This is actually RFC, so comments are welcome! On 21.03.2018 16:21, Kirill Tkhai wrote: > Imagine a big node with many cpus, memory cgroups and containers. > Let we have 200 containers, and every container has 10 mounts > and 10 cgroups. All container tasks don't touch foreign containers > mounts. > > In case of global reclaim, a task has to iterate all over the memcgs > and to call all the memcg-aware shrinkers for all of them. This means, > the task has to visit 200 * 10 = 2000 shrinkers for every memcg, > and since there are 2000 memcgs, the total calls of do_shrink_slab() > are 2000 * 2000 = 4000000. > > 4 million calls are not a number operations, which can takes 1 cpu cycle. > E.g., super_cache_count() accesses at least two lists, and makes arifmetical > calculations. Even, if there are no charged objects, we do these calculations, > and replaces cpu caches by read memory. I observed nodes spending almost 100% > time in kernel, in case of intensive writing and global reclaim. Even if > there is no writing, the iterations just waste the time, and slows reclaim down. > > Let's see the small test below: > $echo 1 > /sys/fs/cgroup/memory/memory.use_hierarchy > $mkdir /sys/fs/cgroup/memory/ct > $echo 4000M > /sys/fs/cgroup/memory/ct/memory.kmem.limit_in_bytes > $for i in `seq 0 4000`; do mkdir /sys/fs/cgroup/memory/ct/$i; echo $$ > /sys/fs/cgroup/memory/ct/$i/cgroup.procs; mkdir -p s/$i; mount -t tmpfs $i s/$i; touch s/$i/file; done > > Then, let's see drop caches time (4 sequential calls): > $time echo 3 > /proc/sys/vm/drop_caches > 0.00user 6.80system 0:06.82elapsed 99%CPU > 0.00user 4.61system 0:04.62elapsed 99%CPU > 0.00user 4.61system 0:04.61elapsed 99%CPU > 0.00user 4.61system 0:04.61elapsed 99%CPU > > Last three calls don't actually shrink something. So, the iterations > over slab shrinkers take 4.61 seconds. Not so good for scalability. > > The patchset solves the problem with following actions: > 1)Assign id to every registered memcg-aware shrinker. > 2)Maintain per-memcgroup bitmap of memcg-aware shrinkers, > and set a shrinker-related bit after the first element > is added to lru list (also, when removed child memcg > elements are reparanted). > 3)Split memcg-aware shrinkers and !memcg-aware shrinkers, > and call a shrinker if its bit is set in memcg's shrinker > bitmap > (Also, there is a functionality to clear the bit, after > last element is shrinked). > > This gives signify performance increase. The result after patchset is applied: > > $time echo 3 > /proc/sys/vm/drop_caches > 0.00user 0.93system 0:00.94elapsed 99%CPU > 0.00user 0.00system 0:00.01elapsed 80%CPU > 0.00user 0.00system 0:00.01elapsed 80%CPU > 0.00user 0.00system 0:00.01elapsed 81%CPU > (4.61s/0.01s = 461 times faster) > > Currenly, all memcg-aware shrinkers are implemented via list_lru. > The only exception is XFS cached objects backlog (which is completelly > no memcg-aware, but pretends to be memcg-aware). See > xfs_fs_nr_cached_objects() and xfs_fs_free_cached_objects() for > the details. It seems, this can be reworked to fix this lack. > > So, the patchset makes shrink_slab() of less complexity and improves > the performance in such types of load I pointed. This will give a profit > in case of !global reclaim case, since there also will be less do_shrink_slab() > calls. > > This patchset is made against linux-next.git tree. > > --- > > Kirill Tkhai (10): > mm: Assign id to every memcg-aware shrinker > mm: Maintain memcg-aware shrinkers in mcg_shrinkers array > mm: Assign memcg-aware shrinkers bitmap to memcg > fs: Propagate shrinker::id to list_lru > list_lru: Add memcg argument to list_lru_from_kmem() > list_lru: Pass dst_memcg argument to memcg_drain_list_lru_node() > list_lru: Pass lru argument to memcg_drain_list_lru_node() > mm: Set bit in memcg shrinker bitmap on first list_lru item apearance > mm: Iterate only over charged shrinkers during memcg shrink_slab() > mm: Clear shrinker bit if there are no objects related to memcg > > > fs/super.c | 8 + > include/linux/list_lru.h | 3 > include/linux/memcontrol.h | 20 +++ > include/linux/shrinker.h | 9 + > mm/list_lru.c | 65 ++++++-- > mm/memcontrol.c | 7 + > mm/vmscan.c | 337 ++++++++++++++++++++++++++++++++++++++++++-- > mm/workingset.c | 6 + > 8 files changed, 418 insertions(+), 37 deletions(-) > > -- > Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> >
