From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758241AbZJLVQg (ORCPT ); Mon, 12 Oct 2009 17:16:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758193AbZJLVQf (ORCPT ); Mon, 12 Oct 2009 17:16:35 -0400 Received: from smtp2.ultrahosting.com ([74.213.174.253]:38819 "EHLO smtp.ultrahosting.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1758191AbZJLVQe (ORCPT ); Mon, 12 Oct 2009 17:16:34 -0400 Date: Mon, 12 Oct 2009 17:08:58 -0400 (EDT) From: Christoph Lameter X-X-Sender: cl@gentwo.org To: Dragoslav Zaric cc: LKML Subject: Re: Real time kernel In-Reply-To: <2d05c4580910121404x7d09ea3dvde1aa49b05d2e3fc@mail.gmail.com> Message-ID: References: <2d05c4580910121100v702cbdddua9bd8de971fecaa2@mail.gmail.com> <2d05c4580910121404x7d09ea3dvde1aa49b05d2e3fc@mail.gmail.com> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 12 Oct 2009, Dragoslav Zaric wrote: > This is definition from http://rt.wiki.kernel.org/ > > ------------------------------------------------------------------------- > Real-time applications have operational deadlines between some > triggering event and the application's response to that event. To meet > these operational deadlines, programmers use real-time operating > systems (RTOS) on which the maximum response time can be calculated or > measured reliably for the given application and environment. > ------------------------------------------------------------------------- What is the calculated maximum response time in this case? > So for specific requirements I would not say, you get slower kernel, > you get purpose-making kernel, because on these kind of systems you > probably do not do anything. It is probably the system that listens > and responds to some events. Ahh.. > Do you want to say, when using preemption in "normal user mode", Linux > works slower then without preemption ? > It makes sense, Preemption increases code and data size of the kernel which causes the processors to have to process more instructions and touch more cachelines to do the same work. > "because the latency of a task only depends on the tasks running at > equal or higher priorities, all other tasks can be ignored." > (http://rt.wiki.kernel.org/index.php/Frequently_Asked_Questions#What_is_real-time.3F) Latency depends on the number of cachelines touched and on the number of instructions that have to be processed by a processor until a reaction to an event can occur. Increasing the latency by make processing more complex can avoid outliers. You trade speed for guarantees in realtime OSes.