[PATCH 0/6] Assorted compaction cleanups and optimizations

Vlastimil Babka
2015-06-10T09:33:30Z

This series is partly the cleanups that were posted as part of the RFC for
changing initial scanning positions [1] and partly new relatively simple
scanner optimizations (yes, they are still possible). I've resumed working
on the bigger scanner changes, but that will take a while, so no point in
delaying these smaller patches.

The interesting patches are 4 and 5, and somewhat patch 6. In 4, skipping of
compound pages in single iteration is improved for migration scanner, so it
works also for !PageLRU compound pages such as hugetlbfs, slab etc. Patch 5
introduces this kind of skipping in the free scanner. The trick is that we
can read compound_order() without any protection, if we are careful to filter
out values larger than MAX_ORDER. The only danger is that we skip too much.
The same trick was already used for reading the freepage order in the migrate
scanner.

Patch 6 avoids some rescanning when compaction restarts from cached scanner
positions, but the benefits are small enough to be lost in the noise.

To demonstrate improvements of Patches 4 and 5 I've run stress-highalloc from
mmtests, set to simulate THP allocations (including __GFP_COMP) on a 4GB system
where 1GB was occupied by hugetlbfs pages. I'll include just the relevant
stats:

Patch 3 Patch 4 Patch 5 Patch 6

Compaction stalls 7523 7529 7515 7495
Compaction success 323 304 322 289
Compaction failures 7200 7224 7192 7206
Page migrate success 247778 264395 240737 248956
Page migrate failure 15358 33184 21621 23657
Compaction pages isolated 906928 980192 909983 958044
Compaction migrate scanned 2005277 1692805 1498800 1750952
Compaction free scanned 13255284 11539986 9011276 9703018
Compaction cost 288 305 277 289

With 5 iterations per patch, the results are still noisy, but we can see that
Patch 4 does reduce migrate_scanned by 15% thanks to skipping the hugetlbfs
pages at once. Interestingly, free_scanned is also reduced and I have no idea
why. Patch 5 further reduces free_scanned as expected, by 15%. Other stats
are unaffected modulo noise. Patch 6 looks like a regression but I believe it's
just the noise. I've verified that compaction now restarts from the exact pfns
it left off, using tracepoints.

[1] https://lkml.org/lkml/2015/1/19/158

Vlastimil Babka (6):
mm, compaction: more robust check for scanners meeting
mm, compaction: simplify handling restart position in free pages
scanner
mm, compaction: encapsulate resetting cached scanner positions
mm, compaction: always skip compound pages by order in migrate scanner
mm, compaction: skip compound pages by order in free scanner
mm, compaction: decouple updating pageblock_skip and cached pfn

mm/compaction.c | 188 ++++++++++++++++++++++++++++++++++----------------------
1 file changed, 115 insertions(+), 73 deletions(-)

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2.1.4