Re: question: HPET for multiple VMs

Ryota OZAKI
2008-03-23T15:32:33Z

Hi Avi,

> If you use the dyntick clock option (the default, IIRC), and a newer
> host kernel, then the kernel provides high-resolution timers, very
> likely using HPET internally or some other high resolution clock and
> event source.

I see. The dyntick clock seems to be more scalable than
the others. I understood that '-clock hpet' is used for
boosting one VM (becuase hpet gains best performance
on virtio), right?

I would like to try dyntick for my multiple VMs environment.

> I think that for newer kernels we already have the desired accuracy.

Yes. In recent versions of kvm, I didn't experience
any time inaccuracy, although I had only tested under
several VMs. I'll try the more number of VMs, and
if time inaccuracy occurs, I would like to report
that.

Many thanks,
ozaki-r

2008/3/23, Avi Kivity <avi@qumranet.com>:
> Ryota OZAKI wrote:
> > Hi all,
> >
> > Current kvm allows only one VM to use HPET. Is
> > there a plan to implement a functionality to
> > allow multiple VMs to use HPET? If so, how
> > about the status of that?
> >
> >
>
>
> If you use the dyntick clock option (the default, IIRC), and a newer
> host kernel, then the kernel provides high-resolution timers, very
> likely using HPET internally or some other high resolution clock and
> event source.
>
>
> > And I would like to ask right and wrong to
> > implement the functionality in terms of need
> > and efficiency (scalability and time accuracy).
>
>
> I think that for newer kernels we already have the desired accuracy.
> We're not always good at exploiting that accuracy; hence the recent
> movement of the PIT implementation from userspace to the kernel. But
> recent discussion leads me to believe it could have been implemented
> with the userspace PIT as well.
>
>
> --
> Any sufficiently difficult bug is indistinguishable from a feature.