Re: semaphores

>>>>> "JNP" == Joshua N Pritikin <[email protected]> writes:

  JNP> It turned out that semaphores were easier than I expected.  Now
  JNP> that I understand them, I'm even more skeptical that they belong
  JNP> in Event.  I'm not saying that I would reject an implementation,
  JNP> but they seem like too specialized a mechanism for commonplace
  JNP> Event'ing.

here's my take on semaphores along with the pound of salt it needs on
the side. i have (almost?) never used true semaphores in many years of
coding event systems. i discovered that pipes make for very effective
semaphores even in a threaded system. the advantage was that i could use
select/poll on those pipes where i couldn't with semaphores. my view is
semaphores are used for sharing some resource between
processes/threads. most designs block on the semaphores as they have
many threads doing other work in the mean time. with an event system
that won't work as well especially in perl with only experimental
threads (so use processes instead).

the only way semaphores would make sense in an event system is for the
semaphore to trigger some event when the other side has counted it down
to zero (or however the semaphore is designed). the problem is that
normally you may have multiple things blocking on the semaphore and they
all get woken up and the first to get the semaphore wins. how do you
translate that to callbacks? and i don't think semaphores have any way
to wakeup a process in a select/poll blocking state which is even more
critical in an event loop (maybe there is a signal?). so the other side
will have to count down the semaphore AND do something to wake up the
other side. hence my liking of pipes which can do both (un/locking a
resource and triggering an event) at the same time.

so i don't feel semaphores need to be supported in event.pm as their use
can be emulated easily with pipes/sockets.

now drink soem water to handle the many grains of salt i sprinkled over
that message.

:-)

uri

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