Re: [Xenomai-help] question about the latency test running on Blackfin

[Jan Kiszka <jan.kiszka@domain.hid>]

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Li Yi wrote:
> Hi Philippe,
> 
> According to your answers, I updated the document on Xenomai on
> Blackfin: (http://docs.blackfin.uclinux.org/doku.php?id=adeos) with the
> test result of different modes, setting  XENO_OPT_TIMING_SCHEDLAT = 1.
> But I still have some question about the test result, hoping for your
> help:
> 
> When running this test in mode 0, the worst case latency is 65 us, (HSD|
> max| 65 - 66 | 1). This result is get when there is no workload. Can I
> say this system is "Hard Real-time" because most of the latency samples
> are in a  limited range?  

Soft RT: the more samples remain under the required limit the higher is
         the quality of the system

Hard RT: ALL samples must be below the limit

So, for hard RT, those 65 us is the interesting number. How did you
create the system load? Typical tests contain some cache benchmarking
tool (I prefer calibrator: http://www.cwi.nl/~manegold/Calibrator) +
network load (e.g. ping -f) + hard disk or other persistent storage
access (e.g. dd if=<your-disk> of=/dev/null).

How much MHz does your board have, and to what can it be compared when
looking at other architectures. 65 us looks quite (too?) good for
low-end, but maybe the blackfin isn't actually low-end.

> 
> That is, from the test result:
> HSS|    min|        59|     31.746|      0.632
> HSS|    avg|    599965|     33.365|      2.151
> HSS|    max|        59|     53.339|      2.739
> 
> Can I make the conclusion that the schedule latency is "determinative" and the system is "hard real-time"? If not,
> what is the expected test result? Xenomai is able to provide hard real-time, right?

Yes, Xenomai is a hard-RT system. But this doesn't depend on a specific
worst-case latency number, rather on the fact that a hardware-dependent
upper limit can be provided that is independent of the (here:) Linux load.

Jan


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