linux-kernel - Re: [External] Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

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Message-ID: <CAMZfGtUMP6mz3DE7DHS55fyto=LZuQpcitt59WuwhZw8m2LqBg@mail.gmail.com>
Date:   Wed, 9 Dec 2020 10:31:55 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Roman Gushchin <guro@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Chris Down <chris@...isdown.name>,
        Yafang Shao <laoar.shao@...il.com>, richard.weiyang@...il.com,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [External] Re: [PATCH v2] mm: memcontrol: optimize per-lruvec
 stats counter memory usage

On Wed, Dec 9, 2020 at 10:21 AM Roman Gushchin <guro@...com> wrote:
>
> On Tue, Dec 08, 2020 at 05:51:32PM +0800, Muchun Song wrote:
> > The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> > of lruvec_stat_cpu is enough. And introduce struct per_cpu_lruvec_stat
> > to optimize memory usage.
> >
> > The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> > is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> > bytes per-memcg per-node where ncpu is the number of the possible CPU.
> > If there are c memory cgroup (include dying cgroup) and n NUMA node in
> > the system. Finally, we can save (152 * ncpu * c * n) bytes.
>
> Honestly, I'm not convinced.
> Say, ncpu = 32, n = 2, c = 500. We're saving <5Mb of memory.
> If the machine has 128Gb of RAM, it's .000000003%.

Hi Roman,

When the cpu hotplug is enabled, the ncpu can be 256 on
some configurations. Also, the c can be more large when
there are many dying cgroup in the system.

So the savings depends on the environment and
configurations. Right?


>
> Using longs (s64) allows not to think too much about overflows
> and can also be slightly faster on 64-bit machines.
>
> Thanks!



--
Yours,
Muchun