Multi-Processing can take place in both parallel and concurrent environments. Parallelism really means the ability to run two or more tasks “simultaneously” at the same time. However, concurrency is different.

Concurrency implies the ability to run two or more tasks in a time-shared manner by switching between one task to another task. Consider a uni-processor machine. It is still capable of running multiple processes concurrently but _not_ parallely. Once the time slice/quanta of a process expires, CPU is given to another “ready to run” process.

However, on a multi-processor/multi-core machine, two or more processes or threads can run at the same time — parallelism and this subsumes concurrency. Therefore, parallelism implies concurrency but vice-versa is not true. I think this should answer the question. Parallel processing is by definition related to parallelism whereas multi-processing can be talked about in the context of both parallelism and concurrency.

As with threading, there are still drawbacks with multiprocessing … you’ve got to pick your poison:

1. There is I/O overhead from data being shuffled around between processes
2. The entire memory is copied into each subprocess, which can be a lot of overhead for more significant programs

What Should You Use?

If your code has a lot of I/O or Network usage:

• Multithreading is your best bet because of its low overhead

If you have a GUI

• Multithreading so your UI thread doesn’t get locked up

If your code is CPU bound:

• You should use multiprocessing (if your machine has multiple cores)