From: Michal Hocko <mhocko@suse.com>
To: Jann Horn <jannh@google.com>
Cc: Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
linux-man <linux-man@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Mark Mossberg <mark.mossberg@gmail.com>
Subject: Re: [PATCH] proc.5: Document inaccurate RSS due to SPLIT_RSS_COUNTING
Date: Mon, 12 Oct 2020 17:33:13 +0200 [thread overview]
Message-ID: <20201012153313.GI29725@dhcp22.suse.cz> (raw)
In-Reply-To: <CAG48ez271-LALwrM3dDr_vvHr8XaoowcQLQGYAnxe1WLECse9A@mail.gmail.com>
On Mon 12-10-20 17:20:08, Jann Horn wrote:
> On Mon, Oct 12, 2020 at 5:07 PM Michal Hocko <mhocko@suse.com> wrote:
> > On Mon 12-10-20 13:49:40, Jann Horn wrote:
> > > Since 34e55232e59f7b19050267a05ff1226e5cd122a5 (introduced back in
> > > v2.6.34), Linux uses per-thread RSS counters to reduce cache contention on
> > > the per-mm counters. With a 4K page size, that means that you can end up
> > > with the counters off by up to 252KiB per thread.
> >
> > Do we actually have any strong case to keep this exception to the
> > accounting?
>
> I have no clue. The concept of "concurrently modified cache lines are
> bad" seemed vaguely reasonable to me... but I have no idea how much
> impact this actually has on massively multithreaded processes.
I do remember some discussion when imprecision turned out to be a real
problem (Android?).
Anyway, I have to say that 34e55232e59f ("mm: avoid false sharing of
mm_counter") sounds quite dubious to me and it begs for re-evaluation.
Btw. thanks for trying to document this weird behavior. This is
certainly useful but I am suspecting that dropping it might be even
better.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-10-12 15:33 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-12 11:49 [PATCH] proc.5: Document inaccurate RSS due to SPLIT_RSS_COUNTING Jann Horn
2020-10-12 14:52 ` Jann Horn
2020-10-27 7:05 ` Michael Kerrisk (man-pages)
2020-10-27 10:35 ` Jann Horn
2020-10-27 12:18 ` Michal Hocko
2020-10-27 13:49 ` Michal Hocko
2020-10-27 13:49 ` Michael Kerrisk (man-pages)
2020-10-12 15:07 ` Michal Hocko
2020-10-12 15:20 ` Jann Horn
2020-10-12 15:33 ` Michal Hocko [this message]
2020-10-27 18:56 ` Vlastimil Babka
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