libeio.setMinParallel(threads): Sets the available number of threads. node's default is 4. Use this function for increasing the number of threads.
libeio.setMaxParallel(threads): Sets the maximum number of threads. This does only cap the number of threads and expires existing threads that are bigger than the maximum.
libeio.usage(): Returns an object with thread usage information.
Based on polotek's thread branch.