From: Linus Torvalds <torvalds@linux-foundation.org>
To: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Andrew Morton <akpm@linux-foundation.org>,
kernel test robot <rong.a.chen@intel.com>, LKP <lkp@01.org>,
LKML <linux-kernel@vger.kernel.org>,
Michal Hocko <mhocko@kernel.org>, Roman Gushchin <guro@fb.com>,
Linux MM <linux-mm@kvack.org>, Cgroups <cgroups@vger.kernel.org>
Subject: Re: [PATCH] mm: memcontrol: don't batch updates of local VM stats and events
Date: Tue, 28 May 2019 10:37:15 -0700 [thread overview]
Message-ID: <CAHk-=wgaLQjZ8AZj76_cwvk_wLPJjr+Dc=Qvac_vHY2RruuBww@mail.gmail.com> (raw)
In-Reply-To: <CALvZod5KFJvfBfTZKWiDo_ux_OkLKK-b6sWtnYeFCY2ARiiKwQ@mail.gmail.com>
On Tue, May 28, 2019 at 9:00 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> I was suspecting the following for-loop+atomic-add for the regression.
If I read the kernel test robot reports correctly, Johannes' fix patch
does fix the regression (well - mostly. The original reported
regression was 26%, and with Johannes' fix patch it was 3% - so still
a slight performance regression, but not nearly as bad).
> Why the above atomic-add is the culprit?
I think the problem with that one is that it's cross-cpu statistics,
so you end up with lots of cacheline bounces on the local counts when
you have lots of load.
But yes, the recursive updates still do show a small regression,
probably because there's still some overhead from the looping up in
the hierarchy. You still get *those* cacheline bounces, but now they
are limited to the upper hierarchies that only get updated at batch
time.
Johannes? Am I reading this right?
Linus
next prev parent reply other threads:[~2019-05-28 17:37 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-20 6:35 [mm] 42a3003535: will-it-scale.per_process_ops -25.9% regression kernel test robot
2019-05-20 21:53 ` Johannes Weiner
2019-05-21 13:46 ` [LKP] " kernel test robot
2019-05-21 15:13 ` Johannes Weiner
2019-05-21 15:16 ` [PATCH] mm: memcontrol: don't batch updates of local VM stats and events Johannes Weiner
2019-05-28 16:00 ` Shakeel Butt
2019-05-28 17:37 ` Linus Torvalds [this message]
2019-05-28 20:32 ` Johannes Weiner
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