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Subject[RFC PATCH 0/6] Improve hugepage allocation success rates under load
Date
Allocation success rates have been far lower since 3.4 due to commit
[fe2c2a10: vmscan: reclaim at order 0 when compaction is enabled]. This
commit was introduced for good reasons and it was known in advance that
the success rates would suffer but it was justified on the grounds that
the high allocation success rates were achieved by aggressive reclaim.
Success rates are expected to suffer even more in 3.6 due to commit
[7db8889a: mm: have order > 0 compaction start off where it left] which
testing has shown to severely reduce allocation success rates under load -
to 0% in one case. There is a proposed change to that patch in this series
and it would be ideal if Jim Schutt could retest the workload that led to
commit [7db8889a: mm: have order > 0 compaction start off where it left].

This series aims to improve the allocation success rates without regressing
the benefits of commit fe2c2a10. The series is based on 3.5 and includes
the commit 7db8889a to illustrate what impact it has to success rates.

Patch 1 updates a stale comment seeing as I was in the general area.

Patch 2 updates reclaim/compaction to reclaim pages scaled on the number
of recent failures.

Patch 3 has kswapd use similar logic to direct reclaim when deciding whether
to continue reclaiming for reclaim/compaction or not.

Patch 4 captures suitable high-order pages freed by compaction to reduce
races with parallel allocation requests.

Patch 5 is an upstream commit that has compaction restart free page scanning
from an old position instead of always starting from the end of the
zone

Patch 6 adjusts patch 5 to restores allocation success rates.

STRESS-HIGHALLOC
3.5.0-vanilla patches:1-2 patches:1-3 patches:1-4 patches:1-5 patches:1-6
Pass 1 36.00 ( 0.00%) 61.00 (25.00%) 49.00 (13.00%) 57.00 (21.00%) 0.00 (-36.00%) 62.00 (26.00%)
Pass 2 46.00 ( 0.00%) 61.00 (15.00%) 55.00 ( 9.00%) 62.00 (16.00%) 0.00 (-46.00%) 63.00 (17.00%)
while Rested 84.00 ( 0.00%) 85.00 ( 1.00%) 84.00 ( 0.00%) 86.00 ( 2.00%) 86.00 ( 2.00%) 86.00 ( 2.00%)

From
http://www.csn.ul.ie/~mel/postings/mmtests-20120424/global-dhp__stress-highalloc-performance-ext3/hydra/comparison.html
I know that the allocation success rates in 3.3.6 was 78% in comparison
to 36% in 3.5. With the full series applied, the success rates are up
to 62% which is still much less but it does not reclaim excessively.
Note what patch 5 which is the upstream commit fe2c2a10 did to allocation
success rates.

MMTests Statistics: vmstat
Page Ins 3037580 3167260 3002720 3120080 2885540 3159024
Page Outs 8026888 8028472 8023292 8031056 8025324 8026676
Swap Ins 0 0 0 0 0 0
Swap Outs 0 0 0 0 0 8

Note that swap in/out rates remain at 0. In 3.3.6 with 78% success rates
there were 71881 pages swapped out.

Direct pages scanned 97106 59600 43926 108327 2109 171530
Kswapd pages scanned 1231288 1419472 1388888 1443504 1180916 1377362
Kswapd pages reclaimed 1231221 1419248 1358130 1427561 1164936 1372875
Direct pages reclaimed 97100 59486 24233 88990 2109 171235
Kswapd efficiency 99% 99% 97% 98% 98% 99%
Kswapd velocity 1001.153 1129.622 1098.647 1080.758 955.967 1084.657
Direct efficiency 99% 99% 55% 82% 100% 99%
Direct velocity 78.956 47.430 34.747 81.105 1.707 135.078

kswapd velocity stays at around 1000 pages/second which is reasonable. In
kernel 3.3.6, it was 8140 pages/second.

include/linux/compaction.h | 4 +-
include/linux/mm.h | 1 +
include/linux/mmzone.h | 4 ++
mm/compaction.c | 142 +++++++++++++++++++++++++++++++++++++-------
mm/internal.h | 7 +++
mm/page_alloc.c | 68 ++++++++++++++++-----
mm/vmscan.c | 29 ++++++++-
7 files changed, 213 insertions(+), 42 deletions(-)

--
1.7.9.2



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