Re: [PATCH] mm: memcg: Use larger chunks for proactive reclaim

["T.J. Mercier" <tjmercier@google.com>]

On Thu, Feb 1, 2024 at 7:34 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Feb 01, 2024 at 02:57:22PM +0100, Michal Koutný wrote:
> > Hello.
> >
> > On Wed, Jan 31, 2024 at 04:24:41PM +0000, "T.J. Mercier" <tjmercier@google.com> wrote:
> > >             reclaimed = try_to_free_mem_cgroup_pages(memcg,
> > > -                                   min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
> > > +                                   max((nr_to_reclaim - nr_reclaimed) / 4,
> > > +                                       (nr_to_reclaim - nr_reclaimed) % 4),
> >
> > The 1/4 factor looks like magic.
>
> It's just cutting the work into quarters to balance throughput with
> goal accuracy. It's no more or less magic than DEF_PRIORITY being 12,
> or SWAP_CLUSTER_MAX being 32.

Using SWAP_CLUSTER_MAX is sort of like having a really large divisor
instead of 4 (or 1 like before).

I recorded the average number of iterations required to complete the
1G reclaim for the measurements I took and it looks like this:
pre-0388536ac291     : 1
post-0388536ac291    : 1814
(reclaim-reclaimed)/4: 17

Given the results with /4, I don't think the perf we get here is
particularly sensitive to the number we choose, but it's definitely a
tradeoff.

<snip>

> > Also IMO importantly, when nr_to_reclaim - nr_reclaimed is less than 8,
> > the formula gives arbitrary (unrelated to delta's magnitude) values.
>
> try_to_free_mem_cgroup_pages() rounds up to SWAP_CLUSTER_MAX. So the
> error margin is much higher at the smaller end of requests anyway.
> But practically speaking, users care much less if you reclaim 32 pages
> when 16 were requested than if you reclaim 2G when 1G was requested.

I like Johannes's suggestion of just a comment instead of the mod op.
It's easier to read, slightly less generated code, and even if we
didn't have the .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX) in
try_to_free_mem_cgroup_pages, memory_reclaim would still get very
close to the target before running out of nr_retries.