Re: [Xenomai-help] CONFIG_PREEMPT & irqbench

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

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Jan Kiszka wrote:
> Markus Franke wrote:
>> Dear all,
>>
>> I am running some latency tests with irqbench/irqloop. I am wondering
>> whether it would be possible to achieve better results when activating
>> CONFIG_PREEMPT and CONFIG_PREEMPT_VOLUNTARILY during the kernel
>> configuration and running irqloop in User Mode over this kernel?
> 
> Nope.
> 
>> Does it make any sense?
> 
> Nope. :)
> 
>> I think in theory it should give better results because
>> this irqloop runs in secondary(linux) domain when started, right? An
>> increasing preemptibility of the linux kernel should be better for the
>> irqloop-task.
> 
> The Linux kernel is already fully preemptible by Xenomai once you
> applied the I-pipe patch. Therefore, you are free to pick the Linux
> preemption strategy according to your *Linux* load, independent of what
> the real-time part needs.
> 
> Unless you have interactive Linux programs running, I can suggest to
> pick PREEMPT_VOLUNTARILY or even PREEMPT_NONE, specifically on low-end
> hardware.
> 
>> All tests were made under heavy I/O and CPU load by means of "dd",
>> "pingflood" and "cpuburn". Nevertheless, I can only achieve worse
>> results when activating CONFIG_PREEMPT.
> 
> Do you have CONFIG_DEBUG_PREEMPT set as well then? This option still as
> a small but measurable impact on Xenomai due to micro-dependencies that
> as scheduled to be removed in the near future.

Hmm, I should have better said "tiny". This experience is based on
I-pipe tracer observations, and I guess you don't have that thing on,
have you?

How much is the difference? How long did you measure to exclude noise.

Jan


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