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

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Date: Tue, 13 Feb 2024 17:45:26 +0000
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 Tue, Feb 13, 2024 at 10:31:16PM +0800, Chengming Zhou wrote:
> On 2024/2/13 06:42, Yosry Ahmed wrote:
> > 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?
> 
> The average of 5 runs. And I just checked/compared each run result,
> the improvement is stable. So yes, it should be a real performance gain.
> 
> > 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.
> 
> Right, percpu_ref use a bit more memory which should be ok for our use case,
> since we almost have only one zswap_pool to be using. The performance gain is
> for zswap_store/load hotpath.
> 
> > 
> > It would also be useful to mention how many threads/CPUs are being used here.
> 
> My bad, the testing uses 32 threads on a 128 CPUs x86-64 machine.

Thanks for the clarification. Please include such details in the commit
message.