[ovirt-devel] [RFC] VM thread pinning for High Performance

[Martin Polednik]

Hey list,

As a part of High Performance VMs feature [1], the oVirt virt team is
planning to introduce a fine grained CPU pinning control for various
VM threads (vcpu, io, emulator).

Current idea for the pinning is to allow per-host configuration of the
number of CPUs per numa node to be reserved for io+emulator threads.

If we assume that the reservation will be done by 'reserved_cpus'
variable, let's use this as an example:

reserved_cpus = [0, 1]

Such configuration would imply that 2 cores, 0th and 1st in each numa
node may be used for io+emulator thread. Additional logic is required
to avoid wasting these cores if single machine spans >1 numa node,
roughly

if numa_node where vcpu0 contains reserved_cpus:
     pin io/emulator thread to reserved_cpus
else:
     reserve reserved_cpus in numa_node with vcpu0
     pin io/emulator thread to reserved_cpus

Such logic won't waste extra vcpus for large VMs.

Any comments on the idea are more than welcome!

[1] https://trello.com/c/MHRDD8ZO/67-high-performance-vm-preset-in-vm-dialog