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Why it slow than physical CPU?

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

cpu_allocation_ratio=1.0
cpu_mode=host-passthrough
ram_allocation_ratio=1.0

Then I create a instance with 8 VCPUs and 8 G 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 cost 2s in the instance.

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

I do not know why, and what's more, I have tried in OpenNebula, same hardware, same cmd, and it cost 1.6s in instance.

could anybody tell me why?