fix: disable swap for executor_linux allocations

[fho]

fix: disable swap for executor_linux allocations

A comment in the nomad source code states that swapping for
executor_linux allocations is disabled but it wasn't.

Nomad wrote -1 to the memsw.limit_in_bytes cgroup file to disable
swapping.
This has the following problems:

1.) Writing -1 to the file does not disable swapping. It sets
    the limit for memory and swap to unlimited.
2.) On common Linux distributions like Ubuntu 16.04 LTS the
    memsw.limit_in_bytes cgroup file does not exist by default.
    The memsw.limit_in_bytes file only exist if the Linux kernel is
    build with CONFIG_MEMCG_SWAP=yes and either
    CONFIG_MEMCG_SWAP_ENABLED=yes or when the kernel parameter
    swapaccount=1 is passed during boot.
    Most Linux distributions disable swap accounting by default because
    of higher memory usage.
    Nomad silently ignores if writing to the memsw.limit_in_bytes file
    fails. The allocation succeeds, no message is logged to notify the
    user.

To ensure that disabling swap works on common Linux kernels, disable
swapping by writing 0 to the memory.swappiness file.
Using the memory.swappiness file only requires that the kernel is
compiled with CONFIG_MEMCG=yes. This is the default in common Linux
kernels.

[schmichael]

Thanks for the fix @fho! I was unaware the swappiness cgroup parameter differed in behavior from the global procfs swappiness parameter!

For the record the swappiness cgroup parameter actually disables swap when set to 0 unlike the global procfs parameter:

Please note that unlike during the global reclaim, limit reclaim
enforces that 0 swappiness really prevents from any swapping even if
there is a swap storage available.

https://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt

[schmichael]

Is there a reason to remove this? Is it possible that in some rare situations it may work while the swappiness approach may not?

[fho]

Is there a reason to remove this?

Yes

1.) Writing -1 to the memsw.limit_in_bytes file does not disable swap. It sets the limit to unlimited.

NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).

https://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt
I'll update my commit message accordingly.

2.) It's sufficient to disable swapping via a single mechanism.

---

Is it possible that in some rare situations it may work while the swappiness approach may not?

Not that I'm aware off. The swappiness value can be set when
CONFIG_MEMCG is enabled and the cggroup memory fs is mounted.
Both are also preconditions for enabling CONFIG_MEMCG_SWAP_ENABLED.

[schmichael]

Thanks again for the excellent fix @fho and working through the details with me!

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.