Messages in this thread | | | From | David Howells <> | Subject | Re: Why Semaphore Hardware-Dependent? | Date | Tue, 29 Aug 2006 19:56:52 +0100 |
| |
Andi Kleen <ak@suse.de> wrote:
> > > BTW maybe it would be a good idea to switch the wait list to a hlist, > > > then the last user in the queue wouldn't need to > > > touch the cache line of the head. Or maybe even a single linked > > > list then some more cache bounces might be avoidable. > > > > You need a list_head to get O(1) push at one end and O(1) pop at the other. > > The poper should know its node address already because it's on its own stack.
No. The popper (__rwsem_do_wake) runs in the context of up_xxxx(), not down_xxxx(). Remember: up() may need to wake up several processes if there's a batch of readers at the front of the queue.
Remember also: rwsems, unlike mutexes, are completely fair.
> > In addition a singly-linked list makes interruptible ops non-O(1) also. > > When they are interrupted I guess? Hardly a problem to make that slower.
Currently interruptible rwsems are not available, but that may change, and whilst I agree making it slower probably isn't a problem, it's still a point that has to be considered.
David - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |