LWLockRelease() currently does something like (simplifying a lot):
acquire lwlock spinlock decrement lock count if lock is free if first waiter in queue is waiting for
exclusivelock, awaken him; else, walk through the queue and awaken all the shared waiters until we reach an
exclusivewaiter end if release lwlock spinlock
This has the nice property that locks are granted in FIFO order. Is it
essential that we maintain that property? If not, we could instead walk
through the wait queue and awaken *all* the shared waiters, and get a
small improvement in throughput.
I can see that this might starve exclusive waiters; however, we allow
the following:
Proc A => LWLockAcquire(lock, LW_SHARED); -- succeeds Proc B => LWLockAcquire(lock, LW_EXCLUSIVE); -- blocks
ProcC => LWLockAcquire(lock, LW_SHARED); -- succeeds
i.e. we don't *really* follow strict FIFO order anyway.
-Neil
