linux-kernel - Re: [PATCH v3 0/2] mm/compaction: allow more aggressive proactive compaction

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Message-Id: <20250127170754.955ecfc1ed3249a027fbebce@linux-foundation.org>
Date: Mon, 27 Jan 2025 17:07:54 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Michal Clapinski <mclapinski@...gle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, Pasha Tatashin <tatashin@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] mm/compaction: allow more aggressive proactive
 compaction

On Mon, 27 Jan 2025 22:50:18 +0100 Michal Clapinski <mclapinski@...gle.com> wrote:

> Our goal is to keep memory usage of a VM low on the host. For that
> reason, we use free page reporting which by default reports free pages
> of order 9 and larger to the host to be freed. The feature works well
> only if the memory in the guest is not fragmented below pages of order
> 9. Proactive compaction can be reused to achieve defragmentation after
> some parameter tweaking.
> 
> When the fragmentation score (lower is better) gets larger than the
> high watermark, proactive compaction kicks in. Compaction stops when
> the score goes below the low watermark (or no progress is made and
> backoff kicks in). Let's define the difference between high and low
> watermarks as leeway. Before these changes, the minimum possible value
> for low watermark was 5 and the leeway was hardcoded to 10 (so minimum
> possible value for high watermark was 15).
> 

It would be helpful to let us know the benefits of this change for our
users.  Some reasonably detailed real-world before-and-after narrative
would suit, please.