Host/Guest SMP: Looking for answers

Host/Guest SMP: Looking for answers

[Peter Meier]

Hello,

I've heard that KVM-33 now supports SMP hosts and SMP guests,
which is cool. However I do have some newbie-level questions
about this one.

- I assume this is implemented by using n threads for n CPUs/CPU
  cores and letting the host OS schedule accordingly? Or does the
  VM somehow automagically bind each of its virtual CPU threads
  to one of the host CPUs/cores?

- I'd be interested to learn how to actually use SMP ;-). What
  options do I need to enable inside the guest kernel's menucon-
  fig beyond the basic SMP one, specifically what SMP sub-archi-
  tecture type?

- For that matter, which Processor Family option is appropriate
  inside KVM? I've been using "Pentium III" so far. Should I be
  using the family of the host system?

- How do I tell the VM to use multiple CPUs/cores? Some command
  line arguments to qemu-system-x86_64, or another way?


Thanks for reading,
Peter

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Re: Host/Guest SMP: Looking for answers

[Avi Kivity]

Peter Meier wrote:
> Hello,
>
> I've heard that KVM-33 now supports SMP hosts and SMP guests,
> which is cool. However I do have some newbie-level questions
> about this one.
>
> - I assume this is implemented by using n threads for n CPUs/CPU
>   cores and letting the host OS schedule accordingly? Or does the
>   VM somehow automagically bind each of its virtual CPU threads
>   to one of the host CPUs/cores?
>   

Every virtual cpu (vcpu) is mapped to a thread.  The scheduler maps 
threads to cores.  It's no different than running a normal multithreaded 
application.

> - I'd be interested to learn how to actually use SMP ;-). What
>   options do I need to enable inside the guest kernel's menucon-
>   fig beyond the basic SMP one, specifically what SMP sub-archi-
>   tecture type?
>   

Er, whichever you like.  At present, NUMA won't buy you anything, but 
that will change one day.  In general any smp-enabled kernel should 
work, and there's no reason to enable any fancy features.

> - For that matter, which Processor Family option is appropriate
>   inside KVM? I've been using "Pentium III" so far. Should I be
>   using the family of the host system?
>   

P3 is a good choice, and so is using the host system.  I doubt it makes 
any measurable difference.

> - How do I tell the VM to use multiple CPUs/cores? Some command
>   line arguments to qemu-system-x86_64, or another way?
>   

Use the -smp N option as documented in the qemu help.


-- 
error compiling committee.c: too many arguments to function


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Re: Host/Guest SMP: Looking for answers

[Peter Meier]

Thanks Avi :). Off to try it out ...


Regards,
Peter

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