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    SubjectRe: [PATCH] readahead:add blk_run_backing_dev

    At 11:37 09/06/01, Wu Fengguang wrote:
    >On Wed, May 27, 2009 at 11:06:37AM +0800, Hisashi Hifumi wrote:
    >>
    >> At 11:57 09/05/27, Wu Fengguang wrote:
    >> >On Wed, May 27, 2009 at 10:47:47AM +0800, Hisashi Hifumi wrote:
    >> >>
    >> >> 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.
    >> >
    >> >I mean, you should get >300MB/s throughput with 7 disks, and you
    >> >should seek ways to achieve that before testing out this patch :-)
    >>
    >> Throughput number of storage array is very from one product to another.
    >> On my hardware environment I think this number is valid and
    >> my patch is effective.
    >
    >What's your readahead size? Is it large enough to cover the stripe width?

    Do you mean strage's readahead size?



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