linux-kernel - Re: [RFC PATCH 03/30] Lazy percpu counters

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Message-ID: <CAJuCfpHpBCUma_=AdTQ+UkfSkfkov2JbKfxLdp5K9_MoonkT7g@mail.gmail.com>
Date:   Wed, 31 Aug 2022 08:37:14 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Davidlohr Bueso <dave@...olabs.net>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R. Howlett" <liam.howlett@...cle.com>,
        David Vernet <void@...ifault.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, mcgrof@...nel.org,
        masahiroy@...nel.org, nathan@...nel.org, changbin.du@...el.com,
        ytcoode@...il.com, Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Christopher Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, 42.hyeyoo@...il.com,
        Alexander Potapenko <glider@...gle.com>,
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        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>, arnd@...db.de,
        jbaron@...mai.com, David Rientjes <rientjes@...gle.com>,
        Minchan Kim <minchan@...gle.com>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        kernel-team <kernel-team@...roid.com>,
        linux-mm <linux-mm@...ck.org>, iommu@...ts.linux.dev,
        kasan-dev@...glegroups.com, io-uring@...r.kernel.org,
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        linux-bcache@...r.kernel.org, linux-modules@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 03/30] Lazy percpu counters

On Wed, Aug 31, 2022 at 3:02 AM Mel Gorman <mgorman@...e.de> wrote:
>
> On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote:
> > From: Kent Overstreet <kent.overstreet@...ux.dev>
> >
> > This patch adds lib/lazy-percpu-counter.c, which implements counters
> > that start out as atomics, but lazily switch to percpu mode if the
> > update rate crosses some threshold (arbitrarily set at 256 per second).
> >
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
>
> Why not use percpu_counter? It has a per-cpu counter that is synchronised
> when a batch threshold (default 32) is exceeded and can explicitly sync
> the counters when required assuming the synchronised count is only needed
> when reading debugfs.

The intent is to use atomic counters for places that are not updated very often.
This would save memory required for the counters. Originally I had a config
option to choose which counter type to use but with lazy counters we sacrifice
memory for performance only when needed while keeping the other counters
small.

>
> --
> Mel Gorman
> SUSE Labs