Concurrency vs Parallelism, Concurrent Programming vs Parallel Programming (Touch Dreams)

By yuanlin on Jun 11, 2006

THIS BLOG HAS BEEN MOVED TO touchdreams.net/blog.

In the danger of hairsplitting, ...

Concurrency and parallelism are NOT the same thing. Two tasks T1 and T2 are concurrent if the order in which the two tasks are executed in time is not predetermined,

• T1 may be executed and finished before T2,
• T2 may be executed and finished before T1,
• T1 and T2 may be executed simultaneously at the same instance of time (parallelism),
• T1 and T2 may be executed alternatively,
• ...

If two concurrent threads are scheduled by the OS to run on one single-core non-SMT non-CMP processor, you may get concurrency but not parallelism. Parallelism is possible on multi-core, multi-processor or distributed systems.

Concurrency is often referred to as a property of a program, and is a concept more general than parallelism.

Interestingly, we cannot say the same thing for concurrent programming and parallel programming. They are overlapped, but neither is the superset of the other. The difference comes from the sets of topics the two areas cover. For example, concurrent programming includes topic like signal handling, while parallel programming includes topic like memory consistency model. The difference reflects the different orignal hardware and software background of the two programming practices.