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[Xen-users] Floating Processors

If the CPUs are "floating" as opposed to "pinned", it's possible for a
busy VM to use all the available processors if the other VMs are idle,

E.G., if I have 4 CPUs and 3 VMs and none of my CPUs are pinned, and
one of my VMs is compiling a kernel while the other two are idle, the
busy kernel-compiling VM will use all 4 CPUs, right?  It won't be
limted to only 2 or something, right?

I'm curious because I've noticed a few people and documents mention
pinning one cpu to each VM.  

I assume is is because they don't want a situation where a
low-priority but high-processor-usage task on one VM causes another VM
with a sporadic but high-priority job to hang out waiting for the
low-prioroty job to finish.  

Ok, now that I've read my own question, I guess my real question is, 
can someone point me to a good document that descibes how Xen allocates 
available processors..?  If there isn't such a document, please take a
shot at the question above...


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