Concurrency and parallelism

Concurrency and parallelism are two terms that are sometimes used interchangeably. However, they are not the same and it is important to understand the differences between them. A program is said to run concurrently if it has multiple individual control flows running during overlapping time periods. In C++, each individual control flow is represented by a thread. The threads may or may not execute at the exact same time, though. If they do, they are said to execute in parallel. For a concurrent program to run in parallel, it needs to be executed on a machine that has support for parallel execution of instructions; that is, a machine with multiple CPU cores.

At first glance, it might seem obvious that we always want concurrent programs to run in parallel if possible, for efficiency reasons. However, that is not necessarily always true. A lot of synchronization primitives (such as mutex locks) covered in this chapter are required only to support the parallel...