Re: Launching background job from Ada.Real_Time.Timing_Events

["Alejandro R. Mosteo" <alejandro@mosteo.com>]

On 25/05/16 09:23, Dmitry A. Kazakov wrote:
> On 24/05/2016 16:22, Alejandro R. Mosteo wrote:
>
>> I'm stuck with something not that strange, so I guess I need another
>> viewpoint on it.
>>
>> I want to trigger, in the future, a possibly long computation in the
>> background. For that I had thought of using the built-in Real_Time
>> events.
>>
>> My idea was to queue the job when the event is triggered, and to have a
>> worker thread waiting for jobs in the queue. However, I've just realized
>> that the event handler is itself a protected action, so I can't call
>> another potentially blocking enqueue from there.
>
> Neither is protected action. To enqueue anything into FIFO you need no
> locking. This is lock-free. To dequeue, provided single consumer there
> is no need to lock anything either. If there are several consumers, you
> use a protected object to dequeue only.
> 1-to-n: The dequeue is a waitable entry. Once you have got the packet
> out of the queue you process it outside any protected actions.
>
> 1-to-1: You use a reset-event implemented using protected object to wake
> up the consumer, which dequeues the packet. Reset event is
> race-condition free to query if the queue is empty and if so, to wait
> for the event.

I'm sorry but I don't follow your explanation. To make things simpler, 
let's consider only the 1-to-1 case. How's this protected used without a 
blocking operation?

Thanks.

>> Any ideas? Basically, how to trigger another task (without creating it)
>> from within a protected handler?
>
> Always by waiting for an entry. E.g. as described above.
>