Concurrency is when two or more tasks can start, run, and complete in overlapping time periods. It doesn’t necessarily mean they’ll ever both be running at the same instant. For example, multitasking on a single-core machine.
Parallelism is when tasks literally run at the same time, e.g., on a multicore processor.
Quoting Sun’s Multithreaded Programming Guide:
- Concurrency: A condition that exists when at least two threads
are making progress. A more generalized form of parallelism that can
include time-slicing as a form of virtual parallelism.
- Parallelism: A condition that arises when at least two threads are executing simultaneously.
short answer: Concurrency is two lines of customers ordering from a single cashier (lines take turns ordering); Parallelism is two lines of customers ordering from two cashiers (each line gets its own cashier).
Concurrency has to do with software structure; parallelism has todo with hardware.
Sequential vs Concurrent Programming Languages:
Sequential : provided concurrency based on OS and using external Libraries
Concurrent : handled by the language, its runtime