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    Subject2.4.0test9 vm: disappointing streaming i/o under load

    Hi,

    Finally got round to checking out 2.4.0test9.

    Unfortunately, 2.4.0test9 exhibits poor streaming i/o performance when
    under a bit of memory pressure.

    The test is this: boot with mem=32M, log onto GNOME and start xmms playing
    a big .wav ripped from a CD (this requires 100-200k read i/o per second).

    Then, I start then kill netscape. I then started a find / and started
    gnumeric firing up at the same time.

    Results
    =======

    2.2 RH7.0: the music skipped maybe twice briefly during the test.

    2.4.0test9: music stuttered repeatedly while netscape started. Worse, when
    firing up gnumeric with the find / on the go, there were big pauses in
    sound output. On pause was over 5 seconds!!!


    So not so hot.

    Could this perhaps be related to the drop_behind magic penalizing
    streaming i/o pages too much? Perhaps the greater ago on the i/o pages
    means that when there is a little memory pressure, they are getting thrown
    out the page cache before the app (xmms) gets a chance to use them!

    Might it be useful for me to try pre10-1, I note it has more "balancing
    fixes".

    Cheers
    Chris


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