Threading vs Parallelism, how do they differ?
Daniel Moth (a former coworker of mine)- Threading/Concurrency vs Parallelism article explains it all.
Quoted:
To take advantage of multiple cores from our software, ultimately threads have to be used. Because of this fact, some developers fall in the trap of equating multithreading to parallelism. That is not accurate...You can have multithreading on a single core machine, but you can only have parallelism on a multi core machine
The quick test: If on a single core machine you are using threads and it makes perfect sense for your scenario, then you are not "doing parallelism", you are just doing multithreading.
There are two different kinds of concurrency:
- Threading: CPU switches between different threads really fast, giving a falsehood of concurrency. Keypoint: only one thread is running at any given time. When one thread is running, others are blocked. You might think, how is this any useful than just running procedurally? Well, think of it as a priority queue. Threads can be scheduled. CPU scheduler can give each thread a certain amount of time to run, pause them, pass data to other threads, then give them different priorities to run at a later time. It's a must for not instant running processes that interact with each other. It's used in servers extensively: thousands of clients can request something at the same time, then getting what they requested at a later time (If done procedurally, only one client can be served at a time). Philosophy: do different things together. It doesn't reduce the total time (moot point for server, because one client doesn't care other clients' total requests).
- Parallelism: threads are running parallel, usually in different CPU core, true concurrency. Keypoint: mlutiple threads are running at any given time. It's useful for heavy computations, super long running processes. Same thing with a fleet of single core machines, split data into sections for each machine to compute, pool them together at the end. Different machines/cores are hard to interact with each other. Philosophy: do one thing in less time.
As you can see, they solve totally different kinds of problems.