Druid 0.9.0 - How to choose worker threads for merging segments

I have a 4 core machine and I am using that to merge segments. I have set 20 worker threads to be started and the CPU usage for all 4 cores is almost 100% with load average being around 21.

So is that the optimal way to use worker threads? Are there any considerations that need to be taken into account while choosing middle manager threads for segment merging?