[Question]: pagecache thrashing and hard to trigger OOM in cgroup

[Question]: pagecache thrashing and hard to trigger OOM in cgroup

[Liu Shixin]

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Hi everyone,

Recently, we meet an IO performance issue which caused by pagecache thrashing in
a cgroup and we found it is introduced by commit 815744d75152 ("mm:  memcontrol:
don't batch updates of local VM stats and events").

The problem can easily reproduced in docker environment. Firstly,create a container
with 4G memory limit and 2G swap limit, then run a program which allocate (6G - 50M)
anon memory so there are only 50M memory can be used and no swap space. Then
do "yum install gcc" and we can observed that the yum program is thrashing and IO
keep high for a long but didn't trigger oom. This affects other processes or containers
in the machine.

After analysis, we found there are large number of readahead failures during this time.
Since page allocation from pagecache readahead have __GFP_NORETRY flag, the oom
will be skipped when reach memcg limit. The pagecache is repeatedly allocated and
reclaimed, and the value of workset_refault_file is high. These readahead take a lot of
time, which consume a lot of IO throughput and impact the entire system. This keeps
for long times until other page allocation trigger oom.

By bisection, we finally found commit 815744d75152("mm:  memcontrol: don't batch
updates of local VM stats and events"). Before the commit, the process will trigger oom
in very short time. We suspect the difference is caused by performance changes.

Is there any good way to fix the problem? we prefer the process to be oom rather
than cause the system to be hung and affect other processes.

Thanks,

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