Concurrency vs Parallelism

Concurrency vs Parallelism

Concurrency and Parallelism are often confused to refer to the ability of a system to run multiple distinct programs at the same time. Though the two terms are somewhat related they mean very different things. To clarify the concept, we'll borrow a juggler from a circus to use as an analogy. Consider the juggler to be a machine and the balls he juggles as processes.

Serial Execution

When programs are serially executed, they are scheduled one at a time on the CPU. Once a task gets completed, the next one gets a chance to run. Each task is run from beginning to end without interruption. The analogy for serial execution is a circus juggler who can only juggle one ball at a time. Definitely not very entertaining!

Concurrency

A concurrent program is one that can be decomposed into constituent parts and each part can be executed out of order or in partial order without affecting the final outcome. A system capable of running several distinct programs or more than one independent unit of the same program in overlapping time intervals is called a concurrent system. The execution of two programs or units of the same program may not happen simultaneously.

A concurrent system can have two programs in progress at the same time where progress doesn't imply execution. One program can be suspended while the other executes. Both programs are able to make progress as their execution is interleaved. In concurrent systems, the goal is to maximize throughput and minimize latency. For example, a browser running on a single core machine has to be responsive to user clicks but also be able to render HTML on screen as quickly as possible. Concurrent systems achieve lower latency and higher throughput when programs running on the system require frequent network or disk I/O.

The classic example of a concurrent system is that of an operating system running on a single core machine. Such an operating system is concurrent but not parallel. It can only process one task at any given point, but all the tasks being managed by the operating system appear to make progress because the operating system is designed for concurrency. Each task gets a slice of CPU time to execute and move forward.

Going back to our circus analogy, a concurrent juggler is one who can juggle several balls at the same time. However, at any one point in time, he can only have a single ball in his hand while the rest are in flight. Each ball gets a time slice during which it lands in the juggler's hand and then is thrown back up. A concurrent system is in a similar sense juggling several processes at the same time.