linux-kernel - Re: [PATCH] mm: memcg: Use larger chunks for proactive reclaim

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Date: Thu, 1 Feb 2024 14:57:22 +0100
From: Michal Koutný <mkoutny@...e.com>
To: "T.J. Mercier" <tjmercier@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Shakeel Butt <shakeelb@...gle.com>, 
	Muchun Song <muchun.song@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>, 
	Efly Young <yangyifei03@...ishou.com>, android-mm@...gle.com, yuzhao@...gle.com, 
	cgroups@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcg: Use larger chunks for proactive reclaim

Hello.

On Wed, Jan 31, 2024 at 04:24:41PM +0000, "T.J. Mercier" <tjmercier@...gle.com> wrote:
>  		reclaimed = try_to_free_mem_cgroup_pages(memcg,
> -					min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
> +					max((nr_to_reclaim - nr_reclaimed) / 4,
> +					    (nr_to_reclaim - nr_reclaimed) % 4),

The 1/4 factor looks like magic. 

Commit 0388536ac291 says:
| In theory, the amount of reclaimed would be in [request, 2 * request).

Doesn't this suggest 1/2 as a better option? (I didn't pursue the
theory.)

Also IMO importantly, when nr_to_reclaim - nr_reclaimed is less than 8,
the formula gives arbitrary (unrelated to delta's magnitude) values.

Regards,
Michal

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