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RE: [Xen-devel] Does dom0 see all physical processors? (RE: [Xen-ia64-de

To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Subject: RE: [Xen-devel] Does dom0 see all physical processors? (RE: [Xen-ia64-devel] SAL INFO virtualization)
From: "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Date: Tue, 4 Apr 2006 12:27:28 -0700
Cc: Tristan Gingold <Tristan.Gingold@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] Does dom0 see all physical processors? (RE: [Xen-ia64-devel] SAL INFO virtualization)
I understand and sympathize with the need for dom0 to
sometimes get and use information from each processor
that is only available if dom0 is running on each processor.

However, AFAIK, SMP guests are always gang-scheduled, correct?
(If not, aren't there some very knotty research issues related
to locking and forward progress?)

So on a 16-processor system, every time dom0 needs to
run (e.g. to handle backend I/O for any one of perhaps hundreds
of domains), *every* domain gets descheduled so that dom0
can be (gang-)scheduled on all 16 processors?

If true, this sounds like a _horrible_ performance hit, so
I hope I'm misunderstanding something...


> -----Original Message-----
> From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx] 
> Sent: Tuesday, April 04, 2006 1:26 AM
> To: Tian, Kevin
> Cc: xen-devel; xen-ia64-devel@xxxxxxxxxxxxxxxxxxx; 
> Magenheimer, Dan (HP Labs Fort Collins); Tristan Gingold
> Subject: Re: [Xen-devel] Does dom0 see all physical 
> processors? (RE: [Xen-ia64-devel] SAL INFO virtualization)
> On 4 Apr 2006, at 03:17, Tian, Kevin wrote:
> >
> > Then consider your question about a large box with many processors.
> > How about the real environment? Is it the case to provide a 
> 16-way SMP
> > box, or a 16-way NUMA box? I prefer to the latter. If it's 
> a NUMA box,
> > dom0 sees physical ACPI table and can be configured as NUMA aware.
> This is a model we must support if we are to have domain0 
> handle other 
> processor-related ACPI activities (e.g., power management). The power 
> information and available settings won't make much sense to the user 
> unless there's an equivalence between VCPUs and PCPUs for domain0.
>   -- Keir

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