multithreading - Why should I use a thread vs. using a process?

Question

Separating different parts of a program into different processes seems (to me) to make a more elegant program than just threading everything. In what scenario would it make sense to make things run on a thread vs. separating the program into different processes? When should I use a thread?

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Edit

Anything on how (or if) they act differently with single-core and multi-core would also be helpful.

Answer 1

You'd prefer multiple threads over multiple processes for two reasons:

1. Inter-thread communication (sharing data etc.) is significantly simpler to program than inter-process communication.
2. Context switches between threads are faster than between processes. That is, it's quicker for the OS to stop one thread and start running another than do the same with two processes.

Example:

Applications with GUIs typically use one thread for the GUI and others for background computation. The spellchecker in MS Office, for example, is a separate thread from the one running the Office user interface. In such applications, using multiple processes instead would result in slower performance and code that's tough to write and maintain.