[Proposal] Better systemd slice memory limit support for OpenJDK

[Severin Gehwolf]

Hi,

Current container awareness for OpenJDK seems to work for systemd
slices too, on some systems. To be precise, this works for older
Kernels e.g. 3.10. However, we've noticed that this breaks for newer
Kernel versions[1] such as the one in F28, currently 4.19.14-200. If
the container support would also look at hierarchical memory limits
exposed via memory.stat in the cgroup file system, it would again
work[2]. A proof of concept implementation is here:

http://cr.openjdk.java.net/~sgehwolf/webrevs/container-systemd-slice-01/webrev/

This enhancement wouldn't change any existing container memory limit
detection as it only kicks in when all other look-ups determined that
there is no limit in place. I've verified this by running Docker
container tests. The idea is to look for hierarchical_memory_limit and
hierarchical_memsw_limit lines in the memory.stat file of the cgroup
tree.

Would it be possible to consider such an enhancement upstream? If so,
I'll file a bug and propose it for review.

This issue has been originally raised here:
https://bugzilla.redhat.com/show_bug.cgi?id=1509371

Thanks,
Severin

[1] Java process gets killed by oom killer.
    See: http://cr.openjdk.java.net/~sgehwolf/webrevs/container-systemd-slice-01/before.txt
[2] Java process throws OutOfMemoryError as expected.
    See: http://cr.openjdk.java.net/~sgehwolf/webrevs/container-systemd-slice-01/after.txt