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

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

[-- Attachment #1: Type: text/plain, Size: 1727 bytes --]

M. Koehrer wrote:
> Hi Jan,
> 
> thanks for your reply.
> 
>>> I am currently checking XENOMAI (V2.2.3 on a 2.6.17.7 kernel P4) to see if
>> I can use it
>>> as replacement for a RTAI 3.3-cv application.
>>> The first thing I did was to run the latency test in the the xenomai's
>> testsuite directory.
>>> The results of the worst time latency are really ugly - about 40µs!
>>> On the very same PC I got a value of about 5µs using RTAI 3.3-cv running
>> the RTAI's
>>> user/latency test.
>> I'm _very_ sceptical about your 5 us. Could you elaborate on how you
>> load your box and how long those tests ran? See also TROUBLESHOOTING in
>> the Xenomai source tree on appropriate load for triggering the worst case.
> 
> My test setup was the same in both cases.
> I connect to the RTAI/Xenomai PC via telnet and let the latency test run for about 5 minutes.

5 min. are far too short.

It takes a while to "arrange" the worst case, i.e. a dirty cache, one
unrelated IRQ or atomic section running while the timer (or any other
event of interest) occurs, and then one or more further IRQs right after
the woken-up task re-enabled interrupts. If you hit such scenario can be
observed via the latency tracer (which has some impact on the numbers,
but not on the structuring).

> No other activity happens (well, there are a couple of services running like
> apache, ... - but they should not cause any load).
> The strange thing, is that in Xenomai even after a few seconds a latency in the range of the
> maximum latency is shown.

Such a test on an unloaded system says almost nothing about a hard
real-time system.

> 
> I can retry the tests using a background stress situation.

Jan


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 250 bytes --]