From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gilles Chanteperdrix Subject: Re: cyclictest better values with system load than without (OMAP3530 target) Date: Fri, 29 Nov 2013 17:58:56 +0100 Message-ID: <5298C7D0.2080300@xenomai.org> References: <5294681E.10406@gmail.com> <20131126101232.21636c8f@sluggy> <20131129125623.GB31099@linutronix.de> <5298AE4B.7070806@osadl.org> <5298C292.2030502@xenomai.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Sebastian Andrzej Siewior , Clark Williams , Stefan Roese , linux-rt-users@vger.kernel.org To: Carsten Emde Return-path: Received: from sd-51317.dedibox.fr ([62.210.215.82]:56262 "EHLO sd-51317.dedibox.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754218Ab3K2Q7F (ORCPT ); Fri, 29 Nov 2013 11:59:05 -0500 In-Reply-To: <5298C292.2030502@xenomai.org> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On 11/29/2013 05:36 PM, Gilles Chanteperdrix wrote: > On 11/29/2013 04:10 PM, Carsten Emde wrote: >> BTW: Power saving and real-time do not necessarily exclude each other. >> If a - still deterministic - but a little longer latency is acceptable, >> some light sleep states and a somewhat lower clock frequency may be >> allowed which still may result in considerable energy saving. If, >> however, the fastest possible real-time response is required, C states >> and P states must be disabled (or set to polling and maximum speed, >> repsectively) and the power bill must be payed. > > Well, I do not fully agree. To be sure that you can clock down the > processor for executing a task which has sufficient time to meet its > deadline, your system must be "time triggered", all the timer events > must be known in advance. Because on a fully dynamic system, you may > make that decision, but a new timer may be scheduled which causes the > system to miss its deadline whereas it would not have missed it if it > had run at full speed. > And a second problem is that you must know the task WCET, which on a modern system: - depends on the task frequency; - depends on the IRQ load. Again, only a time triggered system seems to make this possible. -- Gilles.