Multithreading vs multiprocessing

Question

I am new to this kind of programming and need your point of view.

I have to build an application but I can't get it to compute fast enough. I have already tried Intel TBB, and it is easy to use, but I have never used other libraries.

In multiprocessor programming, I am reading about OpenMP and Boost for the multithreading, but I don't know their pros and cons.

In C++, when is multi threaded programming advantageous compared to multiprocessor programming and vice versa?Which is best suited to heavy computations or launching many tasks...? What are their pros and cons when we build an application designed with them? And finally, which library is best to work with?

Answer 1

Multithreading means exactly that, running multiple threads. This can be done on a uni-processor system, or on a multi-processor system.

On a single-processor system, when running multiple threads, the actual observation of the computer doing multiple things at the same time (i.e., multi-tasking) is an illusion, because what's really happening under the hood is that there is a software scheduler performing time-slicing on the single CPU. So only a single task is happening at any given time, but the scheduler is switching between tasks fast enough so that you never notice that there are multiple processes, threads, etc., contending for the same CPU resource.

On a multi-processor system, the need for time-slicing is reduced. The time-slicing effect is still there, because a modern OS could have hundred's of threads contending for two or more processors, and there is typically never a 1-to-1 relationship in the number of threads to the number of processing cores available. So at some point, a thread will have to stop and another thread starts on a CPU that the two threads are sharing. This is again handled by the OS's scheduler. That being said, with a multiprocessors system, you can have two things happening at the same time, unlike with the uni-processor system.

In the end, the two paradigms are really somewhat orthogonal in the sense that you will need multithreading whenever you want to have two or more tasks running asynchronously, but because of time-slicing, you do not necessarily need a multi-processor system to accomplish that. If you are trying to run multiple threads, and are doing a task that is highly parallel (i.e., trying to solve an integral), then yes, the more cores you can throw at a problem, the better. You won't necessarily need a 1-to-1 relationship between threads and processing cores, but at the same time, you don't want to spin off so many threads that you end up with tons of idle threads because they must wait to be scheduled on one of the available CPU cores. On the other hand, if your parallel tasks requires some sequential component, i.e., a thread will be waiting for the result from another thread before it can continue, then you may be able to run more threads with some type of barrier or synchronization method so that the threads that need to be idle are not spinning away using CPU time, and only the threads that need to run are contending for CPU resources.