On Oct 7, 2016, at 21:06 , Gerriet M. Denkmann <g...@mdenkmann.de> wrote:
>
> But, alas, it is also much slower: overhead almost 40 sec (whereas my
> admittedly rather hackish way took less then half a second).
That may indicate a lot of contention — e.g. you were running multiple copies
of that loop in different threads, and each loop was doing an increment every
iteration. I’d actually try doing it this way:
> // One-time initialization, globally
>
> dsema = dispatch_semaphore_create (1);
>
> NSUInteger counter = 0;
>
>
> // Per-thread code, locally
>
> NSUInteger perThreadCounter = 0;
>
> for …
> {
> …
> perThreadCounter++;
> }
>
> dispatch_semaphore_wait (dsema, <forever>);
> counter += perThreadCounter;
> dispatch_semaphore_signal (dsema);
That should diminish the overhead to, effectively, zero.
But this is a bit academic, if you have a non-deprecated method of atomic
incrementing available. This is about what you might do for a more complicated
calculation that is not inherently atomic.
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