linux-kernel - Re: [PATCH v3 2/2] mm/compaction: make proactive compaction high watermark configurable via sysctl

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Message-ID: <e524abae-918b-4b23-8648-bf56f78464de@suse.cz>
Date: Thu, 30 Jan 2025 15:18:35 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Michal Clapinski <mclapinski@...gle.com>
Cc: Pasha Tatashin <tatashin@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] mm/compaction: make proactive compaction high
 watermark configurable via sysctl

On 1/28/25 02:18, Andrew Morton wrote:
> On Mon, 27 Jan 2025 22:50:20 +0100 Michal Clapinski <mclapinski@...gle.com> wrote:
> 
>> Currently, the difference between the high and low watermarks for
>> proactive compaction is hardcoded to 10. This hardcoded difference is
>> too large for free page reporting to work well.
>> 
>> Add a new sysctl, `compaction_proactiveness_leeway`, to control the
>> difference between the high and low watermarks.
>> 
> 
> Oh dear, yet another tunable.  Is there any way in which we can
> acceptably improve the kernel without adding this?

compaction_proactiveness between 0 and 90 works as usual,
thus up to low watermark of 10 and high watermark of 20

compaction_proactiveness between 90 and 100 additionally reduces leeway,
with value of 100 resulting of low = high = 0

or some similar scheme, as long as a value of 100 does low = high = 0

It's rather arbitrary but AFAIU does what Michal needs and higher
proactiveness means more aggressive compaction.

Question is, would anyone else find it useful to have low_watermark of 0 and
high watermark of 10?