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    SubjectRe: [PATCH] readahead:add blk_run_backing_dev
    On Wed, 27 May 2009 11:21:53 +0900 Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp> 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)
    >
    > 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.
    >

    Ah. So it's likely to be some strange interaction with the RAID setup.

    I assume that you're using the SANNet 2's "hardware raid"? Or is the
    array set up as jbod and you're using kernel raid0?



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