# Parallel vs Concurrent

Tags: computing, parallel, concurrent

## Parallel vs Concurrent

In short, Concurrency and Parallelism are **NOT** the same thing, they are related but quite distinct concepts.

### What's the Difference

From the [Quora answers](https://www.quora.com/What-is-the-difference-between-concurrent-computing-parallel-computing-and-distributed-computing)

> **Parallelism** is generally concerned with accomplishing a particular computation as fast as possible, exploiting multiple processors. The scale of the processors may range from multiple arithmetical units inside a single processor, to multiple processors sharing memory, to distributing the computation on many computers. On the side of models of computation, parallelism is generally about using multiple simultaneous threads of computation internally, in order to compute a final result. Parallelism is also sometimes used for real-time reactive systems, which contain many processors that share a single master clock; such systems are fully deterministic.
>
> **Concurrency** is the study of computations with multiple threads of computation. Concurrency tends to come from the architecture of the software rather than from the architecture of the hardware. Software may be written to use concurrency in order to exploit hardware parallelism, but often the need is inherent in the software's behavior, to react to different asynchronous events (e.g. a computation thread that works independently of a user interface thread, or a program that reacts to hardware interrupts by switching to an interrupt handler thread).
>
> **Distributed** computing studies separate processors connected by communication links. Whereas parallel processing models often (but not always) assume shared memory, distributed systems rely fundamentally on message passing. Distributed systems are inherently concurrent. Like concurrency, distribution is often part of the goal, not solely part of the solution: if resources are in geographically distinct locations, the system is inherently distributed. Systems in which partial failures (of processor nodes or of communication links) are possible fall under this domain.

### Conclusion

> In programming, **concurrency** is the composition of independently executing processes, while **parallelism** is the simultaneous execution of (possibly related) computations. **Concurrency** is about dealing with lots of things at once. **Parallelism** is about doing lots of things at once.
>
> \-- The Go Blog, Rob Pike [Link](https://blog.golang.org/concurrency-is-not-parallelism)
>
> **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.
>
> \-- Yuan Lin [Link](https://blogs.oracle.com/yuanlin/entry/concurrency_vs_parallelism_concurrent_programming)
>
> **Concurrency** is a property of the program and **parallel** execution is a property of the machine.
>
> \-- Douglas Eadline, Ph.D. [Link](http://www.linux-mag.com/id/7411/)

### Reference

* [Wikipedia: Parallel Computing](https://en.wikipedia.org/wiki/Parallel_computing)
* [Wikipedia: Concurrent Computing](https://en.wikipedia.org/wiki/Concurrent_computing)
* [Linux-Mag: Concurrent and Parallel Are Not The Same *By Douglas Eadline, Ph.D.*](http://www.linux-mag.com/id/7411/)
* [The Go Blog: Concurrency is not parallelism](https://blog.golang.org/concurrency-is-not-parallelism) | [Slides](https://talks.golang.org/2012/waza.slide#1)
* [Yuan Lin: Concurrency vs Parallelism, Concurrent Programming vs Parallel Programming](https://blogs.oracle.com/yuanlin/entry/concurrency_vs_parallelism_concurrent_programming)
* [bytearcher: Parallel vs Concurrent in Node.js](http://bytearcher.com/articles/parallel-vs-concurrent/)
* [Quora: What is the difference between concurrent computing, parallel computing and distributed computing?](https://www.quora.com/What-is-the-difference-between-concurrent-computing-parallel-computing-and-distributed-computing)
* [HelpLib: Difference between concurrent programming and parallel programming](http://qa.helplib.com/191292)

#### Misc

* [Concurrency can bite you even in Node](https://glebbahmutov.com/blog/concurrency-can-bite-you-even-in-node/)