Re: Assuming that TAS() will succeed the first time is verboten

* Tom Lane <tgl@sss.pgh.pa.us> [001228 14:25] wrote:
> ncm@zembu.com (Nathan Myers) writes:
> > I wonder about the advisability of using spinlocks in user-level code 
> > which might be swapped out any time.
> 
> The reason we use spinlocks is that we expect the lock to succeed (not
> block) the majority of the time, and we want the code to fall through
> as quickly as possible in that case.  In particular we do *not* want to
> expend a kernel call when we are able to acquire the lock immediately.
> It's not a true "spin" lock because we don't sit in a tight loop when
> we do have to wait for the lock --- we use select() to delay for a small
> interval before trying again.  See src/backend/storage/buffer/s_lock.c.
> 
> The design is reasonable, even if a little bit offbeat.

It sounds pretty bad, if you have a contested lock you'll trap into
the kernel each time you miss, crossing the protection boundry and
then waiting.  It's a tough call to make, because on UP systems
you loose bigtime by spinning for your quantum, however on SMP
systems there's a chance that the lock is owned by a process on
another CPU and spinning might be benificial.

One trick that may help is calling sched_yield(2) on a lock miss,
it's a POSIX call and quite new so you'd need a 'configure' test
for it.

http://www.freebsd.org/cgi/man.cgi?query=sched_yield&apropos=0&sektion=0&manpath=FreeBSD+4.2-RELEASE&format=html

-- 
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
"I have the heart of a child; I keep it in a jar on my desk."