4.6 Processes Vs. Threads

For some programs that benefit from concurrency, the decision whether to use processes or threads can be difficult. Here are some guidelines to help you decide which concurrency model best suits your program:

·         All threads in a program must run the same executable. A child process, on the other hand, may run a different executable by calling an exec function.

·         An errant thread can harm other threads in the same process because threads share the same virtual memory space and other resources. For instance, a wild memory write through an uninitialized pointer in one thread can corrupt memory visible to another thread.

An errant process, on the other hand, cannot do so because each process has a copy of the program's memory space.

·         Copying memory for a new process adds an additional performance overhead relative to creating a new thread. However, the copy is performed only when the memory is changed, so the penalty is minimal if the child process only reads memory.

·         Threads should be used for programs that need fine-grained parallelism. For example, if a problem can be broken into multiple, nearly identical tasks, threads may be a good choice. Processes should be used for programs that need coarser parallelism.

·         Sharing data among threads is trivial because threads share the same memory. (However, great care must be taken to avoid race conditions, as described previously.) Sharing data among processes requires the use of IPC mechanisms, as described in Chapter 5. This can be more cumbersome but makes multiple processes less likely to suffer from concurrency bugs.