[RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2

Mel Gorman
2016-02-23T13:45:34Z

This is a revisit of an RFC series from last year that moves LRUs from
the zones to the node. It is based on mmotm from February 9th as it had
to be rebased on top of work there and will not apply cleanly to 4.5-rc*
Conceptually, this is simple but there are a lot of details. Some of the
broad motivations for this are;

1. The residency of a page partially depends on what zone the page was
allocated from. This is partially combatted by the fair zone allocation
policy but that is a partial solution that introduces overhead in the
page allocator paths.

2. Currently, reclaim on node 0 behaves slightly different to node 1. For
example, direct reclaim scans in zonelist order and reclaims even if
the zone is over the high watermark regardless of the age of pages
in that LRU. Kswapd on the other hand starts reclaim on the highest
unbalanced zone. A difference in distribution of file/anon pages due
to when they were allocated results can result in a difference in
again. While the fair zone allocation policy mitigates some of the
problems here, the page reclaim results on a multi-zone node will
always be different to a single-zone node.
it was scheduled on as a result.

3. kswapd and the page allocator scan zones in the opposite order to
avoid interfering with each other but it's sensitive to timing. This
mitigates the page allocator using pages that were allocated very recently
in the ideal case but it's sensitive to timing. When kswapd is allocating
from lower zones then it's great but during the rebalancing of the highest
zone, the page allocator and kswapd interfere with each other. It's worse
if the highest zone is small and difficult to balance.

4. slab shrinkers are node-based which makes it harder to identify the exact
relationship between slab reclaim and LRU reclaim.

The reason we have zone-based reclaim is that we used to have
large highmem zones in common configurations and it was necessary
to quickly find ZONE_NORMAL pages for reclaim. Today, this is much
less of a concern as machines with lots of memory will (or should) use
64-bit kernels. Combinations of 32-bit hardware and 64-bit hardware are
rare. Machines that do use highmem should have relatively low highmem:lowmem
ratios than we worried about in the past.

Conceptually, moving to node LRUs should be easier to understand. The
page allocator plays fewer tricks to game reclaim and reclaim behaves
similarly on all nodes.

It was tested on a UMA (16 cores single socket) and a NUMA machine (48 cores,
2 sockets). However, many of these results are from the UMA machine as the
NUMA machine had a bug that was causing numa balancing to push everything
out to swap. A fix for that issue has already been posted.

In many benchmarks, there is an obvious difference in the number of
allocations from each zone as the fair zone allocation policy is removed
towards the end of the series. For example, this is the allocation stats
when running blogbench that showed no difference in headling performance

mmotm-20160209 nodelru-v2
DMA allocs 0 0
DMA32 allocs 7218763 608067
Normal allocs 12701806 18821286
Movable allocs 0 0

bonnie
------

This was configured to do an IO test with a working set 2*RAM using the
ext4 filesystem. For both machines, there was no significant performance
difference between them but this is the result for the UMA machine

bonnie
4.5.0-rc3 4.5.0-rc3
mmotm-20160209 nodelru-v2
Hmean SeqOut Char 85457.62 ( 0.00%) 85376.69 ( -0.09%)
Hmean SeqOut Block 87031.13 ( 0.00%) 87523.40 ( 0.57%)
Hmean SeqOut Rewrite 36685.66 ( 0.00%) 36006.64 ( -1.85%)
Hmean SeqIn Char 76766.34 ( 0.00%) 75935.63 ( -1.08%)
Hmean SeqIn Block 105405.02 ( 0.00%) 105513.21 ( 0.10%)
Hmean Random seeks 333.03 ( 0.00%) 332.82 ( -0.07%)
Hmean SeqCreate ops 5.00 ( 0.00%) 4.62 ( -7.69%)
Hmean SeqCreate read 4.62 ( 0.00%) 4.62 ( 0.00%)
Hmean SeqCreate del 1622.44 ( 0.00%) 1633.46 ( 0.68%)
Hmean RandCreate ops 5.00 ( 0.00%) 5.00 ( 0.00%)
Hmean RandCreate read 4.62 ( 0.00%) 4.62 ( 0.00%)
Hmean RandCreate del 1664.51 ( 0.00%) 1672.79 ( 0.50%)

4.5.0-rc3 4.5.0-rc3
mmotm-20160209 nodelru-v2
User 892.43 896.96
System 160.86 156.56
Elapsed 5990.52 6005.04

However, the overall VM stats are interesting

4.5.0-rc3 4.5.0-rc3
mmotm-20160209 nodelru-v2
Swap Ins 8 0
Swap Outs 705 52
Allocation stalls 6480 0
DMA allocs 0 0
DMA32 allocs 38287801 35274742
Normal allocs 64983682 67494335
Movable allocs 0 0
Direct pages scanned 1334296 0
Kswapd pages scanned 77617741 78643061
Kswapd pages reclaimed 77493866 78481909
Direct pages reclaimed 1334220 0
Kswapd efficiency 99% 99%
Kswapd velocity 12956.762 13096.176
Direct efficiency 99% 100%
Direct velocity 222.735 0.000
Percentage direct scans 1% 0%

Note that there were no allocation stalls with this patch applied and no
direct reclaim activity.

tiobench
--------

tiobench is a flawed benchmark but it's very important in this case. tiobench
benefited from a bug prior to the fair zone allocation policy that allowed
old pages to be artificially preserved. The visible impact was that performance
exceeded the physical capabilities of the disk. With this patch applied the results are

tiobench Throughput
4.5.0-rc3 4.5.0-rc3
mmotm-20160209 nodelru-v2
Hmean PotentialReadSpeed 91.27 ( 0.00%) 89.89 ( -1.51%)
Hmean SeqRead-MB/sec-1 84.97 ( 0.00%) 84.33 ( -0.75%)
Hmean SeqRead-MB/sec-2 75.18 ( 0.00%) 75.02 ( -0.20%)
Hmean SeqRead-MB/sec-4 77.05 ( 0.00%) 77.07 ( 0.03%)
Hmean SeqRead-MB/sec-8 68.13 ( 0.00%) 67.90 ( -0.33%)
Hmean SeqRead-MB/sec-16 61.64 ( 0.00%) 61.99 ( 0.57%)
Hmean RandRead-MB/sec-1 0.92 ( 0.00%) 0.86 ( -6.49%)
Hmean RandRead-MB/sec-2 1.06 ( 0.00%) 1.09 ( 2.53%)
Hmean RandRead-MB/sec-4 1.49 ( 0.00%) 1.47 ( -1.54%)
Hmean RandRead-MB/sec-8 1.64 ( 0.00%) 1.73 ( 5.72%)
Hmean RandRead-MB/sec-16 2.02 ( 0.00%) 1.91 ( -5.45%)
Hmean SeqWrite-MB/sec-1 83.03 ( 0.00%) 82.91 ( -0.15%)
Hmean SeqWrite-MB/sec-2 77.46 ( 0.00%) 77.43 ( -0.03%)
Hmean SeqWrite-MB/sec-4 80.92 ( 0.00%) 80.90 ( -0.02%)
Hmean SeqWrite-MB/sec-8 77.71 ( 0.00%) 77.36 ( -0.45%)
Hmean SeqWrite-MB/sec-16 79.23 ( 0.00%) 79.36 ( 0.17%)
Hmean RandWrite-MB/sec-1 1.19 ( 0.00%) 1.16 ( -2.29%)
Hmean RandWrite-MB/sec-2 1.00 ( 0.00%) 1.07 ( 7.03%)
Hmean RandWrite-MB/sec-4 0.96 ( 0.00%) 1.05 ( 8.67%)
Hmean RandWrite-MB/sec-8 0.94 ( 0.00%) 0.97 ( 2.76%)
Hmean RandWrite-MB/sec-16 0.95 ( 0.00%) 0.93 ( -2.42%)

Note that the performance is almost identical allowing us to conclude that
the correct reclaim behaviour granted by the fair zone allocation policy
is preserved.

stutter
-------

stutter simulates a simple workload. One part uses a lot of anonymous
memory, a second measures mmap latency and a third copies a large file.
The primary metric is checking for mmap latency.

stutter
4.5.0-rc3 4.5.0-rc3
mmotm-20160209 nodelru-v2
Min mmap 12.5114 ( 0.00%) 13.5315 ( -8.15%)
1st-qrtle mmap 14.4985 ( 0.00%) 14.3907 ( 0.74%)
2nd-qrtle mmap 14.7093 ( 0.00%) 14.5478 ( 1.10%)
3rd-qrtle mmap 15.7381 ( 0.00%) 14.7581 ( 6.23%)
Max-90% mmap 16.4561 ( 0.00%) 15.6516 ( 4.89%)
Max-93% mmap 16.9571 ( 0.00%) 15.8844 ( 6.33%)
Max-95% mmap 17.2948 ( 0.00%) 16.3679 ( 5.36%)
Max-99% mmap 21.1054 ( 0.00%) 19.9593 ( 5.43%)
Max mmap 2815.7509 ( 0.00%) 2717.4201 ( 3.49%)
Mean mmap 16.6965 ( 0.00%) 14.9653 ( 10.37%)

There is a consistent improvement in mmap latency and some of this may be due
to less direct reclaim and more kswapd activity

4.5.0-rc3 4.5.0-rc3
mmotm-20160209 nodelru-v2
Minor Faults 89868559 78842249
Major Faults 1037 899
Swap Ins 362 583
Swap Outs 0 0
Allocation stalls 65758 31410
DMA allocs 0 0
DMA32 allocs 1196649783 2633682376
Normal allocs 2227851590 1110162400
Movable allocs 0 0
Direct pages scanned 28776006 15074415
Kswapd pages scanned 13051818 30529292
Kswapd pages reclaimed 12936208 26704609
Direct pages reclaimed 28774473 15074044

Best1%Mean mmap 14.0438 ( 0.00%) 13.7945 ( 1.77%)

Other pagereclaim workloads were tested but the results are often repetitive

lmbench lat_mmap: no major performance difference, less direct reclaim scanning
parallelio: This measures how much an anonymous memory workload is affected by
large amounts of background IO. Impact on workload is roughly comparable.
fsmark: This created large numbers of zero-length files to target the shrinkers.
Shrinker activity was comparable.

Page allocator intensive workloads showed little difference as the cost
of the fair zone allocation policy does not dominate from a userspace perspective
but a microbench of just the allocator shows a difference

4.5.0-rc3 4.5.0-rc3
mmotm-20160209 nodelru-v2
Min total-odr0-1 1075.00 ( 0.00%) 606.00 ( 43.63%)
Min total-odr0-2 786.00 ( 0.00%) 456.00 ( 41.98%)
Min total-odr0-4 383.00 ( 0.00%) 377.00 ( 1.57%)
Min total-odr0-8 355.00 ( 0.00%) 554.00 (-56.06%)
Min total-odr0-16 312.00 ( 0.00%) 293.00 ( 6.09%)
Min total-odr0-32 309.00 ( 0.00%) 284.00 ( 8.09%)
Min total-odr0-64 283.00 ( 0.00%) 269.00 ( 4.95%)
Min total-odr0-128 292.00 ( 0.00%) 274.00 ( 6.16%)
Min total-odr0-256 305.00 ( 0.00%) 292.00 ( 4.26%)
Min total-odr0-512 335.00 ( 0.00%) 333.00 ( 0.60%)
Min total-odr0-1024 347.00 ( 0.00%) 347.00 ( 0.00%)
Min total-odr0-2048 361.00 ( 0.00%) 356.00 ( 1.39%)
Min total-odr0-4096 371.00 ( 0.00%) 366.00 ( 1.35%)
Min total-odr0-8192 376.00 ( 0.00%) 368.00 ( 2.13%)
Min total-odr0-16384 377.00 ( 0.00%) 368.00 ( 2.39%)

Documentation/cgroup-v1/memcg_test.txt | 4 +-
Documentation/cgroup-v1/memory.txt | 4 +-
arch/s390/appldata/appldata_mem.c | 2 +-
arch/tile/mm/pgtable.c | 18 +-
drivers/base/node.c | 73 +--
drivers/staging/android/lowmemorykiller.c | 12 +-
fs/fs-writeback.c | 4 +-
fs/fuse/file.c | 8 +-
fs/nfs/internal.h | 2 +-
fs/nfs/write.c | 2 +-
fs/proc/meminfo.c | 14 +-
include/linux/backing-dev.h | 2 +-
include/linux/memcontrol.h | 30 +-
include/linux/mm_inline.h | 4 +-
include/linux/mm_types.h | 2 +-
include/linux/mmzone.h | 156 +++---
include/linux/swap.h | 15 +-
include/linux/topology.h | 2 +-
include/linux/vm_event_item.h | 11 +-
include/linux/vmstat.h | 106 +++-
include/linux/writeback.h | 2 +-
include/trace/events/vmscan.h | 40 +-
include/trace/events/writeback.h | 10 +-
kernel/power/snapshot.c | 10 +-
kernel/sysctl.c | 4 +-
mm/backing-dev.c | 15 +-
mm/compaction.c | 28 +-
mm/filemap.c | 14 +-
mm/huge_memory.c | 14 +-
mm/internal.h | 11 +-
mm/memcontrol.c | 235 ++++-----
mm/memory-failure.c | 4 +-
mm/memory_hotplug.c | 7 +-
mm/mempolicy.c | 2 +-
mm/migrate.c | 35 +-
mm/mlock.c | 12 +-
mm/mmap.c | 4 +-
mm/nommu.c | 4 +-
mm/page-writeback.c | 119 ++---
mm/page_alloc.c | 269 +++++-----
mm/page_idle.c | 4 +-
mm/rmap.c | 15 +-
mm/shmem.c | 12 +-
mm/swap.c | 66 +--
mm/swap_state.c | 4 +-
mm/vmscan.c | 828 ++++++++++++++----------------
mm/vmstat.c | 363 ++++++++++---
mm/workingset.c | 51 +-
48 files changed, 1455 insertions(+), 1198 deletions(-)

--
2.6.4