Zack's Kernel News

Container-Aware Cgroups

Roman Gushchin didn't like the way the out-of-memory (OOM) killer targeted individual processes for termination. On a system with many virtual systems on top, he said, the current OOM killer would not behave ideally. It would not recognize individual processes as belonging to particular containers, so it might unexpectedly kill some random process within the container. Or a very large container might not be recognized as a proper target for the OOM killer if it simply contained a large number of very small processes. The OOM killer might target a much smaller container instead, only because that container had a couple of large processes.

Roman wanted to address these problems by creating an OOM killer that would treat a single container as having the size of all processes running within it. Then the OOM killer might properly target that container and kill all the processes associated with it. In cases where no such containers existed, the OOM killer would fall back to its traditional per-process targeting system.

He posted a patch to implement this, but Michal Hocko objected. The real problem with the OOM killer is similar to the problem with context switching, in which the kernel switches rapidly between processes to give the illusion that they are all running simultaneously. The problem with context-switching algorithms is that different user behaviors call for different switching algorithms; the same is true for the OOM killer. There's no obviously correct way to choose which process to kill during OOM conditions.

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