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Understanding the OOM killer

Hi,

I am hoping to get some help in understanding how the OOM killer works in general and any macOS specifics.

I have a docker container which I am allocating 2GiB memory. Within the container I run a Java process which is also being given a max memory of 2GiB (too much I know but I don’t think that is relevant at the moment).

When I run the container and monitor it with docker stats, the memory usage gets to about 45% before it exits. The Java process exits with error code 137 which is from receiving a SIGKILL.

If I run docker inspect on the container it shows:

    "State": {
        "Status": "exited",
        "Running": false,
        "Paused": false,
        "Restarting": false,
        "OOMKilled": false,
        "Dead": false,
        "Pid": 0,
        "ExitCode": 137,
        "Error": "",
        "StartedAt": "2020-05-24T21:39:39.5365223Z",
        "FinishedAt": "2020-05-24T21:41:33.6247207Z"
    },
    ...
    "HostConfig": {
        ...
        "Memory": 2147483648,
        ...
    }

I commit the container and spin it back up to run dmesg and it shows:

[336736.836392] Out of memory: Kill process 90118 (java) score 350 or sacrifice child
[336736.838454] Killed process 90118 (java) total-vm:7548324kB, anon-rss:1060364kB, file-rss:0kB, shmem-rss:0kB
[336737.071919] oom_reaper: reaped process 90118 (java), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB

So the Java process was using just over 1GiB of memory before it was killed.

I tried setting --oom-kill-disable but this makes no difference.

I should also mention I have 32GiB of RAM on my host and there is no memory pressure there.

Q1. Why is the Java process being killed?
Q2. Why does docker inspect say "OOMKilled": false when in fact it was killed by the OOM killer?
Q3. Why does --oom-kill-disable have no affect?
Q4. Where does the total-vm:7548324kB value come from and what does it represent?

I think I misunderstood how docker for mac works.

I didn’t realise that there was a single VM which is the actual docker host and that had been left at the default 2GB memory settings. I guess it is the kernel of that VM which is deciding to kill the process.

So that answers Q1 but my other questions still remain unanswered.

Q4 I am guessing is the memory of the VM but why is it 7GB and not the 2GB specified in docker desktop?