Xen devel - Apr 2006 - RE: 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.

Ian

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