[libvirt] Suboptimal default cpu Cgroup

[Daniel P. Berrange]

On Thu, Aug 14, 2014 at 01:55:11PM -0400, Andrew Theurer wrote:
> 
> 
> ----- Original Message -----
> > From: "Radim Krčmář" <rkrcmar at redhat.com>
> > To: libvir-list at redhat.com
> > Cc: "Daniel P. Berrange" <berrange at redhat.com>, "Andrew Theurer" <atheurer at 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?

Realistically libvirt can't change what it does by default for VMs wrt
to this cgroups setting, because it would cause an immediate functional
change for any who has deployed current libvirt versions & upgrades.

Management apps like oVirt or OpenStack should explicitly set the policy
they desire in this respect.

Regards,
Daniel
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