[Xen-devel] Odd behaviour of Xen guests on multiprocessor systems ?

[Arjun <cse.syslab@xxxxxxxxx>]

I noticed an odd thing recenlty when running some experiments with several xen guests on a multiprocessor host:-

I was running some tests with infinite while loops inside the guests and used xentop on the host to see what was happening.
Firstly, xentop doesn't give per-physical-cpu information for multiprocessors, it gives VCPU information (have a separate posted query on this).
During the tests I noticed that in some cases I, when I started the infinite loops inside the guests, 2 guests appeared to be on the same 
CPU - I inferred this because xentop would show 49.9% VCPU usage for each of the 2 guests. In other cases, the 2 guests appeared to be on different
CPUs because xentop would show 99.9% VCPU usage for each. This state would remain the same for however long I ran the experiment.

Can anyone explain why this is the case ? Once a guest VM is started on a multiprocessor system, does it always stick to one particular physical
CPU ? If so why and doesn't this have some serious implications for provisioning for mulitple colocated VMs ?

Thanks and regards
Arjun



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