When to use processes, multithreading or neither

While using Chrome I notice a great many processes. As I understand it Chrome's design entails each plugin, tab and extension to be in its own process.

Is this a more effective approach to just using multithreading such as internet explorer does, or having everything in one process such as Firefox?

I feel the different browsers are a good example for my question. When is it appr开发者_高级运维opriately to use a process for everything vs multithreading vs nothing?

In Chrome, this multi-processing is done to achieve robustness and security. If one process hangs or crashes, it has absolutely no affect on the other processes (unless it's the process responsible for the UI). This is crucial in an environment where 3rd party extensions might hang, crash or even try to hack your program (different processes don't share the same address space, as opposed to different threads).

Using this approach adds a significant overhead for Inter-Process Communication. When multithreading, passing messages is not a problem, because all threads share the same address space. Passing messages between processes is much harder. Also, the total memory consumption of the program is higher when using processes instead of threads. And then there's the state of the program which is also harder to maintain - if the UI crashes, for instance, all other processes must be shut down. This is not a problem in multithreading, where the main thread exits.

Bottom line - on most cases, using multiple processes is an overkill. However, for better separation of the various tasks, it might be the solution on some cases.