[v4,0/2] mm/compaction: allow more aggressive proactive compaction

Message

Michal Clapinski April 4, 2025, 11:11 a.m. UTC

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

v4:
- Replace the leeway tunable with an equation based on proactiveness.
- Add graphs to prove it works.
v3: Remove gerrit ids from commit msgs.
v2: Change commit msgs and document the new sysctl.

Michal Clapinski (2):
  mm/compaction: remove low watermark cap for proactive compaction
  mm/compaction: reduce the difference between low and high watermarks

 Documentation/admin-guide/sysctl/vm.rst |  6 ++++++
 mm/compaction.c                         | 12 ++++--------
 2 files changed, 10 insertions(+), 8 deletions(-)