I actually don't recommend doing this. But it is set at the OS level and you would look for setting processor affinity. But, with the 64 bit version of java 8 the OS will automatically use the processor cores to its best advantage. The best example of this is my server that is running 3 modded minecraft servers (AGS2, Infinity and a custom) on 8 core CPU. If I watch the CPU I see the usage pretty well balanced. BTW: All three servers have no issues keeping up but you need really good hardware to do this. Mine is xeon 3.4 GHz cpu with 32GB of RAM and two SSD configured for raid 0.