Re: [Qemu-devel] cpufreq and QEMU guests

[Benoît Canet]

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.

Best regards

Benoît

> 
> > 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.
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