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    SubjectRe: qsbench, interesting results
    Lorenzo Allegrucci wrote:
    > qsbench is a VM benchmark based on sorting a large array
    > by quick sort.
    > Below are some results of qsbench sorting a 350Mb array
    > on a 256+400Mb RAM+swap machine.
    > Tested kernels: 2.4.19, 2.5.38 and 2.5.39

    Thanks for pointing this out. It's happening because the VM in
    2.5.39 tries to avoid stalling tasks for too long.

    That works well, so qsbench just gets in and submits more reads
    against the swapdevice much earlier than it used to. The new IO
    scheduler then obligingly promotes the swap reads ahead of the
    swap writes and we end up doing a ton of seeking.

    The -mm patchset has some kswapd improvements which pull most
    of the difference back.

    Stronger fixes for this would be a) penalise heavily-faulting
    tasks and b) tag swap writeout as needing higher priority at the
    block level.

    I'll take a look at some preferential throttling later on. But
    I must say that I'm not hugely worried about performance regression
    under wild swapstorms. The correct fix is to go buy some more
    RAM, and the kernel should not be trying to cater for underprovisioned
    machines if that affects the usual case.
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      Last update: 2005-03-22 13:29    [W:0.029 / U:6.688 seconds]
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