Using the EDF scheduler

[Jonathan Brandmeyer]

On Fri, Jan 17, 2020 at 4:03 PM Joel Sherrill <joel at rtems.org> wrote:

>
>
> On Fri, Jan 17, 2020 at 3:00 PM Gedare Bloom <gedare at rtems.org> wrote:
>
>>
>> From the theory side, this problem is usually addressed using a
>> (sporadic) server. Basically, you create some periodic task at the
>> prioritization that you want (e.g., the shortest relative deadline in
>> the system to usually get the highest priority) and then the server
>> uses its budget to execute the event-based tasks. There are several
>> variations with pros/cons. However, these do not allow for the
>> event-based tasks to be the most responsive in general; with EDF,
>> unless you use relative deadline = 1, there can always be some other
>> task with a shorter absolute deadline. Also, if the server budget gets
>> consumed you have to wait for replenishment. I'm not up-to-date with
>> the latest in SMP schedulers with servers. (Shame on me.)
>>
>
> I wondered about a band of high rate periodic tasks that check
> for the condition and do it or go to sleep. Binds them to periods
> which introduces latency so that's bad.
>
> This would work but likely not be acceptable.
>

A tickless scheduler would allow for a much finer scheduling quantum.  At
that point, an ISR could waken a task and hand it a new deadline.
rtems_event_receive_periodic() combined with rtems_event_send_deadline()?
Or perhaps some kind of rate_monotonic_interrupt_server?  At least in my
case, the task really is rate-monotonic, but the timing source is both
asynchronous with respect to the kernel tick and much finer.

-- 
Jonathan Brandmeyer
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