Re: [Xenomai-help] Handling interrupt - jitter

[Wolfgang Grandegger <wg@domain.hid>]

Gilles Chanteperdrix wrote:
> On Fri, Apr 18, 2008 at 4:33 PM, Bosko Radivojevic
> <bosko.radivojevic@domain.hid> wrote:
>> Hi,
>>
>>  I'm having (from my perspective) too big jitter on handling
>>  interrupts. System is: ARM9, AT91SAM9260, Kernel 2.6.20.21, Adeos
>>  1.8-03, Xenomai-2.4.2, interrupt is generated every 1ms, externally.
> 
> It's funny to notice how much you asked for the 2.6.24 patch for
> actually not using it.
> 
>>  Interrupt source (ET1200) is connected to IRQ1 pin (PC15). I'm using
>>  native skin from user space. Usually, handler 'reacts' after ~110us,
>>  but, worst case delay is around 250us! Is this performance
>>  normal/expected on my platform? Why the difference is so huge?
> 
>>  Is there anything I can do to get better results (with this hw:)?
> 
> 250 us is the user-space dispatch latency you obtain with the
> "latency" test on a properly loaded target. It is the time for a
> user-space context switch with a full cache flush. To reduce this
> latency you have to:
> - handle interrupts in kernel space, so that you will be limited by
> interrupt latency, not user-space scheduling latency
> - use trunk so that user-space context switches involving real-time
> threads do not happen with hardware interrupts off.

I started to do some external interrupt latency measurements with
gpioirqbench showing also the effect of handling the interrupt in
user-space, kernel-space or even the interrupt handler on two PowerPC
systems. For further information, have a look to

  http://www.denx.de/wiki/DULG/AN2008_03_Xenomai_gpioirqbench

Wolfgang.