Difference between Concurrency and Parallelism

Concurrency and parallelism are related concepts in the context of software development, but they have different meanings. Concurrency is the ability of a system to execute multiple tasks using a single processing unit. Parallelism, on the other hand, uses multiple processing units to execute multiple tasks simultaneously, in parallel.

Read this article to learn more about concurrency and parallelism and how they are different from each other.

What is Concurrency?

The technique used for reducing the response time of a system using a single CPU is termed as concurrency. Concurrency is the method that requires only a single processing unit to accomplish a task.

In this technique, a single task is divided into multiple parts and the CPU processes all the parts concurrently, switching back and forth quickly enough to give the impression that all the parts are running simultaneously. Hence, concurrency is a basically a false parallelism. Sometimes, concurrency is also referred to as a sequential processing.

Concurrency is achieved by context switching. It performs a lot of things simultaneously, but not at the same instant.

What is Parallelism?

The technique in which multiple processors are used to accomplish multiple computations simultaneously is known as parallelism. In parallelism, multiple tasks are executed at the same instant.

The primary objective of parallelism is to increase the computational speed by employing multiple independent processing units. Hence, parallelism basically causes the overlapping of processors and input-output activities of one process with another process. Parallelism does a lot of things simultaneously, and hence improves the throughput of the system.

Now, let us discuss the differences between concurrency and parallelism in detail.

Difference between Concurrency and Parallelism

The following table highlights all the major differences between concurrency and parallelism −

S.No.	Concurrency	Parallelism
1.	Concurrency is a technique of executing and managing multiple processes at the same instant.	Parallelism is the technique of executing multiple processes simultaneously.
2.	Concurrency is achieved by using context switching, or interleaving operation of processes on the central processing unit.	Parallelism is achieved by using multiple central processing units.
3.	Concurrency requires only a single processing unit.	Parallelism requires multiple processing units.
4.	Concurrency is used to increase the amount of work completed at a time.	Parallelism is used to increases the computational speed and throughput of a system.
5.	Concurrency performs a lot of things simultaneously.	Parallelism performs a lot of things simultaneously.
6.	Concurrency is a nondeterministic control flow strategy.	Parallelism is a deterministic control flow strategy.
7.	The debugging is difficult in the case of concurrency.	In parallelism, the debugging is simpler than concurrency.
8.	Example of concurrency is using multiple applications at the same instant.	Example of parallelism is the running a web-crawler on a cluster.

Conclusion

The most significant difference between the two is that concurrency is the action of running and managing multiple computations concurrently using a single processing unit, while parallelism is the action of running multiple computations simultaneously using multiple processing units. Note that parallelism requires concurrency, but concurrency doesn't necessarily require parallelism.