Re: [patch 1/4] prepare_to_wait/finish_wait sleep/wakeup API

[Andrew Morton]

Linus Torvalds wrote:
> 
> On Wed, 25 Sep 2002, David S. Miller wrote:
> >
> > Ok, so if the condition retest fails at wakeup (someone got to the
> > event before us), it's ok because we add ourselves back to the wait
> > queue first, mark ourselves as sleeping, _then_ retest.
> 
> Right. The looping case (if somebody else was first) is slowed down
> marginally, but the common case is sped up and needs one less time through
> the waitqueue lock.
> 

Most of the gain I saw in Badari's profiles (dd to 60 disks) was
in fact in __wake_up.  60 tasks parked on a waitqueue, waiting
for memory to come clean, wakeups being delivered to them faster
than they can wake up and get off the queue.

Yeah, my code is bust ;)  The heavy __wake_up cost in there seems
to be specific to the profusion chipset, which is two quads joined
by wet string, but the principle still applies.

I expect a decent win would come from using this technique in
select/poll, but that code relies on the remains-on-the-waitqueue
semantics, and would need some fiddling.