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Date: Sat, 5 Apr 2025 13:53:00 +0200
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From: Ancoron Luciferis <ancoron.luciferis@googlemail.com>
Subject: Kubernetes, cgroups v2 and OOM killer - how to avoid?
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Hi,

I've been investigating this topic every now and then but to this day 
have not come to a setup that consistently leads to a PostgreSQL backend 
process receiving an allocation error instead of being killed externally 
by the OOM killer.

Why this is a problem for me? Because while applications are accessing 
their DBs (multiple services having their own DBs, some high-frequency), 
the whole server goes into recovery and kills all backends/connections.

While my applications are written to tolerate that, it also means that 
at that time, esp. for the high-frequency apps, events are piling up, 
which then leads to a burst as soon as connectivity is restored. This in 
turn leads to peaks in resource usage in other places (event store, 
in-memory buffers from apps, ...), which sometimes leads to a series of 
OOM killer events being triggered, just because some analytics query 
went overboard.

Ideally, I'd find a configuration that only terminates one backend but 
leaves the others working.

I am wondering whether there is any way to receive a real ENOMEM inside 
a cgroup as soon as I try to allocate beyond its memory.max, instead of 
relying on the OOM killer.

I know the recommendation is to have vm.overcommit_memory set to 2, but 
then that affects all workloads on the host, including critical infra 
like the kubelet, CNI, CSI, monitoring, ...

I have already gone through and tested the obvious:

https://www.postgresql.org/docs/current/kernel-resources.html#LINUX-MEMORY-OVERCOMMIT

And yes, I know that Linux cgroups v2 memory.max is not an actual hard 
limit:

https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#memory-interface-files


Any help is greatly appreciated!


Cheers,

	Ancoron