[CentOS-virt] What are KVM guest cores?

Fri Nov 19 00:01:24 UTC 2010
Kenni Lund <kenni at kelu.dk>

> Do you have a rule of thumb as to how many core to assign
> to a guest?  For instance, with an Intel x5650 with 6 real
> and 12 hyperthreaded cores, how many cores would you assign
> to the guest?

It fully depends on the load of your guests and how many guests you
want/need to run on a single server.

You need to perform some testing to know what will work the best in
your case. One thing to remember though: In your case, if you create
two guests with 12 virtual CPUs each, and one of them crashes and take
all its 12 virtual CPUs up to 100%, it will essentially take most of
the processing power away from the second guest, leaving the second
guest in a close-to-useless state (depending on your scheduler, but
you get the point). If you on the other hand had assigned 6 cores to
each of them, the second guest would have remained unaffected, since
it just uses the 6 cores with no load. So if your guest will not
utilize the extra CPUs anyway, then don't assign them.

Best regards