How many virtual cores to allocate for a virtual machine?

Question

Virtualization

Now steal climbs up to 50% of the processor power. The more cores to give, the greater the expectation of the neighbors. It seems that if you give everyone 1 core, happiness will come. How to distribute cores to virtual machines? Maybe there is a special CPU setting to control performance?

Intel® Xeon E3 1220 host has 4 cores 4 threads. I stuffed 5 virtual machines into it, 2 cores each. Then he gave three of them 4 cores each, steal (waiting for CPU neighbors) got up to 50% of the processor power, that is, two cores, roughly speaking, do not work anyway. And if you return and put only 2 cores, the expectation drops to 20%.

Answer 1

When requesting physical threads, the priority is given to a virtual machine with a large number of vCPUs, because will access the hypervisor more often than a VM with fewer processors. So the prioritization of processor resources should be considered only in the case of several multiprocessor machines. When the total number of vCPUs of all virtual machines on the host is equal to the number of processor threads (including those obtained using Hyper-Threading), the mapping is 1:1, i.e. per vCPU - one core. If the total number of virtual processors exceeds the number of physical threads - a part of the processor time of the physical socket is allocated to the virtual processor - this is where prioritization for each VM comes into play.