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I'm using docker and docker-compose to conduct a few performance tests on a Postgres instance. I've read somewhere that the /dev/shm mounted inside the container is a separate space from the host's one. I can't seem to find any explanation on how does that relate to the memory limits set on the container.

Minimal example follows:

version: "3.9"
services:
  postgres:
    image: postgres:14
    shm_size: '2gb'
    deploy:
      resources:
        limits:
          memory: '24gb'

Will the container use:

• up to 24gb (2 for shm and remaining 22 for applications in the container)
• up to 26gb (2 for shm from a separate pool, deducted from the host and 24 for the app)
• ? some other scenario

Answers

TLDR: With cgroupsv2 --memory will also take into account the size of /dev/shm.

On cgroupsv2 the size of contents inside /dev/shm get added to /sys/fs/cgroup/memory.current

Thus the --memory flag applies to ram used by the container + the contents inside /dev/shm.

Also a post to reference (using cgroupsv1 tho) https://www.sobyte.net/post/2022-04/k8s-pod-shared-memory/

Example:

podman run -it --entrypoint /bin/bash --shm-size=3GB --memory=512MB -v /tmp:/tmp1 bash


dd if=/dev/random of=/dev/shm/test      bs=1024 count=102400
#total system free mem -100MB
dd if=/dev/random of=/dev/shm/test      bs=1024 count=1024000
#total system free mem -400MB+write_error (or crash)