Re: unfair VCPU scheduling: slow HVM guest boot

[Christoph Egger <Christoph.Egger@amd.com>]

On Friday 15 August 2008 13:39:45 George Dunlap wrote:
> The fact that there's different amounts of cpu time isn't evidence
> that the scheduler is unfair.  The vcpus may be blocked, or may be
> coming up at different times.
>
> Is there a particular reason you want to run with more VCPUs than physical
> cpus?

Yes. You can have a virtual test/crash box for some development work, for
example.

> Given that cpu synchronization primitives like spinlocks and IPIs were
> generally designed with the assumption that they're running on bare
> metal and are not pre-empted, it's not surprising that when vcpus are
> trying to work together but not able to run at the same time, there
> will be performance problems.

Yes, overcommitting always causes performance problems.
The thing I am observing with xentop is that the last activated VCPU
seems to be prefered over the others and the virtual BSP runs very rarely. 
That is what I mean by "unfair".

Christoph

>  -George
>
> On Fri, Aug 15, 2008 at 12:29 PM, Christoph Egger
>
> <Christoph.Egger@amd.com> wrote:
> > Hi,
> >
> > Launch a HVM guest with twice as many VCPUs as physical CPUs are in the
> > machine. You will notice the guest boots slow.
> >
> > With xentop you see, the first VCPU is rarely scheduled once the other
> > VCPUs are up in the guest.
> > If the boot process is just waiting for the first VCPU to finish
> > something (e.g. handling an interrupt), then the whole boot process
> > "freezes" until the first VCPU gets scheduled.
> >
> > Here is an xentop line showing how unfair the VCPUs are scheduled:
> >
> > VCPUs(sec):   0:         44s  1:         94s  2:         96s  3:       
> > 140s
> >
> >
> > Christoph


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