Re: [Xenomai-help] Results of xenomai's latency test vs. RTAI's

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

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M. Koehrer wrote:
> Hi everybody,
> 
> I have repeated the test using a parallel stress test (dd if=/dev/sda of=/dev/null, I have
> a SATA harddisk).
> The results are that my RTAI latency of 4µs is not valid, here the latency is about 26µs.
> However, xenomai shows a higher latency of about 40µs.

How long did the tests run this time? To my experience anything below 30
min. is not meaningful. And the faster the machines get, the lower the
probability for certain constellations becomes. So each additional hour
runtime gives a little bit more confidence.

> The interesting thing is, that without load RTAI seems to run very long in a very 
> low latency region, at the same setup xenomai shows large latencies earlier.
> 
> 
> Well, I have repeated these tests on a different PC that has a server mainboard and 
> a Pentium D 4 (real dual core) CPU.
> The results (measured under load) are much better here:
> RTAI shows a latency of about 6µs
> Xenomai shows a latency of about 10µs.
> Both values are acceptable, however I wonder where the difference comes from?

RTAI contains a few shortcuts that may explain parts of this gap, but
not all.

One of these shortcuts is the immediate IRQ dispatching mode (the reason
why they have their own patch...). In that mode the registered timer IRQ
handler gets called without caring about any I-pipe abstraction.

Some others I recall are a missing clean timer subsystem abstraction
(tasked are timed, and then there are also some kind of timer tasklets)
and the immediate reschedule from IRQ context instead of doing this on
IRQ return. Some of those features should be controllable via magic
knobs, maybe it's worth a try what they actually buy you here.

In any case, to really understand the differences, it takes the tracer.
I will check if I can find my old RTAI hack to instrument their latency
tool - if you are still interested in digging deeper on your hardware.

Jan


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