Add support for hosts that have both CGroups v1 and v2

[HenrikBengtsson]

The availableCores() function queries CGroups for CPU limits. It supports both CGroups v1 and CGroups v2. However, it does not handle hosts that have both CGroups v1 and CGroups v2 enabled at the time. When such a host is discovered, a warning is produced;

> ncores <- parallelly::availableCores()
Warning message:
In getCGroupsRoot(controller = controller) :
  Mixed CGroups versions are not supported: ‘cgroup2’, ‘cgroup’

This warning means that it detected that both CGroups v1 (cgroups) and CGroups v2 (cgroups2) were discovered on the current machine, but that the current implementation does not know how to navigate that and will simply ignore both of them. One reason for not support this case is that I do have access to a machine is configured this way.

Now, today, @skertberg reported (futureverse/future#758) that they experienced this on one of their machines, which provides an opportunity to implement support for this mixed case.

@skertberg, I've updated the internals of parallelly to allow us to gather some more information about your CGroups v1+v2 setup, which would allow me to start working on this. Could you please install parallelly (>= 1.41.0-9004) by calling:

remotes::install_github("futureverse/parallelly", ref="develop")

Then, after restarting R, call:

> parallelly:::cloneCGroups()
[1] "cgroups.tar.gz"

This saves a file cgroups.tar.gz to the current directory. Could you please send that file to me? That way I'll be able to implement and test support for your machine too.

[HenrikBengtsson]

@cmicek1, in #124 (comment) you mentioned you also get this warning. If so, could you please install the develop version;

remotes::install_github("futureverse/parallelly", ref="develop")

restart R, and call:

> parallelly:::cloneCGroups()
[1] "cgroups.tar.gz"

Could you please send me the cgroups.tar.gz to file that was saved to the current directory?

[cmicek1]

Here you are!

cgroups.tar.gz