Re: [PATCH]: Free pages from local pcp lists under tight memory conditions

Andrew Morton
2005-11-23T21:26:47Z

Rohit Seth <rohit.seth@intel.com> wrote:
>
> > I don't think Martin was able to demonstrate much benefit from the lock
> > contention reduction on 16-way NUMAQ either.
> >
> > So I dithered for months and it was a marginal merge, so it's appropriate
> > to justify the continued presence of the code.
> >
>
> May be the limits on the number of pages hanging on the per_cpu_pagelist
> was (or even now is) too small (for them to give any meaningful gain).
> May be we should have more physical contiguity in each of these pcps to
> give better cache spread.

Could be. The initial settings were pretty arbitrary - I assumed that
someone would get in and tune them up, but nothing much happened. Perhaps
we should expose the thresholds in /proc/sys/vm so they're easier to play
with.

> > We didn't measure for any coloring effects though. In fact, I didn't know
> > that this feature actually provided any benefit in that area.
>
> I thought Nick et.al came up with some of the constant values like batch
> size to tackle the page coloring issue specifically. In any case, I
> think one of the key difference between 2.4 and 2.6 allocators is the
> pcp list. And even with the minuscule batch and high watermarks this is
> helping ordinary benchmarks (by reducing the variation from run to run).

OK, useful info, thanks.