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[libvirt] Suboptimal default cpu Cgroup



Hello,

by default, libvirt with KVM creates a Cgroup hierarchy in 'cpu,cpuacct'
[1], with 'shares' set to 1024 on every level.  This raises two points:

1) Every VM is given an equal amount of CPU time. [2]
   ($CG/machine.slice/*/shares = 1024)

   Which means that smaller / less loaded guests are given an advantage.

2) All VMs combined are given 1024 shares. [3]
   ($CG/machine.slice/shares)

   This is made even worse on RHEL7, by sched_autogroup_enabled = 0, so
   every other process in the system is given the same amount of CPU as
   all VMs combined.

It does not seem to be possible to tune shares and get a good general
behavior, so the best solution I can see is to disable the cpu cgroup
and let users do it when needed.  (Keeping all tasks in $CG/tasks.)

Do we want cgroups in the default at all?
(Is OpenStack dealing with these quirks?)

Thanks.


---
1: machine.slice/
    machine-qemu\\x2d${name}.scope/
     {emulator,vcpu*}/

2: To reproduce, run two guests with > 1 VCPU and execute two spinners
   on the first and one on the second.
   The result will be 50%/50% CPU assignment between guests;  66%/33%
   seems more natural, but it could still be considered as a feature.

3: Run a guest with $n VCPUs and $n spinners in it, and $n spinners in
   the host
    - RHEL7: 1/($n + 1)% CPU for the guest -- I'd expect 50%/50%.
    - Upstream: 50%/50% between guest and host because of autogrouping;
      if you run $n more spinners in the host, it will still be 50%/50%,
      instead of seemingly more fair 33%/66%.  (And you can run spinners
      from different groups, so it would be the same as in RHEL7 then.)

      And it also works the other way: if the host has $n CPUs, then
      $n/2 tasks in the host suffice to minimize VMs' performance,
      regardless of the amount of running VCPUs.


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