Re: [PATCH 0/3] KVM-userspace: add NUMA support for guests

[Andi Kleen <andi@firstfloor.org>]

On Sat, Nov 29, 2008 at 08:43:35PM +0200, Avi Kivity wrote:
> Andi Kleen wrote:
> >It depends -- it's not necessarily an improvement. e.g. if it leads to
> >some CPUs being idle while others are oversubscribed because of the
> >pinning you typically lose more than you win. In general default
> >pinning is a bad idea in my experience.
> >
> >Alternative more flexible strategies:
> >
> >- Do a mapping from CPU to node at runtime by using getcpu()
> >- Migrate to complete nodes using migrate_pages when qemu detects
> >node migration on the host.
> >  
> 
> Wouldn't that cause lots of migrations?  Migrating a 1GB guest can take 

I assume you mean the second one (the two points were orthogonal)
The first one is an approximate method, also has advantages
and disadvantages.

> a huge amount of cpu time (tens or even hundreds of milliseconds?) 
> compared to very high frequency activity like the scheduler.

Yes migration is expensive, although you can do it on demand of course, 
but the scheduler typically has pretty strong cpu affinity so it shouldn't 
happen too often. Also it's only a temporary cost compared to the 
endless overhead of running forever non local or running forever with 
some cores idle.

Another strategy would be to tune the load balancer in the scheduler
for this case and make it only migrate in extreme situations.

Anyways it's not ideal either, but in my mind would be all preferable
to default CPU pinning.

-Andi

-- 
ak@linux.intel.com