Re: [Xen-devel] Supported #of CPUs/VMs per CPUs

["George Dunlap " <dunlapg@xxxxxxxxx>]

On 9/18/06, whit <whitson22@xxxxxxxxxxx> wrote:
> 1.  Maximum number of VMs is 32 per CPU
> If above is true, how can domains/partitions can be unlimited.
#1 is not true.  What Keir said was that the number of Virtual CPUs
per domain was 16*.

(* Amending to 16 from 32)

There is no maximum number of domains per cpu.  The maximum number of
domains *per machine* (regardless of the # of CPUs) depends on the
amount of resources available, mainly memory.

As an analogy, consider how many processes can you have per CPU in
Linux.  Well, it depends on how much memory you have, but generally a
whole lot.  (My non-loaded laptop has 141 right now.)  I've never
tried to max out the numer if idle domains in Xen, but in general, the
answer is quite a bit.

The other important answer to the question, "How many domains can I
have running on one CPU" is "One at a time."  If you have 50 domains
not doing anything, you'll probably be fine.  If you have 50 domains
all trying to do I/O- and cpu-intensive workloads on a 1-cpu system,
your performance will be awful.

OTOH, if you have 4 single-vcpu domains doing cpu-intensive stuff on a
4-cpu box, your performance will be just fine.   If you have 4
single-vcpu domains doing cpu-intensive stuff on a 1-cpu box, your
performance won't be that great.  If you have 8 single-vcpu domains
moderately loaded, active about 40-50% of the time, on a 4-cpu box,
you should have decent performance too.

Does that make sense?

-George

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