RE: [Linux-ia64] Re: web page on O(1) scheduler

Davide Libenzi
2003-05-24T00:20:13Z

On Fri, 23 May 2003, Boehm, Hans wrote:

> Pthread_spin_lock() under the NPTL version in RH9 does basically what my
> custom locks do in the uncontested case, aside from the function call.
> But remember that this began with a discussion about whether it was
> reasonable for user locking code to explicitly yield rather than relying
> on pthreads to suspend the thread. I don't think pthread_spin_lock is
> relevant in this context, for two reasons:
>
> 1) At least the RH9 version of pthread_spin_lock in NPTL literally spins
> and makes no attempt to yield or block. This only makes sense at user
> level if you are 100% certain that the processors won't be
> overcommitted. Otherwise there is little to be lost by blocking once you
> have spun for sufficiently long. You could use pthread_spin_trylock and
> block explicitly, but that gets us back to custom blocking code.

Yes, that would be a spinlock. Your code was basically a spinlock that
instead of spinning was doing abort() in contention case. Again, you
measured two different things. Even if the pthread mutex does something
very simple like :

spinlock(mtx->lock);
while (mtx->busy) {
spinunlock(mtx->lock);
waitforunlocks();
spinlock(mtx->lock);
}
mtx->busy++;
spinunlock(mtx->lock);

Only the fact that this code likely reside inside a deeper call lever will
make you pay in a tight loop like your.

> 2) AFAICT, pthread_spin_lock is currently a little too bleeding edge to
> be widely used. I tried to time it, but failed. Pthread.h doesn't
> include the declaration for pthread_spin_lock_t by default, at least not
> yet. It doesn't seem to have a Linux man page, yet. I tried to define
> the magic macro to get it declared, but that broke something else.

$ gcc -D_GNU_SOURCE ...

- Davide