Re: [Xen-devel] SMP guest support in unstable tree.

[Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>]

On Tue, Jan 04, 2005 at 10:41:57AM -0600, Andrew Theurer wrote:
> Do you think there would be any room for "dedicated" cpus in a domain, 
> like a one-to-one mapping of physical cpu to domain cpu?  I am asking 
> because I think there would be situations where (a) one would want to 
> discretely divide a large system, in particular one with numa 
> characteristics where one could dedicate cpus and memory close to each 
> other and

We have a one-to-one mapping (pinning) of virtual cpus to physical cpus --
if you don't allocate multiple virtual cpus to the same physical cpu, then
the physical cpu becomes implicitly dedicated to that domain.

This mapping can be changed dynamically, at least at the Xen level -- the
tools don't have support for changing the mapping of SMP guests yet.  We
also don't support enforcing allocation policies yet.

> (b) perhaps in this one to one mapping, there might be less 
> overhead of managing cpus in a domain, vs (assuming) some sort of 
> timesharing of a physical cpu to many domains, and even more than one 
> virtual cpu in just one domain. 

I don't think there's significant overhead if there's only a single
virtual cpu pinned to one physical cpu so I wouldn't expect a noticeable
performance advantage if we handled this case differently.

> Anyway, I am mostly curious at this point.  This is just what I have 
> seen in the ppc/power5 world, a choice of dedicated cpus (however, if 
> they are idle that cpu can be "shared" if desired) or virtual cpus (up 
> to 64 I think) backed by N physical cpus.

I think we need load balancing software and we also need to get
measurements to see what's the cost of moving virtual cpus between
physical cpus (or hyperthreads) and what impact service domains have
on the scheduling and load balancing decisions.

    christian



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