On Tue, Apr 4, 2017 at 8:38 AM, Henrik Austad <henrik@xxxxxxxxx> wrote: > 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 Thanks for these comments, Henrik. I have reviewed examples of how to use SCHED_DEADLINE, yet I am not sure: Does it use POSIX or non-POSIX timers ? I ask because we've seen some issue with POSIX timers in some kernel version, architectures. Ran -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
