----- Original Message -----
> From: "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
> To: libvir-list@xxxxxxxxxx
> Cc: "Daniel P. Berrange" <berrange@xxxxxxxxxx>, "Andrew Theurer" <atheurer@xxxxxxxxxx>
> Sent: Thursday, August 14, 2014 9:25:05 AM
> Subject: 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.)
Could we have each VM's shares be nr_vcpu * 1024, and the share for $CG/machine.slice be sum of all VM's share?
> Do we want cgroups in the default at all?
> (Is OpenStack dealing with these quirks?)
>
> Thanks.
>
>
> ---
> 1: machine.slice/
> machine-qemu\\x2d${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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