Re: How many threads should a kvm vm be starting?

[Thomas Fjellstrom <thomas@fjellstrom.ca>]

On September 28, 2011, Daniel P. Berrange wrote:
> On Tue, Sep 27, 2011 at 04:04:41PM -0600, Thomas Fjellstrom wrote:
> > On September 27, 2011, Avi Kivity wrote:
> > > On 09/27/2011 03:29 AM, Thomas Fjellstrom wrote:
> > > > I just noticed something interesting, a virtual machine on one of my
> > > > servers seems to have 69 threads (including the main thread). Other
> > > > guests on the machine only have a couple threads.
> > > > 
> > > > Is this normal? or has something gone horribly wrong?
> > > 
> > > It's normal if the guest does a lot of I/O.  The thread count should go
> > > down when the guest idles.
> > 
> > Ah, that would make sense. Though it kind of defeats assigning a vm a
> > single cpu/core. A single VM can now DOS an entire multi-core-cpu
> > server. It pretty much pegged my dual core (with HT) server for a couple
> > hours.
> 
> You can mitigate these problems by putting each KVM process in its own
> cgroup, and using the 'cpu_shares' tunable to ensure that each KVM
> process gets the same relative ratio of CPU time, regardless of how
> many threads it is running. With newer kernels there are other CPU
> tunables for placing hard caps on CPU utilization of the process as
> a whole too.

I'll have to look into how to set that up with libvirt. A brief search leads 
me to believe its rather easy to set up, so I'll have to do that asap :)

> Regards,
> Daniel


-- 
Thomas Fjellstrom
thomas@fjellstrom.ca