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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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