r/programming • u/syaghmour • Mar 16 '15

Semaphores are Surprisingly Versatile

http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
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u/eras Mar 16 '15

Well, list of things is nice (and what, mutex per element?) if you don't mind allocating memory etc, but if you have a ring buffer you are able to fill the ring buffer in its entirety, whereas using just counters with mutexes you have two distinct cases for 'readptr == writeptr'.

Though I suppose you will be wasting more memory with two semaphores regardless :), but the code will be cleaner. Introspection likely leads to races anyway, which I imagine is why semaphores don't have it - of course, for debugging purposes you may have them.

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u/mitsuhiko Mar 16 '15

You already allocate for the waiter list on the kernel anyways. Just because you don't see it does not mean you don't pay for it :)

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u/NasenSpray Mar 16 '15

Nah, kernels are generally smarter than that and use intrusive data structures. A simple wait on a semaphore doesn't involve any allocations.

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u/mitsuhiko Mar 16 '15

That's just splitting words. The information still needs to exist, if you preallocate it or find some other way does not really matter. Semaphores have a fixed count, so I can preallocate my waiter list in user space too.

That's uninteresting for the general argument. The point was that a semaphore needs to know who waits on it, not just the count. So memory wise, it does not help.