Among the more unique features of Xen 4.2 is a feature called cpupools, designed and implemented by Jürgen Groß at Fujitsu. At its core it’s a simple idea, but one that allows it to be a flexible and powerful solution to a number of different problems.

The core idea behind cpupools is to divide the physical cores on the machine into different pools. Each of these pools has an entirely separate cpu scheduler, and can be set with different scheduling parameters. At any time, a given logical cpu can be assigned to only one of these pools (or none). A VM is assigned to one pool at a time, but can be moved from pool to pool.

There are a number of things one can do with this functionality. Suppose you are a hosting or cloud provider, and you have a number of customers who have multiple VMs with you. Instead of selling based on CPU metering, you want to sell access to a fixed number of cpus for all of their VMs: e.g. a customer with 6 single-vcpu VMs might buy 2 cores worth of computing space which all of the VMs share.

You could solve this problem by using cpu masks to pin all of the customer’s vcpus to a single set of cores. However, cpu masks do not work well with the scheduler’s weight algorithm — the customer wont’ be able to specify that VM A should get twice the cpu as VM B. Solving the weight issue in a general way is very difficult, since VMs can have any combination of overlapping cpu masks. Furthermore, this extra complication would be there for all users of the credit algorithm, regardless of whether they use this particular mode or not.
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