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In the release notes for Xen 4.2 this statement appears:

Large Systems: Following on from the improvements made in 4.1 Xen now supports 
even larger systems, with up to 4095 host CPUs and up to 512 guest CPUs.    

How is 'system' defined here? A cabinet of servers? One huge box?

And how is the division made between a 'host CPU' and a 'guest CPU'?

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up vote 4 down vote accepted

A "system" is a single physical or virtual computer.

The limit of 4095 host CPUs indicates that Xen can take full advantage of a physical host system with up to 4095 CPU cores. But, if you had a (theoretical) 8192-core server, Xen could use only half of those CPU cores.

The guest CPU limitation applies to virtual machines. If you have a server with 1024 cores, you can assign only 512 of them to any single virtual machine.

What does this really mean, in 2012? If you are working with typical commercially available server hardware, you will not have the opportunity to reach these limits. So, why are these limits important? Because other popular hypervisors have CPU limits that are far smaller. For example, Hyper-V R2 allows only 4 virtual CPUs per guest operating system, and even vSphere 5 allows only 32.

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So this doesn't imply that xen would cluster multiple machines into a larger 'system'? Ok. I can go with that. – ethrbunny Oct 1 '12 at 17:25
There are some arm based CPU's coming out now with very high core counts. So these limits could be reached sooner than thought. There is a variant of xen on arm. – Matt May 13 '13 at 1:08

System: A physical server/hypervisor (eg. Dell R420)

Host CPUs: Number of physical CPU threads available to the "System" (CPUs/cores/hyper threading/etc.) (eg. An Intel Xeon E7-8870 is one processor, 10 cores, and 2 threads per core = 20 "Host CPUs")

Guest CPUs: Number of "Host CPUs" usable by a given guest (eg. You might "give" 4 "Host CPUs" to a Windows Server guest running Exchange. The guest machine would think it has a quad-core processor available to it.)

(I've made this answer into a community wiki, feel free to improve it.)

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Why would you make this CW ? – Iain Oct 1 '12 at 16:06
@Iain Because I would like to see this become a "glossary of terms," and I don't know them all... – Soviero Oct 1 '12 at 16:19

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