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

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Date:   Tue, 8 Dec 2020 19:52:35 -0800
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.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 09, 2020 at 10:31:55AM +0800, Muchun Song wrote:
> 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.

Actually the threshold can be as big as MEMCG_CHARGE_BATCH * PAGE_SIZE.
It still fits into s32, but without explicitly saying it it's hard to
understand why not choosing s8, as in vmstat.c.

> > >
> > > 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?

Of course, but machines with more CPUs tend to have more RAM as well.

Thanks!