linux-kernel - Re: [PATCH v2 1/2] memcg: flush stats only if updated

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Message-Id: <20211104142751.5ab290d5cf4be1749c9c87ed@linux-foundation.org>
Date:   Thu, 4 Nov 2021 14:27:51 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     mkoutny@...e.com, cgroups@...r.kernel.org, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...nel.org
Subject: Re: [PATCH v2 1/2] memcg: flush stats only if updated

On Thu, 14 Oct 2021 09:31:46 -0700 Shakeel Butt <shakeelb@...gle.com> wrote:

> Hi Michal,
> 
> On Wed, Oct 13, 2021 at 11:01 AM Michal Koutný <mkoutny@...e.com> wrote:
> >
> > On Fri, Oct 01, 2021 at 12:00:39PM -0700, Shakeel Butt <shakeelb@...gle.com> wrote:
> > > In this patch we kept the stats update codepath very minimal and let the
> > > stats reader side to flush the stats only when the updates are over a
> > > specific threshold.  For now the threshold is (nr_cpus * CHARGE_BATCH).
> >
> > BTW, a noob question -- are the updates always single page sized?
> >
> > This is motivated by apples vs oranges comparison since the
> >         nr_cpus * MEMCG_CHARGE_BATCH
> > suggests what could the expected error be in pages (bytes). But it's mostly
> > wrong since: a) uncertain single-page updates, b) various counter
> > updates summed together. I wonder whether the formula can serve to
> > provide at least some (upper) estimate.
> >
> 
> Thanks for your review. This forces me to think more on this because each
> update does not necessarily be a single page sized update e.g. adding a hugepage
> to an LRU.
> 
> Though I think the error is time bounded by 2 seconds but in those 2 seconds
> mathematically the error can be large.

Sounds significant?

> What do you think of the following
> change? It will bound the error better within the 2 seconds window.

This didn't seem to go anywhere.  I'll send "memcg: flush stats only if
updated" Linuswards, but please remember to resurrect this idea soonish
(this month?) if you think such a change is desirable.