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Re: [libvirt] Suboptimal default cpu Cgroup



> On Thu, Aug 14, 2014 at 01:55:11PM -0400, Andrew Theurer wrote:
> > 
> > 
> > ----- 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?
> 
> 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.

Is this another way of saying, "we have already set a bad precedent, so we need to keep it"?  I am concerned that anyone who may be experiencing this problem may be unsure of what is causing it, and is not aware of how to fix it.
 
> Management apps like oVirt or OpenStack should explicitly set the policy
> they desire in this respect.

Shouldn't a user or upper level mgmt have some expectation of sane defaults?  A user or mgmt app has already specified a preference in the number of vcpus -shouldn't that be enough?  Why have this fix need to be pushed to multiple upper layers when it can be remedied in just one (libvirt)?  Honestly, I don't understand how this even got out the way it is.

-Andrew

> 
> Regards,
> Daniel
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