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Xen 3.0 Virtualization User Guide
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7. CPU Management

Xen allows a domain's virtual CPU(s) to be associated with one or more host CPUs. This can be used to allocate real resources among one or more guests, or to make optimal use of processor resources when utilizing dual-core, hyperthreading, or other advanced CPU technologies.

Xen enumerates physical CPUs in a `depth first' fashion. For a system with both hyperthreading and multiple cores, this would be all the hyperthreads on a given core, then all the cores on a given socket, and then all sockets. I.e. if you had a two socket, dual core, hyperthreaded Xeon the CPU order would be:

socket0 socket1
core0 core1 core0 core1
ht0 ht1 ht0 ht1 ht0 ht1 ht0 ht1
#0 #1 #2 #3 #4 #5 #6 #7

Having multiple vcpus belonging to the same domain mapped to the same physical CPU is very likely to lead to poor performance. It's better to use `vcpus-set' to hot-unplug one of the vcpus and ensure the others are pinned on different CPUs.

If you are running IO intensive tasks, its typically better to dedicate either a hyperthread or whole core to running domain 0, and hence pin other domains so that they can't use CPU 0. If your workload is mostly compute intensive, you may want to pin vcpus such that all physical CPU threads are available for guest domains.

Xen 3.0 Virtualization User Guide
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  Published under the terms of the GNU General Public License Design by Interspire