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Re: [libvirt] Suboptimal default cpu Cgroup




----- Original Message -----
> From: "Radim Krčmář" <rkrcmar redhat com>
> To: libvir-list redhat com
> Cc: "Daniel P. Berrange" <berrange redhat com>, "Andrew Theurer" <atheurer redhat com>
> 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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