[PATCH 00/27] Move LRU page reclaim from zones to nodes v6

Mel Gorman
2016-06-09T18:05:01Z

This is only lightly tested as I've had stability problems during boot
that have nothing to do with the series. It's based on mmots as of June
6th. Very little has changed with the big exception of "mm, vmscan:
Move LRU lists to node" because it had to adapt to per-zone changes in
should_reclaim_retry and compaction_zonelist_suitable.

Changelog since v5
o Rebase and adjust to changes

Changelog since v4
o Rebase on top of v3 of page allocator optimisation series

Changelog since v3
o Rebase on top of the page allocator optimisation series
o Remove RFC tag

This is the latest version of a series that moves LRUs from the zones to
the node that is based upon 4.6-rc3 plus the page allocator optimisation
series. 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.

The series got basic testing this time on a UMA machine. The page allocator
microbenchmark highlights the gain from removing the fair zone allocation
policy

4.7.0-rc2 4.7.0-rc2
mmotm-20160606 nodelru-v6r2
Min total-odr0-1 500.00 ( 0.00%) 475.00 ( 5.00%)
Min total-odr0-2 358.00 ( 0.00%) 343.00 ( 4.19%)
Min total-odr0-4 292.00 ( 0.00%) 279.00 ( 4.45%)
Min total-odr0-8 253.00 ( 0.00%) 242.00 ( 4.35%)
Min total-odr0-16 275.00 ( 0.00%) 226.00 ( 17.82%)
Min total-odr0-32 225.00 ( 0.00%) 215.00 ( 4.44%)
Min total-odr0-64 219.00 ( 0.00%) 210.00 ( 4.11%)
Min total-odr0-128 216.00 ( 0.00%) 207.00 ( 4.17%)
Min total-odr0-256 243.00 ( 0.00%) 246.00 ( -1.23%)
Min total-odr0-512 276.00 ( 0.00%) 265.00 ( 3.99%)
Min total-odr0-1024 290.00 ( 0.00%) 287.00 ( 1.03%)
Min total-odr0-2048 303.00 ( 0.00%) 296.00 ( 2.31%)
Min total-odr0-4096 312.00 ( 0.00%) 310.00 ( 0.64%)
Min total-odr0-8192 320.00 ( 0.00%) 308.00 ( 3.75%)
Min total-odr0-16384 320.00 ( 0.00%) 308.00 ( 3.75%)
Min total-odr1-1 737.00 ( 0.00%) 707.00 ( 4.07%)
Min total-odr1-2 547.00 ( 0.00%) 521.00 ( 4.75%)
Min total-odr1-4 620.00 ( 0.00%) 418.00 ( 32.58%)
Min total-odr1-8 386.00 ( 0.00%) 367.00 ( 4.92%)
Min total-odr1-16 361.00 ( 0.00%) 340.00 ( 5.82%)
Min total-odr1-32 352.00 ( 0.00%) 328.00 ( 6.82%)
Min total-odr1-64 345.00 ( 0.00%) 324.00 ( 6.09%)
Min total-odr1-128 347.00 ( 0.00%) 328.00 ( 5.48%)
Min total-odr1-256 347.00 ( 0.00%) 329.00 ( 5.19%)
Min total-odr1-512 354.00 ( 0.00%) 332.00 ( 6.21%)
Min total-odr1-1024 355.00 ( 0.00%) 337.00 ( 5.07%)
Min total-odr1-2048 358.00 ( 0.00%) 345.00 ( 3.63%)
Min total-odr1-4096 360.00 ( 0.00%) 346.00 ( 3.89%)
Min total-odr1-8192 360.00 ( 0.00%) 347.00 ( 3.61%)

A basic IO benchmark based on varying numbers of dd running in parallel
showed nothing interesting other than differences in what zones were
scanned due to the fair zone allocation policy being removed.

This series is not without its hazards. There are at least three areas
that I'm concerned with even though I could not reproduce any problems in
that area.

1. Reclaim/compaction is going to be affected because the amount of reclaim is
no longer targetted at a specific zone. Compaction works on a per-zone basis
so there is no guarantee that reclaiming a few THP's worth page pages will
have a positive impact on compaction success rates.

2. The Slab/LRU reclaim ratio is affected because the frequency the shrinkers
are called is now different. This may or may not be a problem but if it
is, it'll be because shrinkers are not called enough and some balancing
is required.

3. The anon/file reclaim ratio may be affected. Pages about to be dirtied are
distributed between zones and the fair zone allocation policy used to do
something very similar for anon. The distribution is now different but not
necessarily in any way that matters but it's still worth bearing in mind.

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 | 2 +-
include/linux/mm_types.h | 2 +-
include/linux/mmzone.h | 157 +++---
include/linux/swap.h | 15 +-
include/linux/topology.h | 2 +-
include/linux/vm_event_item.h | 14 +-
include/linux/vmstat.h | 111 +++-
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 | 39 +-
mm/filemap.c | 14 +-
mm/huge_memory.c | 33 +-
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/page-writeback.c | 124 +++--
mm/page_alloc.c | 271 +++++-----
mm/page_idle.c | 4 +-
mm/rmap.c | 15 +-
mm/shmem.c | 12 +-
mm/swap.c | 66 +--
mm/swap_state.c | 4 +-
mm/util.c | 4 +-
mm/vmscan.c | 829 +++++++++++++++---------------
mm/vmstat.c | 374 +++++++++++---
mm/workingset.c | 52 +-
47 files changed, 1489 insertions(+), 1217 deletions(-)

--
2.6.4