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

  • To: "Christoph Egger" <Christoph.Egger@xxxxxxx>
  • From: "George Dunlap" <George.Dunlap@xxxxxxxxxxxxx>
  • Date: Fri, 15 Aug 2008 12:39:45 +0100
  • Cc: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Fri, 15 Aug 2008 04:40:08 -0700
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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?

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.


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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