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    SubjectRe: [REGRESSION] 2.6.19/2.6.20-rc3 buffered write slowdown
    David Chinner wrote:
    > On Thu, Jan 11, 2007 at 10:13:55AM +1100, Nick Piggin wrote:
    >
    >>David Chinner wrote:
    >>
    >>>On Wed, Jan 10, 2007 at 03:04:15PM -0800, Christoph Lameter wrote:
    >>>
    >>>
    >>>>On Thu, 11 Jan 2007, David Chinner wrote:
    >>>>
    >>>>
    >>>>
    >>>>>The performance and smoothness is fully restored on 2.6.20-rc3
    >>>>>by setting dirty_ratio down to 10 (from the default 40), so
    >>>>>something in the VM is not working as well as it used to....
    >>>>
    >>>>dirty_background_ratio is left as is at 10?
    >>>
    >>>
    >>>Yes.
    >>>
    >>>
    >>>
    >>>>So you gain performance by switching off background writes via pdflush?
    >>>
    >>>
    >>>Well, pdflush appears to be doing very little on both 2.6.18 and
    >>>2.6.20-rc3. In both cases kswapd is consuming 10-20% of a CPU and
    >>>all of the pdflush threads combined (I've seen up to 7 active at
    >>>once) use maybe 1-2% of cpu time. This occurs regardless of the
    >>>dirty_ratio setting.
    >>
    >>Hi David,
    >>
    >>Could you get /proc/vmstat deltas for each kernel, to start with?
    >
    >
    > Sure, but that doesn't really show the how erratic the per-filesystem
    > throughput is because the test I'm running is PCI-X bus limited in
    > it's throughput at about 750MB/s. Each dm device is capable of about
    > 340MB/s write, so when one slows down, the others will typically
    > speed up.

    But you do also get aggregate throughput drops? (ie. 2.6.20-rc3-worse)

    > So, what I've attached is three files which have both
    > 'vmstat 5' output and 'iostat 5 |grep dm-' output in them.

    Ahh, sorry to be unclear, I meant:

    cat /proc/vmstat > pre
    run_test
    cat /proc/vmstat > post

    It might just give us a hint what is changing (however vmstat doesn't
    give much interesting in the way of pdflush stats, so it might not
    show anything up).

    Thanks,
    Nick

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