Re: Delays on usleep calls

[Dario Faggioli <dario.faggioli@citrix.com>]

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On mer, 2014-02-05 at 16:30 -0500, Robbie VanVossen wrote:
> On 1/20/2014 10:05 AM, Dario Faggioli wrote:
> > What about giving a try to it yourself? I think standardizing on one (a
> > set of) specific tool could be a good thing.
> 
> Dario,
> 
Hey! :-)

> We thought we would try to get some similar readings for the Arinc653 scheduler.
> We followed your suggestions from this thread and have gotten some readings for
> the following configurations:
> 
That's cool, thanks for doing this and sharing the results.

> We used the following command to get results for a 30 millisecond (30,000us)
> interval with 500 loops:
> 
> cyclictest -t1 -i 30000 -l 500 -q
> 
> Results:
> 
> +--------+--------+-----------+-------+-------+-------+
> | Config | Domain | Scheduler |      Latency (us)     |
> |        |        |           |   Min |   Max |   Avg |
> +--------+--------+-----------+-------+-------+-------+
> |      1 |      0 |  Arinc653 |    20 |   163 |    68 |
> |      2 |      0 |  Arinc653 |    21 |   173 |    68 |
> |      3 |      1 |  Arinc653 |    20 |   155 |    75 |
> +--------+--------+-----------+-------+-------+-------+
> 
> It looks like we get negligible latencies for each of these simplistic
> configurations.
> 
It looks indeed. You're right, the configuration are simplistic. Yet, as
stated before, latency and jitter in scheduling/event response has two
major contributors: one is the scheduling algorithm itself, the other is
the interrupt/event delivery latency of the platform (HW + HYP + OS).
This means that, of course, you need to pick the right scheduler and
configure it properly, but there may be other sources of latency and
delay, and that is what sets the lowest possible limit, unless you go
chasing and fix these 'platform issues'.

From your experiments (an from some other numbers I also have) it looks 
like this lower bound is not terrible in Xen, which is something good to
know... So thanks again for taking the time of running the benchmarks
and sharing the results! :-D

That being said, especially if we compare to baremetal, I think there is
some room for improvements (I mean, there always will be an overhead,
but still...). Do you, by any chance, have the figures for cyclictest on
Linux baremetal too (on the same hardware and kernel, if possible)?

Thanks a lot again!
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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