Re: question: HPET for multiple VMs

Dor Laor
2008-03-23T22:33:29Z

On Mon, 2008-03-24 at 00:32 +0900, Ryota OZAKI wrote:
> 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.
>

The problem is not inaccuracy of guest clock (which we do suffer from in
some guests and there is work in progress to fix). The problem is that
qemu_timer is not accurate, thus the virtio tx timer is too slow leading
to not optimized performance for virtio-net.

Try host kernel >= 2.6.24 with dyntick.

> 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.
> >
> >
>
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