Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 28 May 2019 10:37:15 -0700 | Subject | Re: [PATCH] mm: memcontrol: don't batch updates of local VM stats and events |
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On Tue, May 28, 2019 at 9:00 AM Shakeel Butt <shakeelb@google.com> wrote: > > I was suspecting the following for-loop+atomic-add for the regression.
If I read the kernel test robot reports correctly, Johannes' fix patch does fix the regression (well - mostly. The original reported regression was 26%, and with Johannes' fix patch it was 3% - so still a slight performance regression, but not nearly as bad).
> Why the above atomic-add is the culprit?
I think the problem with that one is that it's cross-cpu statistics, so you end up with lots of cacheline bounces on the local counts when you have lots of load.
But yes, the recursive updates still do show a small regression, probably because there's still some overhead from the looping up in the hierarchy. You still get *those* cacheline bounces, but now they are limited to the upper hierarchies that only get updated at batch time.
Johannes? Am I reading this right?
Linus
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