From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [Xenomai-help] testing interrupt latency From: Philippe Gerum In-Reply-To: <17618.14793.304543.952079@domain.hid> References: <1154615459.9835.62.camel@domain.hid> <17618.14793.304543.952079@domain.hid> Content-Type: text/plain Date: Thu, 03 Aug 2006 20:18:34 +0200 Message-Id: <1154629114.5010.161.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Reply-To: rpm@xenomai.org List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: xenomai@xenomai.org On Thu, 2006-08-03 at 20:00 +0200, Gilles Chanteperdrix wrote: > Bart Jonkers wrote: > > Hey, > > > > I have written a small module to test the interrupt response time of > > Xenomai to compare it with the time of Linux. > > > > I generate an interrupt on a GPIO pin and toggles another GPIO pin in > > the interrupt service routine. (code below) > > The time between the generated interrupt and the value change on the > > GPIO pin is taken as interrupt response time. > > > > Everything works well, but I notice a strange behavior. > > I thought that the interrupt response time of xenomai should be (as good > > as) constant all the time. But I see that this is not the case. > > > > Did I do something wrong in my code or did I made a wrong assumption? > > What should be guaranteed by Xenomai is that the response time is > bounded, not that it is constant. Of course, we make all we can to make > the bound as small as possible, but each hardware has its > limits. > Indeed. Not to speak of the basic issue all real-time Linux infrastructures (native or not) have to deal with, compared to purely standalone RTOSes: the hardware is shared by GPOS services which do not give a damn of our efforts to keep the latencies close to the hardware limits. See TLB flushes, significant cache trashing, power saving tricks, funky processor modes and friends. This is the price to pay for having real-time and general purpose services integrated on a single and commoditized hardware and software platform. -- Philippe.