aio completion notification?

So perusing the aio code in preparation to disassocate it from the Squid
fs code, I found that there's a pipe being used as a notification/wakeup
mechanism.

Basically, thread completion writes a ! into the pipe; this wakes up the
main squid loop and calls a PF callback.

All the callback does is read into a temp buffer, and re-schedules for
read IO.

We then call aioCheckCallbacks() per storedir each time through the
comm loop. This can be hundreds of times a second; add 10 + store_dir's
to the mix and this ends up being thousands of times a second at minimum.

I then wondered why we weren't calling aioCheckCallbacks() here.
I modified the code to do so and am successfully running polygraph
against it.

http://www.creative.net.au/diffs/20080711-cacheboy-aufs-callback.diff

It should apply against 2.6, 2.7 and 2.HEAD as well as Cacheboy-HEAD.
aioCheckCallbacks() is now being called much, much less frequently than
it was before; more in line with how many operation requests there are.

Could some people please test this out and let me know if it works?

Adrian

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