[libdispatch-dev] Using libdispatch/GCD in a real-time context

[Stéphane Letz]

> I would start by filing an RFE bug to the libdispatch tracker, since this is a complex request that may take some time to implement. Here's the link:
> 
> 	http://libdispatch.macosforge.org/trac/newticket
> 
> The dispatch_queue_create() function and the dispatch_queue_attr struct could be extended to specify a realtime queue [1]. The pthread_workqueue_attr struct could be extended to support a time_constraint_policy for the worker threads [2]. Each realtime dispatch_queue would use a separate pthread_workqueue whose threads would run with the desired time_constraint_policy.
> 
> For additional context, take a look at the pthread_workqueue code I pulled from the xnu and darwin sources:
> 
> 	http://mark.heily.com/src/libpthread_workqueue.tar
> 
> Regards,
> 
> - Mark
> 
> [1]
> 
> typedef struct dispatch_queue_attr {
>    int thread_policy;
>    struct thread_time_constraint_policy ttcpolicy;
> } dispatch_queue_attr_t;
> 
> dispatch_queue_attr_t attr = {
> 	.thread_policy = THREAD_TIME_CONSTRAINT_POLICY,
> 	.ttcpolicy = {
> 		.period = ?,
> 		.computation = ?,
> 		.constraint = ?,
> 		.preemptible = 1,
> 	},
> };
> 
> my_queue = dispatch_queue_create("org.jack.realtime.audio", &attr);
> 	
> [2]
> 
> typedef struct {
>        uint32_t sig;
>        int queueprio;
>        int overcommit;
> +	int thread_policy;
> +	struct thread_time_constraint_policy *ttcpolicy;
>        unsigned int resv2[13];
> } pthread_workqueue_attr_t;
> 
> +int pthread_workqueue_attr_settimeconstraintpolicy_np(
> +	pthread_workqueue_attr_t * attr,
> +	struct thread_time_constraint_policy *ttcpolicy);
> 
> 


Thanks.

Before going further, I've adapted our "fine-grained" parallel code (that is using an ad-hoc Work Stealing Scheduler and a pool or RT threads..) to use libdispatch. Performances are currently much worse. 

- I see that using blocks is also allocating memory, which is usually forbidden in RT code (I guess "dispatch_async_f" should be used instead of "dispatch_async" right ?)

- The parallel queue is DISPATCH_QUEUE_PRIORITY_HIGH, I guess that I cannot change the  GCD thread priorities on the fly right?

Thanks.

Stéphane Letz