Re: [HACKERS] spin locks

On Sun, 15 Feb 1998, Bruce Momjian wrote:

> > spin-lock.patch
> >
> > I'm not sure if this is really useful, but it seems stupid to have
> > a backend wasting cpu cycles in a busy loop while the process which
> > should release the lock is waiting for the cpu. So I added a call
> > to process_yield() if the spin lock can't obtained.
> > This has been implemented and tested only on Linux. I don't know if
> > other OS have process_yield(). If someone can check please do it.
>
> Massimo brings up a good point. Most of our s_lock.h locking does asm
> mutex loops looking for a lock. Unless we are using a multi-cpu
> machine, there is no way this is going to change while we are spinning.

I'm not quite sure I follow this...in a multi-cpu environment,
would process_yield() introduce a problem? *raised eyebrow*

> Linux has process_yield(), but most OS's don't. Is there a
> platform-independent way to relinquish the cpu if the first attempt at
> the spinlock fails? Would a select() of 1 microsecond work?

There is nothing wrong with introducing an OS specific
optimization to the code...we can add a HAVE_PROCESS_YIELD to config.h and
if a system has it, use it...

Marc G. Fournier
Systems Administrator @ hub.org
primary: scrappy(at)hub(dot)org secondary: scrappy(at){freebsd|postgresql}.org