Re: [9fans] qunlock

[Sape Mullender <sape@huygens.org>]

> The purpose is to have multiple producers and consumers on an queue
> with exclusive access, with priority always given to consumers if there
> are waiting ones and the queue is not empty.

I can imagine one would want to give priority to consumers over producers,
but the danger is that the performance gained by such a thing is more than
offset by the cost of the mechanism.

Putting something on a queue or taking something off is not an expensive
operation.  The number of instructions needed is typically somewhere between
a dozen and a gross.

The queue must be protected by a lock (unless one uses a non-blocking queue
implementation, but that's another story).  QLocks themselves also have queues
and they, in turn, are protected by spin locks.

If your queue is fairly light weight, I would recommend just using lock() and
unlock() to protect them.  When you try to lock when it's already locked, you'll
waste cycles by spinning (but fewer than you might think), but when you use
qlocks — and especially when you use three of them — you'll end up wasting far
more cycles.

I'd say forget about giving priority to consumers; it's not worth it.

	Sape