Re: VM disk cache behavior.

[Andy Isaacson <adi@hexapodia.org>]

On Tue, Feb 08, 2005 at 12:06:14PM -0500, jon ross wrote:
> 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.

Could we get some quant here?  How small is "small"?  How large is
"large"?  What are you measuring?  What are the results?  Does the app
re-use the same data, or is its use a one-time deal?

> Do I need to set/compile anything to allow the VM to use the memory?

No, the Linux VM system should automatically cache for you.

> 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.

Check out the commands "free", "vmstat 1", "top", the contents of
/proc/meminfo, the output of Sysrq-M.

Most likely is that your app isn't behaving in a cache-friendly way.  If
your file will fit in memory, just fault it in sequentially (wc -l file)
and then your app should cook.  If you're not going to fit in memory,
the vm caching will probably only help if you have some reuse; you could
develop a pre-faulter to get your IO started ahead of time, but that's
generally nontrivial.

-andy