VM disk cache behavior.

[jon ross]

I have an app with a small fixed memory footprint that does a lot of
random reads from a large file. I thought if I added more memory to
the machine the VM would do more caching of the disk, but added memory
does not seem to make any difference. I played with some of the params
in /proc/sys/vm and none of them seem to have any effect.

I tired both a 2.4.20 & 2.6.10 kernels with no difference.

The machine is a Dell 2560. I tired memory configs of 512M, 1G, 4G and
the average read-times do not change.

Do I need to set/compile anything to allow the VM to use the memory?
If is was a way to tell how much memory the VM is using for a drive
cache I could at least tell if my kernel is miss-configured or my app
sucks.

Thanks,

-Jon
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