Anthony Williams <anthony_w.geo@yahoo.com> writes:
Howard Hinnant <hinnant@twcny.rr.com> writes:
With both the previous release and this one it looks like those threads waiting on upgradable or exclusive access are essentially polling if another thread already enjoys such access. What I'm not seeing is a signal or condition variable upon which a thread sleeps after it enters the first gate and tries to acquire upgradable or exclusive access but discovers such access is not immediately available. I fear the polling behavior will defeat the optimizations that read/write/upgradable exist for in the first place. But I have not measured your code, so I could well be mistaken.
You're half right! If the threads block on trying to enter a new state, then they wait on the mutex_state_sem. This semaphore is triggered every time a lock is released, and again if a waiting thread is awakened but cannot satisfy it's condition.
This can lead to a thread polling, if it is the only waiting thread, because once it has been awakened, it will keep releasing the semaphore and reacquiring it until it's condition can be satisfied.
With thanks to Roland Schwarz, who's condition implementation gave me the inspiration I needed, I attach a new implementation of read-write-mutexes for win32. The idea is to have a second gate on the mutex states --- if a thread is unable to make the change it requires immediately, then it must enter the gate. This allows the thread to again check whether it can enter the new state, and if not, increment the "waiting threads" count without fear of interruption, or of missing a wakeup. It can then exit the gate, and wait for notification. When a thread transitions into a state that allows other threads in other states (i.e. every state except the exclusive writing state, and the acquiring_XXX or releasing_XXX states), then it must notify waiting threads. To do this, it must first enter the gate, and check for waiting threads. If there are none, it exits the gate. If there are some, then it leaves the gate locked, and notifies the waiting threads (using a semaphore to release the appropriate number of threads). When a thread is woken up, it decreases the waiting thread count. If it is the last thread to be woken, then it unlocks the gate. This ensures that only one notification is in progress at once. Whereas the problems Howard described above were real --- in long running tests, my old implementation consumed near 100% CPU usage --- the new implementation does not suffer from this problem, and consumes very little CPU time. Not only that, but the overall runtime is less, too. Any comments? Anthony -- Anthony Williams Software Developer Just Software Solutions Ltd http://www.justsoftwaresolutions.co.uk