Re: [Starpu-devel] Is StarPU busy-waiting for tasks ?

[Samuel Thibault <samuel.thibault@ens-lyon.org>]

Benoît Lizé, le Thu 07 Nov 2013 10:39:40 +0100, a écrit :
> +/* Condition variable and mutex used to pause/resume. */
> +static starpu_pthread_cond_t pause_cond = STARPU_PTHREAD_COND_INITIALIZER;
> +static starpu_pthread_mutex_t pause_mutex = 
> STARPU_PTHREAD_MUTEX_INITIALIZER;
>  unsigned _starpu_machine_is_running(void)
>  {
>       unsigned ret;
> -     /* running is just protected by a memory barrier */
> +     unsigned should_pause;
> +     /* running and paused are just protected by a memory barrier */
>       STARPU_RMB();
> +     should_pause = config.paused;
> +
> +     if (STARPU_UNLIKELY(should_pause)) {
> +    STARPU_PTHREAD_MUTEX_LOCK(&pause_mutex);

You need to check again should_pause here, otherwise someone may have
run resume in-between.

> +             STARPU_PTHREAD_COND_WAIT(&pause_cond, &pause_mutex);
> +             STARPU_PTHREAD_MUTEX_UNLOCK(&pause_mutex);
> +     }
> +
>       ANNOTATE_HAPPENS_AFTER(&config.running);
>       ret = config.running;
>       ANNOTATE_HAPPENS_BEFORE(&config.running);
>       return ret;
>  }
>  
> +void starpu_pause()
> +{
> +  config.paused = 1;

Mmm, I'd rather see pause/resume work as a push/pop mechanism, i.e.
paused is the number of calls to pause() minus the number of calls to
resume.

> @@ -967,6 +993,9 @@ void starpu_shutdown(void)
>       initialized = CHANGING;
>       STARPU_PTHREAD_MUTEX_UNLOCK(&init_mutex);
>  
> +     /* If the workers are frozen, no progress can be made. */
> +     starpu_resume();

And I'd rather avoid this, and use an ASSERT(!paused) instead.

Samuel