Re: [PATCH] (1/2) reverse mapping VM for 2.5.23 (rmap-13b)

Martin J. Bligh
2002-06-24T21:35:36Z

>> I'll try a more varied set of tests tonight, with cpu usage tabulated.
>
> Please do a few non-swap tests too.
>
> Swapping is the thing that rmap is supposed to _help_, so improvements in
> that area are good (and had better happen!), but if you're only looking at
> the swap performance, you're ignoring the known problems with rmap, ie the
> cases where non-rmap kernels do really well.
>
> Comparing one but not the other doesn't give a very balanced picture..

It would also be interesting to see memory consumption figures for a benchmark
with many large processes. With this type of load, memory consumption
through PTEs is already a problem - as far as I can see, rmap triples the
memory requirement of PTEs through the PTE chain's doubly linked list
(an additional 8 bytes per entry) ... perhaps my calculations are wrong?
This is particular problem for databases that tend to have thousands of
processes attatched to a large shared memory area.

A quick rough calculation indicates that the Oracle test I was helping out
with was consuming almost 10Gb of PTEs without rmap - 30Gb for overhead
doesn't sound like fun to me ;-(

M.