Re: [PATCH] readahead:add blk_run_backing_dev

[Wu Fengguang <fengguang.wu@intel.com>]

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 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.

OK. You should mention the single disk and RAID performance earlier.

Thanks,
Fengguang