6.30-rc6-patched
> > 1048576+0 records in
> > 1048576+0 records out
> > 17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s
>
>I'd suggest you to configure the array properly before coming back to
>measuring the impact of this patch.

I created 16GB file to this disk array, and mounted to testdir, dd to this directory.

Thanks.