Parallelism vs Concurrency

Parallelism.

Parallelism is about doing multiple things at the same time.

For this, it uses multiple CPUs/cores to operate multiple processes/threads (Multiprocessing vs Multihreading). Thus, results in overlaping of CPU and I/O activities in one process/thread with the CPU and I/O activities of another process/thread.

Concurrency

Concurrency is about dealing with multiple things at the same time (giving the illusion of simultaneity).

Scheduler interleaves threads (context switching), and how much time each thread gets is non-deterministic and not in developers control, so which instructions are being run by a thread is also non-deterministic. Thus, in concurrency scenarios, it’s important to look for threads to coordinate, generally using locks.

• Shared resource is to be access/updates.
• Multiple tasks need to coordinate.

Concurrency + Parallelism

• Split the sequential flow into independent components.
• Use threads/threadpools to parallelize (speed up).
• Whenever share resource is to be updated, use concurrency tools to manage state.
• Whenever independent components need to coordinate, use concurrency tools.