From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <4808C5E4.2000603@domain.hid>
Date: Fri, 18 Apr 2008 18:01:40 +0200
From: Wolfgang Grandegger <wg@domain.hid>
MIME-Version: 1.0
References: <d6c8ef150804180733k3e2b2bc0p1255b84e16062f05@domain.hid>
	<2ff1a98a0804180748o4dc88c91of90e8e26c36d32ed@domain.hid>
In-Reply-To: <2ff1a98a0804180748o4dc88c91of90e8e26c36d32ed@domain.hid>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: Re: [Xenomai-help] Handling interrupt - jitter
List-Id: Help regarding installation and common use of Xenomai
	<xenomai.xenomai.org>
List-Unsubscribe: <https://mail.gna.org/listinfo/xenomai-help>,
	<mailto:xenomai-help-request@domain.hid>
List-Archive: </public/xenomai-help>
List-Post: <mailto:xenomai@xenomai.org>
List-Help: <mailto:xenomai-help-request@domain.hid>
List-Subscribe: <https://mail.gna.org/listinfo/xenomai-help>,
	<mailto:xenomai-help-request@domain.hid>
To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: xenomai-help <xenomai@xenomai.org>

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.