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From: Muchun Song <songmuchun@bytedance.com>
To: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	 Vladimir Davydov <vdavydov.dev@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	 Shakeel Butt <shakeelb@google.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	 Chris Down <chris@chrisdown.name>,
	Yafang Shao <laoar.shao@gmail.com>,
	richard.weiyang@gmail.com,  LKML <linux-kernel@vger.kernel.org>,
	Cgroups <cgroups@vger.kernel.org>,
	 Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [External] Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage
Date: Wed, 9 Dec 2020 15:05:49 +0800	[thread overview]
Message-ID: <CAMZfGtVJnZESZQZmPzk=mBqK0KH4L=-s_+ebTr-6Z76zdXXn7g@mail.gmail.com> (raw)
In-Reply-To: <20201209031115.GA2390587@carbon.lan>

On Wed, Dec 9, 2020 at 11:52 AM Roman Gushchin <guro@fb.com> wrote:
>
> 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@fb.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.

Yeah, here I need to update the commit log.

>
> > > >
> > > > 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.

Here I mean possible CPU not online CPU. The number of possible
CPUs may be greater than online CPUs. The per-cpu allocator is based
on the number of possible CPUs. Right?

Thanks.

>
> Thanks!



-- 
Yours,
Muchun


      reply	other threads:[~2020-12-09  7:06 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-08  9:51 [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage Muchun Song
2020-12-08 18:20 ` Shakeel Butt
2020-12-09  2:21 ` Roman Gushchin
2020-12-09  2:28   ` Roman Gushchin
2020-12-09  2:31   ` [External] " Muchun Song
2020-12-09  3:52     ` Roman Gushchin
2020-12-09  7:05       ` Muchun Song [this message]

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