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How can I make VCPU use full power of phsical CPU?

I configs /etc/nova/nova.conf like this:


Then I create a instance with 8 VCPUs and 8G Memory .

To test the CPU , I use sysbench, here is how I test:

sysbench --test=cpu --cpu-max-prime=10000 --num-threads=8 run

It costs 2s in the instance, and it use all of 8 CPUs, I can see it in top, I mean all of 8 phsical CPU.

But at the compute node in which the instance running, same cmd , it only cost 1.5s .

And what's more, I have tried in OpenNebula, same hardware, same cmd, and it cost 1.6s in instance.

What Magic behinds this? Could anybody tell me why?