From: Michal Clapinski Date: Fri, 4 Apr 2025 11:11:02 +0000 (+0200) Subject: mm/compaction: remove low watermark cap for proactive compaction X-Git-Url: http://git.monstr.eu/?a=commitdiff_plain;h=bb317f00b9b7f59de5a2ef512a76a1ae285bd23f;p=linux-2.6-microblaze.git mm/compaction: remove low watermark cap for proactive compaction Patch series "mm/compaction: allow more aggressive proactive compaction", v4. 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). To test this, I created a VM with 19GB of memory and free page reporting enabled. The VM was ~idle. I meassured the memory usage from inside the guest (/proc/meminfo) and from the host (provided by the hypervisor). Before: https://drive.google.com/file/d/1Xw23lRry_PgEH3f6QRnSGvoHh2u9UHyI/view?usp=sharing After: https://drive.google.com/file/d/1wMhpIzepx6t44F70yCPA50n1S5V2AT-a/view?usp=sharing This patch (of 2): 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 (19GB of memory) has been perfectly fine with both watermarks hardcoded to 0. Link: https://lkml.kernel.org/r/20250404111103.1994507-1-mclapinski@google.com Link: https://lkml.kernel.org/r/20250404111103.1994507-2-mclapinski@google.com Signed-off-by: Michal Clapinski Cc: Vlastimil Babka Cc: Mel Gorman Signed-off-by: Andrew Morton --- diff --git a/mm/compaction.c b/mm/compaction.c index dd868c861774..f9ee06d55726 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -2251,12 +2251,7 @@ static unsigned int fragmentation_score_wmark(bool low) { unsigned int wmark_low; - /* - * Cap the low watermark to avoid excessive compaction - * 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); }