How many parallels to use?

• First of all, I call them "parallels" as this applies to forks, threads, spawns, and even to async code.

• Overhead of creating new parallel.

• Overhead of communication (sending job input to parallel, receiving results).

• Total number of items to process.

• Time it takes to process an item.

• Distribution of processing times. (e.g. one long and many short jobs.)

• Number of cores (CPUs).