From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nivedita Singhvi Subject: Re: preempt rt in commercial use Date: Wed, 15 Sep 2010 07:09:22 -0700 Message-ID: <4C90D392.8040808@us.ibm.com> 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> <4C907A51.1050305@steinhoff.de> 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: Armin Steinhoff Return-path: Received: from e34.co.us.ibm.com ([32.97.110.152]:60849 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754043Ab0IOOJj (ORCPT ); Wed, 15 Sep 2010 10:09:39 -0400 Received: from d03relay04.boulder.ibm.com (d03relay04.boulder.ibm.com [9.17.195.106]) by e34.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id o8FE0PmF011131 for ; Wed, 15 Sep 2010 08:00:25 -0600 Received: from d03av03.boulder.ibm.com (d03av03.boulder.ibm.com [9.17.195.169]) by d03relay04.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id o8FE9UPQ066018 for ; Wed, 15 Sep 2010 08:09:31 -0600 Received: from d03av03.boulder.ibm.com (loopback [127.0.0.1]) by d03av03.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id o8FE9TIZ003346 for ; Wed, 15 Sep 2010 08:09:30 -0600 In-Reply-To: <4C907A51.1050305@steinhoff.de> Sender: linux-rt-users-owner@vger.kernel.org List-ID: Armin Steinhoff wrote: >> 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. At some stage this might have been a pretty good response time. But HW improves by leaps and bounds, and what was considered "fast" or "real-time" 25 years ago might be your average vanilla desktop box speed of today. So, you'd have to define _exactly_ what operation completes in under 15us? Again, it's these kinds of ambiguities that make this a very woollen and fuzzy way to talk about subjects and needs which are usually very precise and critical. If your OS can support sub-us response times for some required operation, I expect you wnat to say that, rather than a generic "hard RT". > 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. Right. Regardless of what you call it, I would want the user to understand very clearly what the OS is capable of, and what it is not. And whether that meets their application's needs or not. > BTW ... we use PREEMPT_RT Linux as a base for our commercial > solft-PLC called DACHSview: http://steinhoff-automation.com/Programming.htm Very interesting! Thanks for sharing that... thanks, Nivedita