Re: cpufreq and QEMU guests

[Gleb Natapov <gleb@redhat.com>]

On Mon, Sep 16, 2013 at 05:05:45PM +0200, Benoît Canet wrote:
> Le Monday 16 Sep 2013 à 09:39:10 (-0500), Alexander Graf a écrit :
> > 
> > 
> > Am 16.09.2013 um 07:15 schrieb Benoît Canet <benoit.canet@irqsave.net>:
> > 
> > > 
> > > Hello,
> > > 
> > > I know a cloud provider worried about the fact that the /proc/cpuinfo of his
> > > guests give a bogus frequency to his customer.
> > > 
> > > QEMU and the guests kernel currently have no way to reflect the host frequency
> > > changes to the guests.
> > > 
> > > The customer compute intensive application then read this information and take
> > > wrong decisions.
> > 
> > Why do they care about the frequency? Is it for scheduling workloads? The only other case I can think of would be the TSC and that should be fixed frequency these days.
> > 
> > If it's scheduling, you could maybe expose the unavailable compute time as steal time to the guest. Exposibg frequency in a virtual environment feels backwards.
> 
> The final customer have a compute intensive workload.
> At startup the code retrieve the cpu cache topology, the cpu model, and various
> informations including the guest cpu frequency before starting the compute job.
> The QEMU instance typicaly use -cpu host.
> 
> The code inspects the cpu frequency has seen by the guests to choose the number
> of vms to instanciate to compute the given task.
I am not sure I understand. They look at guest cpu frequency to estimate
guest's performance?

> They even destroy and recreate some vms that would be underperforming to
> mitigate the high inter vm communication costs.
> 
> Do you think the steal time trick would work for this ?
> 

--
			Gleb.