How can multithreading speed up an application (when threads can't run concurrently)?

Question

I\'m learning about multithreading, but after reading some tutorials I\'m sort of confused. I don\'t understand how multithreading can speed up an application.

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Answer 1

because you constantly have to wait for those semaphores.

Only in a poorly-designed program or in one designed for parallel work on a single-processor machine. In a well-designed program, the threads do useful work in parallel in between the synchronization points, and enough of it to outweigh the overhead of synchronization.

Even without parallel (multicore/multiprocessor) processing, multithreading can be beneficial when the threads do blocking I/O. E.g., the good old CVSup programs used multithreading in the single-core era to make full use of network connections' duplex capabilities. While one thread was waiting for data to arrive over the link, another would be pushing data the other way. Due to network latency, both threads necessarily had to spend a lot of time waiting, during which the other threads could do useful work.