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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 31 Aug 2022 08:37:14 -0700
Message-ID: <CAJuCfpHpBCUma_=AdTQ+UkfSkfkov2JbKfxLdp5K9_MoonkT7g@mail.gmail.com>
Subject: Re: [RFC PATCH 03/30] Lazy percpu counters
To:     Mel Gorman <mgorman@suse.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Kent Overstreet <kent.overstreet@linux.dev>,
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On Wed, Aug 31, 2022 at 3:02 AM Mel Gorman <mgorman@suse.de> wrote:
>
> On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote:
> > From: Kent Overstreet <kent.overstreet@linux.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@linux.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