linux-kernel - Re: [PATCH v2 3/6] mm: zswap: proactive shrinking before pool size limit is hit

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Message-ID: <CAKEwX=M8vTfBsptOF=GdbFKK5V++5Lki+XSHkOmVKWYXPVbkOA@mail.gmail.com>
Date: Fri, 12 Jul 2024 16:18:52 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Takero Funaki <flintglass@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Jonathan Corbet <corbet@....net>, 
	Andrew Morton <akpm@...ux-foundation.org>, 
	Domenico Cerasuolo <cerasuolodomenico@...il.com>, linux-mm@...ck.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/6] mm: zswap: proactive shrinking before pool size
 limit is hit

On Fri, Jul 5, 2024 at 7:25 PM Takero Funaki <flintglass@...il.com> wrote:
>
> This patch implements proactive shrinking of zswap pool before the max
> pool size limit is reached. This also changes zswap to accept new pages
> while the shrinker is running.
>
> To prevent zswap from rejecting new pages and incurring latency when
> zswap is full, this patch queues the global shrinker by a pool usage
> threshold between 100% and accept_thr_percent, instead of the max pool
> size.  The pool size will be controlled between 90% to 91% for the
> default accept_thr_percent=90.  Since the current global shrinker
> continues to shrink until accept_thr_percent, we do not need to maintain
> the hysteresis variable tracking the pool limit overage in
> zswap_store().
>
> Before this patch, zswap rejected pages while the shrinker is running
> without incrementing zswap_pool_limit_hit counter. It could be a reason
> why zswap writethrough new pages before writeback old pages.  With this
> patch, zswap accepts new pages while shrinking, and zswap increments
> the counter when and only when zswap rejects pages by the max pool size.
>
> Now, reclaims smaller than the proactive shrinking amount finish
> instantly and trigger background shrinking.  Admins can check if new
> pages are buffered by zswap by monitoring the pool_limit_hit counter.
>
> The name of sysfs tunable accept_thr_percent is unchanged as it is still
> the stop condition of the shrinker.
> The respective documentation is updated to describe the new behavior.
>
> Signed-off-by: Takero Funaki <flintglass@...il.com>

Code-wise and idea-wise, I like this patch :) This is dependent on
other changes (as you have pointed out), but if the series goes
through, please feel free to include:

Reviewed-by: Nhat Pham <nphamcs@...il.com>