linux-kernel - [RFC PATCH 0/6] Improve hugepage allocation success rates under load

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Date:	Tue,  7 Aug 2012 13:31:11 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Linux-MM <linux-mm@...ck.org>
Cc:	Rik van Riel <riel@...hat.com>, Minchan Kim <minchan@...nel.org>,
	Jim Schutt <jaschut@...dia.gov>,
	LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: [RFC PATCH 0/6] Improve hugepage allocation success rates under load

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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