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

To: "Magenheimer, Dan \(HP Labs Fort Collins\)" <dan.magenheimer@xxxxxx>, "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: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Tue, 4 Apr 2006 21:06:07 +0100
Cc: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Tristan Gingold <Tristan.Gingold@xxxxxxxx>
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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?

No, there's no need to strictly gang schedule, and the current scheduler makes 
no attempt to do so. It may generally be a decent thing to do, though.

> (If not, aren't there some very knotty research issues 
> related to locking and forward progress?)

You could end up preempting a vCPU holding a lock which could lead to daft 
behaviour of naïve spin locks. A number of possible workarounds have been 
prototyped, but since it doesn't seem to be much of a problem in practice 
nothing has been checked in.

> 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...

This isn't an issue.

After booting you probably want dom0 to give up all but 1 vCPU anyway.


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