Re: [Qemu-devel] cpufreq and QEMU guests

[Gleb Natapov]

On Mon, Sep 16, 2013 at 05:46:04PM +0200, Benoît Canet wrote:
> Le Monday 16 Sep 2013 à 18:32:39 (+0300), Gleb Natapov a écrit :
> > 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 <address@hidden>:
> > > > 
> > > > > 
> > > > > 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?
> 
> Yes they take guest cpu count, model and frequency to estimate the performance
> of the guest.
> Next they cluster enough guests to be able to compute the job in a given time 
> by
> using this estimate.
> 
They do it wrong. They should take guest cpu count, host cpu model and
frequency, pcpu/vcpu over commit (if any), guest/host memory overcommit
(if any) and estimate performance based on this. For pure computational
performance guest core performance should be close to host core
performance if there is not cpu/memory overcommit. With a lot of IO
things become more complicated.

--
                        Gleb.