linux-kernel - Re: [PATCH 1/2] mm/compaction: remove low watermark cap for proactive compaction

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Message-ID: <a5bcc8f6-11a4-42f5-b332-7f4d3079e7fb@suse.cz>
Date: Mon, 27 Jan 2025 11:38:08 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Michal Clapinski <mclapinski@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Nitin Gupta <nigupta@...dia.com>,
 Pasha Tatashin <tatashin@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/compaction: remove low watermark cap for proactive
 compaction

On 1/24/25 19:21, Michal Clapinski wrote:
> Previously a min cap of 5 has been set in the commit introducing
> proactive compaction. This was to make sure users don't hurt themselves
> by setting the proactiveness to 100 and making their system
> unresponsive. But the compaction mechanism has a backoff mechanism that
> will sleep for 30s if no progress is made, so I don't see a significant
> risk here. My system (20GB of memory) has been perfectly fine with
> proactiveness set to 100 and leeway set to 0.

What if you don't set the leeway to 0? In other words, should we keep the
cap in some sense but make it depend on the leeway?

> Signed-off-by: Michal Clapinski <mclapinski@...gle.com>
> ---
>  mm/compaction.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index a2b16b08cbbff..29524242a16ef 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2253,7 +2253,7 @@ static unsigned int fragmentation_score_wmark(bool low)
>  	 * activity in case a user sets the proactiveness tunable
>  	 * close to 100 (maximum).
>  	 */
> -	wmark_low = max(100U - sysctl_compaction_proactiveness, 5U);
> +	wmark_low = 100U - sysctl_compaction_proactiveness;
>  	return low ? wmark_low : min(wmark_low + 10, 100U);
>  }
>