Re: Some facts about Erlang and SMP

I'm having some difficulties with the "it's only when locks conflict
that locking time is significant" idea. My understanding with the
pthreads locks is that you start with a CAS, and if that fails then you
spin for a while (with CAS), and if you spin for too long then you go a
kernel lock. If that's wrong then I'm sorry, as most of the following
will also be wrong!

Now assuming the above is correct (or at least close enough for jazz),
in the conflict case, sure, you're going to block, which is the point,
and then some time in the future either the kernel will wake you up
(pretty slow), or the spinning finally succeeds in which case the
wakeup should be really fast.

If there's not a conflict then the first CAS will succeed. But a CAS is
many hundreds of cycles as you've got to do cache-coherency and then
really go all the way out to RAM and load the values in fresh,
especially in a multisocket box. So hundreds of cycles does not seem
like "not significant", at least to me. Or have I missed something
obvious here?