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Re: [Xen-devel] unfair VCPU scheduling: slow HVM guest boot

To: "George Dunlap" <George.Dunlap@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] unfair VCPU scheduling: slow HVM guest boot
From: Christoph Egger <Christoph.Egger@xxxxxxx>
Date: Fri, 15 Aug 2008 13:50:07 +0200
Cc: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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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

> 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".


>  -George
> On Fri, Aug 15, 2008 at 12:29 PM, Christoph Egger
> <Christoph.Egger@xxxxxxx> 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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