Re: [PATCH 2/2] mm/memcontrol: split local and nested atomic vmstats/vmevents counters

[Konstantin Khlebnikov]

On 17.07.2019 20:53, Johannes Weiner wrote:
> On Wed, Jul 17, 2019 at 03:29:19PM +0300, Konstantin Khlebnikov wrote:
> > This is alternative solution for problem addressed in commit 815744d75152
> > ("mm: memcontrol: don't batch updates of local VM stats and events").
> >
> > Instead of adding second set of percpu counters which wastes memory and
> > slows down showing statistics in cgroup-v1 this patch use two arrays of
> > atomic counters: local and nested statistics.
> >
> > Then update has the same amount of atomic operations: local update and
> > one nested for each parent cgroup. Readers of hierarchical statistics
> > have to sum two atomics which isn't a big deal.
> >
> > All updates are still batched using one set of percpu counters.
> >
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
>
> Yeah that looks better. Note that it was never about the atomics,
> though, but rather the number of cachelines dirtied. Your patch should
> solve this problem as well, but it might be a good idea to run
> will-it-scale on it to make sure the struct layout is still fine.

Looks like this patch shows 2% regression for 24 core 2 numa node
machine I have. Compete remove of these counters gives 2% boost.
Also I cannot reproduce regression fixed by commit 815744d75152 - revert
have no effect.

So, feel free to ignore second patch. I'll play with this a little more.

Maybe atomic per-numa counters could give nice balance between scalability add overhead.
Ideally this memory could be mapped in per-cpu manner to give atomic access via fs/gs.