linux-kernel - Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu

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Date: Mon, 12 Feb 2024 14:42:09 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Nhat Pham <nphamcs@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/zswap: change zswap_pool kref to percpu_ref

On Sun, Feb 11, 2024 at 5:58 AM Chengming Zhou
<zhouchengming@...edance.com> wrote:
>
> All zswap entries will take a reference of zswap_pool when
> zswap_store(), and drop it when free. Change it to use the
> percpu_ref is better for scalability performance.
>
> Testing kernel build in tmpfs with memory.max=2GB
> (zswap shrinker and writeback enabled with one 50GB swapfile).
>
>         mm-unstable  zswap-global-lru
> real    63.20        63.12
> user    1061.75      1062.95
> sys     268.74       264.44

Are these numbers from a single run or the average of multiple runs?
It just seems that the improvement is small, and percpu refcnt is
slightly less intuitive (and uses a bit more memory), so let's make
sure there is a real performance gain first.

It would also be useful to mention how many threads/CPUs are being used here.