[libvirt-users] cputune shares with multiple cpu and pinning

Hi, I have found the cpu time partitioning based on cpu shares weights not very intuitive. On RHEL64, I deployed two qemu/kvm VMs VM1 with 1 vcpu and 512 cpu shares VM2 with 2 vcpus and 1024 cpu shares I pinned their vcpus to specific host pcpus: VM1 vcpu 0 to host pcpu1 VM2 vcpu 0 to host pcpu1, VM2 vcpu 1 to host pcpu2 I executed inside the VMs a simple process that consume all available cpu, eg # cat /dev/zero > /dev/null on the host, using 'top', the reported cpu usage per qemu process is : with 1 process in VM1 and 1 process on vcpu1 in VM2 VM1 = 100%, VM2=100% explanation - without contention for pcpu2 shares are to be irrelevant (that's ok!) with 1 process in VM1 and 1 process on vcpu0 in VM2 VM1=33%, VM2=66% explanation - with contention on pcpu2, host cpu usage is partitioned according to shares (that's ok!) with 1 process in VM1, and 2 processes in VM2 (one on vcpu0 and on vcpu1 - launched with taskset) VM1=50%, VM2=150% This result was a bit unexpected to me: adding load on VM2 resulted in more cpu time allowed for VM1 - can anyone please explain the logic ? Changing the pinning so that both VMs can use the host pcpuset 1-2 then the CPU usage is 100%/100% when VM2 is executing one task only ( on whatever vcpu ) 50%/150% when VM2 is executing two tasks. again, not intuitive just by looking at the shares weights, I think. -------------------------------------------------- Edoardo Comar WebSphere Application Service Platform for Networks (ASPN) ecomar(a)uk.ibm.com +44 (0)1962 81 5576 IBM UK Ltd, Hursley Park, SO21 2JN Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU