Only half of CPUs are using?

During test, only half of CPUs are using, and the rest of CPUs will be picked when the current using CPU are over 50% busy. Why? CPU affinity?

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1. CPU affinity?

2. CFS scheduler? 

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when do application deployment, the CPU usage leverage all CPUs, so it  won't be CPU affinity.

CPU goes down

It appears the CPU usage decreasing, and all response times also go down, but there is no effect on the overall throughput. What will causes CPU decreasing?

-- The reason is the run-time data was decreasing.