[dev] spinlock implementation

[Igor Sysoev]

On Sat, Aug 16, 2008 at 02:09:58PM +0200, Manlio Perillo wrote:

> I'm reading the nginx spinlock implementation, since I would like to use 
> it in a separate project I'm working on.
> 
> The code is quite simple, however there is a thing I'm not sure to 
> unserstand:
> why ngx_cpu_pause is called only if Nginx detects that more than 1
> cpu are in use?

If nginx detects more than 1 CPU, then it tries to acquire lock in short
loop. If system has the single CPU then the loop is useless: another
process/thead holding the lock does not run at the moment and will not
release the lock in nearest future.

ngx_cpu_pause is P4 hyperthreading optmization for such loops.

The spinlock is optimized to run on SMP systems. For example,

        if (*lock == 0 && ngx_atomic_cmp_set(lock, 0, value)) {
            return;
        }

is better than simple:

        if (ngx_atomic_cmp_set(lock, 0, value)) {
            return;
        }


> Moreover the number of CPU is only detected for Free BSD.

Yes, this bug should be fixed.


-- 
Igor Sysoev
http://sysoev.ru/en/