[Cbe-oss-dev] [PATCH 1/5] MARS: yield in workload queue wait busy loop

[Arnd Bergmann]

On Tuesday 15 July 2008, Yuji Mano wrote:
> @@ -194,9 +195,8 @@ int workload_queue_wait(struct mars_work
>         MARS_CHECK_RET(queue->block[block].state[index] !=
>                         MARS_WORKLOAD_STATE_NONE, MARS_ERROR_STATE);
>  
> -       while (*(volatile uint8_t *)&queue->block[block].state[index] !=
> -               MARS_WORKLOAD_STATE_FINISHED) {
> -       }
> +       while (queue->block[block].state[index] != MARS_WORKLOAD_STATE_FINISHED)
> +               sched_yield();
>  
>         return MARS_SUCCESS;
>  }

I think you should try to avoid busy loops altogether. Doing sched_yield()
here (or anywhere else) will not give you the right behaviour. When you are
waiting for a value in memory to change, basing your waiting primitive on
futex should give you the best performance.

If you don't want to write the futex code yourself, you can use a
pthread_cond_t variable for waiting, but you can't currently use those
from an SPU, unlike futexes, which you can access using the SPU syscall
support in the kernel.

	Arnd <><