[dev] spinlock implementation

[Manlio Perillo]

Igor Sysoev ha scritto:
> 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.
> 

What about a single CPU with hyperthreading enabled?

> ngx_cpu_pause is P4 hyperthreading optmization for such loops.
> 

Ok.

> 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.
> 


Thanks   Manlio Perillo