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Difference between spin locks and semaphores--Some Notes

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Both manage a limited resource. I'll first describe difference between binary semaphore (mutex) and spin lock.
Spin locks perform a busy wait - i.e. it keeps running loop(In turn heavy CPU usage example is
in Reactor usage of AsyncTaskExecutors):
while (try_acquire_resource ());
...
release();
It performs very lightweight locking/unlocking but if the locking thread will be preempted by other which will try to access the same resouce the second one will simply try to acquitre resource untill it run out of it CPU quanta.
On the other hand mutex behave more like:
if (!try_lock()) {
    add_to_waiting_queue ();
    wait();
}
...
process *p = get_next_process_from_waiting_queue ();
p->wakeUp ();
Hence if the thread will try to acquire blocked resource it will be suspended till it will be avaible for it. Locking/unlocking is much more heavy but the waiting is 'free' and 'fair'.
Semaphore is a lock that is allowed to be used multiple (known from initialization) number of times - for example 3 threads are allowed to simultainusly hold the resource but no more. It is used for example in producer/consumer problem or in general in queues:
P(resources_sem)
resource = resources.pop()
...
resources.push(resources)
V(resources_sem)

Comments

  1. Disha21 January 2016 at 02:06

    Can you please explain the difference between waiting threads and suspended threads?
    I mean both are waiting in the end..we are not able to use them?

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    Replies
    1. Abhisar21 January 2016 at 19:31

      It doesn't mean much of a difference.The only thing in depending on programming language there's a little variance. Wait basically means your thread is active and actively using your CPU cycles waiting on some conditions.Suspended means your thread context is saved and currently your thread is dormant and not burning off CPU cycles.

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