Multithreading in Uniprocessor

Question

I wish to know how multi-threading in a uniprocessor system is helpful my doubt is when you create the thread it is going to take the execution time slice from the main thread only and other thing is scheduling of threads (context switch between the threads) will also takes considerable amount of time (preemptive kernel) and at a time processor is going to execute only one thread.

Answer 1

Many processes have their speed bound by the slow speed of I/O devices such as disks. Using multiple threads, you can do useful work even while waiting for a slow disk access to complete. Of course, if your process is not I/O bound, then multi-threading on a single processor can cause slow-downs, rather than speed-ups - it's a question of horses for courses.

Answer 2

It can also be used to Simulate something working in parallel

Answer 3

It can also be helpful to the user experience to use multiple threads, even if things don't actually run faster because of it.

Nothing worse than seeing an entire window refuse to repaint when an operation is going off in the background, especially when there's a progress bar which of course becomes useless.

Answer 4

It depends on the OS, but the scheduler usually considers thread priority as well. For example, for 'real-time' audio applications (e.g. recording the audio with some processing), the processing and recording is more important than the UI refreshment, since the audio signal is lost forever if you miss even a few samples.

Most "pro-grade" audio applications used multi-threading long before multi-core CPU became common-place.

Answer 5

If you put the heavy work in separate threads, the gui is still responsive.

Answer 6

Because sometimes threading is the most natural way to express your program. Threads provide a way for you to represent tasks that should conceptually run at the same time. Even though, on single processors they obviously can't run at the same time.

One common area to use threading is GUIs, for example. You don't want your GUI to be unresponsive just because there is a lot of work going on in another area of the program. So by splitting off the GUI into another thread, you can still have your GUI responsive despite a lot of computation somewhere else in your program.

Answer 7

Multithreading was invented because it was found that most of the time a program is waiting for I/O. If the processor is shared among other programs this idle time can be made use of. Even though some processor time is spent managing thread/processes this practice was found to be more productive than running one program at a time to the end in sequence.

Answer 8

With Uniprocessor systems, multithreading helps in sharing the CPU among multiple tasks so that no one task hogs the CPU till it gets completed.

Answer 9

A good example is a game, where you have to do many things concurrently.

The common approach is to have a main loop where you process events, game logic, physics, graphics and sound; but if those task need to be interleaved in a non static-deterministic way, because some of them take more than one iteration to complete (for example, you're dropping some frames, but the game logic is still running) or you need to sample sound more frequently because otherwise glitches can be heard; the scheduler of you game is likely to become more and more complex...

In that case, you could just split your tasks in threads and let the OS to do the scheduling job for you. But you'll need to design that very carefully because it's very probable that all the threads have to read the same data (the world state) and one or two of them also write it (the game logic and physics) so it's imperative to stablish the proper locks.