On Mon, Apr 03, 2017 at 03:53:27PM +0300, Ran Shalit wrote: > On Mon, Apr 3, 2017 at 8:06 AM, Ran Shalit <ranshalit@xxxxxxxxx> wrote: > > On Mon, Apr 3, 2017 at 5:38 AM, Alison Chaiken <alison@xxxxxxxxxxxxxxxx> wrote: > >> Ran Shalit <ranshalit@xxxxxxxxx> wrote: > >>> I have found in > >>> https://wiki.linuxfoundation.org/realtime/documentation/howto/applications/cyclic > >>> that there is EDF scheduler option for creating a periodic RT thread. > >>> Yet, I could not understand what's behind the EDF scheduler > >> > >> > >> Have a look at "Using SCHED_DEADLINE" at > >> > >> http://elinux.org/ELC_Europe_2016_Presentations > >> > >> Also, look for papers on this topic by Juri Lelli. > >> > >> HTH, > >> Alison > > > > Hi Alison, > > > > Thank you very much. > > I reviewed the material, which is very interesting, but I am not yet > > sure which timer are used internally in SCHED_DEADLINE. > > Is it posix or non-posix ? > > > > Thanks, > > Ran > > I am trying to understand which scheduler is better used with RT. > In the wiki there is an example for SCHED_FIFO, periodic time. > But in sched_deadline wiki ( https://wiki.automotivelinux.org/sched_deadline ) > it is said as following: > " > Using SCHED_FIFO/SCHED_RR, instead, we can give that kind of > guarantee only using a global period, with the constraint that “a > subgroup must have a smaller or equal period to its parent”. > The latency experienced by a task (i.e., the time between two > consecutive executions of a task) is not deterministic and cannot be > bound, since it highly depends on the number of tasks running in the > system at that time. Using EDF, instead, this time is deterministic, > bounded and known at any instant of time." > > I am not sure now. Does it mean that using more than one SCHED_FIFO > periodic thread is not deterministic compared with SCHED_DEADLINE? It means that when you use priorities, you basically do a one-way mapping between the inherit deadlines in your tasks to a set of static priorities. As a developer you know *when* the tasks needs to complete, right? So then the job is to pick a priority and hope that no other higher-prioritized thread preempts you and cause you to miss your deadline. And that is the problem with a complicated system, many tasks lead to an intractable problem of getting all the priorities right, causing you no end to the headache. With SCHED_DEADLINE, you express your requirements directly as deadlines, thus avoid that whole one-way mapping and instead the tasks relative priority will adjust as the scheduler now has a _much_ more comprehensive view of the requirements. -- Henrik Austad
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