Message-ID: <e52f00fc-ffe3-4745-8082-b492f44c6693@joeconway.com>
Date: Wed, 12 Mar 2025 22:35:17 -0400
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Subject: Re: Getting out ahead of OOM
To: Joseph Hammerman <joe.hammerman@datadoghq.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-admin@lists.postgresql.org
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From: Joe Conway <mail@joeconway.com>
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On 3/12/25 18:21, Joseph Hammerman wrote:
> Joe, can you expand on your recommendation to use cgroup-v2? We're 
> trying to collect our complete rationale for our request to our internal 
> team that is tasked with rolling out this configuration change.

cgroup-v2 has a much better measure of memory pressure (see PSI[1]), 
better ability to reclaim memory pages[4], safer delegation, and other 
advantages. When I last looked the kube support for it was still brand 
new, but it appears to be well supported now [2][3]. In particular, this 
statement from [3] is important:

   Memory QoS uses memory.high to throttle workload approaching its
   memory limit, ensuring that the system is not overwhelmed by
   instantaneous memory allocation.

With cgroup-v1 a kube memory limit would set memory.limit and usage of 
the pod (sum across all processes in the pod cgroup) was tracked with 
memory.usage_in_bytes. Whenever the latter exceeds the former, the OOM 
killer will whack the process in the pod cgroup with the highest 
oom_score, irrespective of how much free memory may be available at the 
host level.

With cgroup-v2 it appears that kube uses memory.high[5], which is more 
of a throttle/soft limit. In cgroup-v2 there is also a new memory.max[6] 
which is essentially the same as what memory.limit was in v1. Exceeding 
memory.max would invoke the OOM killer, but since kubernetes limits the 
pod memory with memory.high, the OOM killer should be avoided.

Note that I cannot claim a bunch of hands on experience with this 
(cgroup-v2 with kubernetes), so please do your own testing and YMMV, etc.

[1] https://docs.kernel.org/accounting/psi.html#psi
[2] https://kubernetes.io/docs/concepts/architecture/cgroups/
[3] 
https://kubernetes.io/docs/concepts/workloads/pods/pod-qos/#memory-qos-with-cgroup-v2
[4] 
https://docs.kernel.org/admin-guide/cgroup-v2.html#:~:text=memory.reclaim
[5] https://docs.kernel.org/admin-guide/cgroup-v2.html#:~:text=memory.high
[6] https://docs.kernel.org/admin-guide/cgroup-v2.html#:~:text=memory.max


-- 
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com