From mboxrd@z Thu Jan 1 00:00:00 1970 From: Armin Steinhoff Subject: Re: preempt rt in commercial use Date: Wed, 15 Sep 2010 09:48:33 +0200 Message-ID: <4C907A51.1050305@steinhoff.de> References: <20100914094411.GB10841@pengutronix.de> <4C8F8500.5070002@theptrgroup.com> <201009141830.03206@zigzag.lvk.cs.msu.su> <4C8F8B79.1010300@theptrgroup.com> <4C8FF52E.1030407@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Jeff Angielski , "Nikita V. Youshchenko" , Robert Schwebel , Raz , linux-rt-users To: Nivedita Singhvi Return-path: Received: from moutng.kundenserver.de ([212.227.17.10]:64294 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751666Ab0IOHqy (ORCPT ); Wed, 15 Sep 2010 03:46:54 -0400 In-Reply-To: <4C8FF52E.1030407@us.ibm.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: Nivedita Singhvi wrote: > On 09/14/2010 07:49 AM, Jeff Angielski wrote: > >>> Isn't any case where preempt-rt does not behave as hard reatlime a >>> bug in >>> preempt-rt, that should be reported to this list and eventually fixed? >> >> That is a philosophical question for the preempt-rt maintainers. >> >> I *believe* that the design goal for the preempt rt code is to minimize >> kernel latency. It's not to make the kernel deterministic to support >> hard realtime. > > Er, sort of, not quite. > > The design goal of the real-time kernel is most certainly to make the > kernel more deterministic, to the extent we can in a general-purpose way. > > Determinism = capping max latencies. Capping max latencies doesn't help without a good real-time, event driven scheduler. But it helps to classify real-time operatings systems as so called hard or soft real-time operating systems. IMHO ... there is a common understanding that a RTOS can be considered as a hard-reatime OS if the max latency is < 15us because it is able to server 80% (?) of all hard real-time applications in the field. All others are considered as soft real-time operating systems. From this point of view is PREEMPT_RT Linux a hard real-time OS ... if the hardware base is appropriate. BTW ... we use PREEMPT_RT Linux as a base for our commercial solft-PLC called DACHSview: http://steinhoff-automation.com/Programming.htm > > It's better to have all hundred iterations of an operation take > 45us each than to have 95 iterations take 30us and 5 iterations > take 75us. You want to be able to say, "this operation will take > at _most_ 50us" and have that be as true as possible. All these numbers have no meaning if you don't specify the hardware environment. --Armin http://www.steinhoff-automation.com > > We sacrifice overall throughput (ave latency) for determinism > (low max latency). > > Not sure if that's what you intended to say, but hope that helps. > > thanks, > Nivedita > > > -- > To unsubscribe from this list: send the line "unsubscribe > linux-rt-users" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >