Thread Throttling

Let's discuss a practical use case of semaphores to control the number of running threads.

One other simple use case for semaphores arises on occasion, and thus we present it here. The specific problem is this: how can a programmer prevent “too many” threads from doing something at once and bogging the system down? Answer: decide upon a threshold for “too many”, and then use a semaphore to limit the number of threads concurrently executing the piece of code in question. We call this approach throttling“Re: NT kernel guy playing with Linux” by Linus Torvalds. June 27, 1999. Available: A response from Linus himself about the utility of semaphores, including the throttling case we mention in the text. As always, Linus is slightly insulting but quite informative. and consider it a form of admission control.

Let’s consider a more specific example. Imagine that you create hundreds of threads to work on some problem in parallel. However, in a certain part of the code, each thread acquires a large amount of memory to perform part of the computation; let’s call this part of the code the memory-intensive region. If all of the threads enter the memory-intensive region at the same time, the sum of all the memory allocation requests will exceed the amount of physical memory on the machine. As a result, the machine will start thrashing (i.e., swapping pages to and from the disk), and the entire computation will slow to a crawl.

A simple semaphore can solve this problem. By initializing the value of the semaphore to the maximum number of threads you wish to enter the memory-intensive region at once, and then putting a sem_wait() and sem_post() around the region, a semaphore can naturally throttle the number of threads that are ever concurrently in the dangerous region of the code.

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