How many parallels to use?

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• 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).