Re: [PATCH v7 00/17] Improve shrink_slab() scalability (old complexity was O(n^2), new is O(n))

[Kirill Tkhai]

On 26.05.2018 20:15, Vladimir Davydov wrote:
> Hello Kirill,
> 
> The whole patch set looks good to me now.
> 
> Acked-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>

Thanks for the review, Vova!

Kirill

> 
> On Tue, May 22, 2018 at 01:07:10PM +0300, Kirill Tkhai wrote:
>> Hi,
>>
>> this patches solves the problem with slow shrink_slab() occuring
>> on the machines having many shrinkers and memory cgroups (i.e.,
>> with many containers). The problem is complexity of shrink_slab()
>> is O(n^2) and it grows too fast with the growth of containers
>> numbers.
>>
>> Let we have 200 containers, and every container has 10 mounts
>> and 10 cgroups. All container tasks are isolated, and they 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. The writer
>> consumes pages fast, but it's need to shrink_slab() before the reclaimer
>> reached shrink pages function (and frees SWAP_CLUSTER_MAX pages). 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 (5 sequential calls):
>> $time echo 3 > /proc/sys/vm/drop_caches
>>
>> 0.00user 13.78system 0:13.78elapsed 99%CPU
>> 0.00user 5.59system 0:05.60elapsed 99%CPU
>> 0.00user 5.48system 0:05.48elapsed 99%CPU
>> 0.00user 8.35system 0:08.35elapsed 99%CPU
>> 0.00user 8.34system 0:08.35elapsed 99%CPU
>>
>>
>> Last four calls don't actually shrink something. So, the iterations
>> over slab shrinkers take 5.48 seconds. Not so good for scalability.
>>
>> The patchset solves the problem by making shrink_slab() of O(n)
>> complexity. There are following functional 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 1.10system 0:01.10elapsed 99%CPU
>> 0.00user 0.00system 0:00.01elapsed 64%CPU
>> 0.00user 0.01system 0:00.01elapsed 82%CPU
>> 0.00user 0.00system 0:00.01elapsed 64%CPU
>> 0.00user 0.01system 0:00.01elapsed 82%CPU
>>
>> The results show the performance increases at least in 548 times.
>>
>> 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.
>>
>> v7: Refactorings and readability improvements.
>>
>> v6: Added missed rcu_dereference() to memcg_set_shrinker_bit().
>>     Use different functions for allocation and expanding map.
>>     Use new memcg_shrinker_map_size variable in memcontrol.c.
>>     Refactorings.
>>
>> v5: Make the optimizing logic under CONFIG_MEMCG_SHRINKER instead of MEMCG && !SLOB
>>
>> v4: Do not use memcg mem_cgroup_idr for iteration over mem cgroups
>>
>> v3: Many changes requested in commentaries to v2:
>>
>> 1)rebase on prealloc_shrinker() code base
>> 2)root_mem_cgroup is made out of memcg maps
>> 3)rwsem replaced with shrinkers_nr_max_mutex
>> 4)changes around assignment of shrinker id to list lru
>> 5)everything renamed
>>
>> v2: Many changes requested in commentaries to v1:
>>
>> 1)the code mostly moved to mm/memcontrol.c;
>> 2)using IDR instead of array of shrinkers;
>> 3)added a possibility to assign list_lru shrinker id
>>   at the time of shrinker registering;
>> 4)reorginized locking and renamed functions and variables.
>>
>> ---
>>
>> Kirill Tkhai (16):
>>       list_lru: Combine code under the same define
>>       mm: Introduce CONFIG_MEMCG_KMEM as combination of CONFIG_MEMCG && !CONFIG_SLOB
>>       mm: Assign id to every memcg-aware shrinker
>>       memcg: Move up for_each_mem_cgroup{,_tree} defines
>>       mm: Assign memcg-aware shrinkers bitmap to memcg
>>       mm: Refactoring in workingset_init()
>>       fs: Refactoring in alloc_super()
>>       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: Export mem_cgroup_is_root()
>>       mm: Set bit in memcg shrinker bitmap on first list_lru item apearance
>>       mm: Iterate only over charged shrinkers during memcg shrink_slab()
>>       mm: Add SHRINK_EMPTY shrinker methods return value
>>       mm: Clear shrinker bit if there are no objects related to memcg
>>
>> Vladimir Davydov (1):
>>       mm: Generalize shrink_slab() calls in shrink_node()
>>
>>
>>  fs/super.c                 |   11 ++
>>  include/linux/list_lru.h   |   18 ++--
>>  include/linux/memcontrol.h |   46 +++++++++-
>>  include/linux/sched.h      |    2 
>>  include/linux/shrinker.h   |   11 ++
>>  include/linux/slab.h       |    2 
>>  init/Kconfig               |    5 +
>>  mm/list_lru.c              |   90 ++++++++++++++-----
>>  mm/memcontrol.c            |  173 +++++++++++++++++++++++++++++++------
>>  mm/slab.h                  |    6 +
>>  mm/slab_common.c           |    8 +-
>>  mm/vmscan.c                |  204 +++++++++++++++++++++++++++++++++++++++-----
>>  mm/workingset.c            |   11 ++
>>  13 files changed, 478 insertions(+), 109 deletions(-)
>>
>> --
>> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>