From George Dunlap – http://lists.xensource.com/archives/html/xen-devel/2010-08/msg00673.html
At the moment, the “weight” parameter for a VM is set on a per-VM basis. This means that when cpu time is scarce, two VMs with the same weight will be given the same amount of total cpu time, no matter how many vcpus it has. I.e., if a VM has 1 vcpu, that vcpu will get x% of cpu time; if a VM has 2 vcpus, each vcpu will get (x/2)% of the cpu time.
I believe this is a counter-intuitive interface. Users often choose to add vcpus; when they do so, it’s with the expectation that a VM will need and use more cpu time. In my experience, however, users rarely change the weight parameter. So the normal course of events is for a user to decide a VM needs more processing power, add more cpus, but doesn’t change the weight. The VM still gets the same amount of cpu time, but less efficiently allocated (because it’s divided).
The attached patch changes the meaning of the “weight” parameter, to be per-vcpu. Each vcpu is given the weight. So if you add an extra vcpu, your VM will get more cpu time as well.
This patch has been in Citrix XenServer for several releases now (checked in June 2008), and seems to fit more with customer expectations.