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multithreadingprocessmultiprocessing
KMint
KMint

Reputation: 43

How does multithreading utilizes multiple cores?

So recently I've learned some basic knowledge about multithreading. What I've understood is that thread is a lightweight process that runs under processes by sharing memory, while one process is running under one CPU core.

Yet by this perspective I couldn't understand some saying that threads utilizes multiple cores and make the whole program executes more effective. From what I've known, threads created by one process should run only under that specific process, which means that it should only run under that very one CPU core. If we want to utilize multiple cores, we should actually use multiprocess to run parallelly. Most of what I've researched is only about the conclusion, i.e multithreading utilizes multiple cores, but none of them explains my question. Did I think anything wrong? Thanks!

Upvotes: 1

Views: 2364

Answers (1)

Patrick
Patrick

Reputation: 1567

Your confusion lies here:

[...] while one process is running under one CPU core.

[...] threads created by one process should run only under that specific process, which means that it should only run under that very one CPU core.

This is not true. I think what the various explanations you have read meant that any process have at least one thread (where a 'thread' is a sequence of instructions ran by a CPU core).

If you have a multithreaded program, the process will have several threads (sequences of instructions ran by a CPU core) that can run concurrently on different CPU cores.

There are many processes executing on your computer at any given time. The Operating System (OS) is the program that allocates the hardware resources (CPU cores) to all these processes and decides which process can use which cores for what amount of time before another process gets to use the CPU. Whether or not a process gets to use multiple cores is not entirely up to the process. More confusing still, multithreaded programs can use more threads than there are cores on the computer's CPU. In that case you can be certain that all your threads do not run in parallel.

One more thing:

[...] threads utilizes multiple cores and make the whole program executes more effective

I am going to sound very pedantic, but it is more complicated than that. It depends on what you mean by "effective". Are we talking about total computation time, energy consumption ..?

A sequential (1 thread) program may be very effective in terms of power consumption but taking a very long time to compute. If you are able to use multiple threads, you may be able to reduce that computation time but it will probably incur new costs (synchronization between threads, additional protection mechanisms against concurrent accesses ...).

Also, multithreading cannot help for certain tasks that fall outside of the CPU realm. For example, unless you have some very specific hardware support, reading a file from the hard-drive with 2 or more concurrent threads cannot be parallelized efficiently.

Upvotes: 3

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