Re: [PATCH] memcg: async flush memcg stats from perf sensitive codepaths

[Michal Koutný]

Making decisions based on up to 2 s old information.

On Tue, Mar 01, 2022 at 09:21:12AM -0800, Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> Without flushing the worst that can happen in the refault path is
> false (or missed) activations of the refaulted page.

Yeah, this may under- or overestimate workingset size (when it's
changing), the result is likely only less efficient reclaim.

> For reclaim code, some heuristics (like deactivating active LRU or
> cache-trim) may act on old information.

Here, I'd be more careful whether such a delay cannot introduce some
unstable behavior (permanent oscillation in the worst case).

> Now, coming to your question, yes, we can remove the flushing from
> these performance critical codepaths as the stats at most will be 2
> second old due to periodic flush. 

Another aspect is that people will notice and report such a narrowly
located performance regression more easily than reduced/less predictable
reclaim behavior. (IMO the former is better, OTOH, it can also be
interpreted that noone notices (is able to notice).)

Michal