On Thu, 22 Dec 2005, Andrew Morton wrote:
> I'd prefer to see mutexes compared with semaphores after you've put as much
> work into improving semaphores as you have into developing mutexes.
There is a fundamental difference between semaphores and mutexes. The
semaphore semantics _require_ atomic increments/decrements where mutexes
do not. This makes a huge difference on ARM where 99% of all ARM
processors out there can only perform atomic swaps which is sufficient
for mutexes but insufficient for semaphores. Therefore on ARM
performing an atomic increment/decrement (the semaphore fast
path) requires extra costly locking
and .text space (23 cycles over 8 instructions) while the mutex fast
path is about 2-3 instructions and
needs 7-8 cycles. I bet many other architectures are in the same camp.
Nicolas
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