Futex wake behavior question

From: Joseph Seigh
Date: Wed Nov 10 2004 - 08:04:39 EST

(2nd try at posting this. from different email acct this time)

I have a question about futex wake behavior. On a uniprocessor is
it supposed to preempt the caller and dispatch the woken thread if
all threads in question have equal priority?

I was playing around with an experimental lock-free implementation
of condvars (no internal lock) and I got a 30% performance improvement
over NPTL's implementation which I didn't expect since the mutex the
condvar is bound to tends to be the gating factor. I did getrusage()
to get some stats and noticed the context switching was proportional
to the condvar signaling, the difference between the NPTL and lock-free
versions being one context switch which I attribute to NPTL's internal

As a side note, since wait morphing doesn't appear to be currently
implemented in NPTL, you get a performance boost from signaling while
not holding the mutex since the first thing the woken thread does is
try to get the lock. Even with this technique, the lock-free condvar
does better.

This behavior would also cause problems for semaphores and tend to turn
semaphore calls into expensive coroutine calls.

So the question is, is this a feature or a bug? I'm on Suse 9.2 on
a uniprocessor. The kernel is 2.6.8-24 and NPTL is 2.3.3.

Joe Seigh

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