Re: lwlocks and starvation

Neil Conway wrote:
> 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 exclusive lock,
>       awaken him; else, walk through the queue and awaken
>       all the shared waiters until we reach an exclusive waiter
>     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
>     Proc C => LWLockAcquire(lock, LW_SHARED); -- succeeds
> 
> i.e. we don't *really* follow strict FIFO order anyway.

My guess is the existing behavior was designed to allow waking of
multiple waiters _sometimes_ without starving of exclusive waiters. 
There should be a comment in the code explaining this usage and I bet it
was intentional.

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