From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757571Ab2C1Bfk (ORCPT ); Tue, 27 Mar 2012 21:35:40 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122]:9618 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755687Ab2C1Bfj (ORCPT ); Tue, 27 Mar 2012 21:35:39 -0400 X-Authority-Analysis: v=2.0 cv=P4S4d18u c=1 sm=0 a=ZycB6UtQUfgMyuk2+PxD7w==:17 a=XQbtiDEiEegA:10 a=3MQ0ZmHyo4IA:10 a=5SG0PmZfjMsA:10 a=Q9fys5e9bTEA:10 a=8pif782wAAAA:8 a=toliARVeFMJUvrcVeOEA:9 a=BbyBtpCogBZ_Cn_j4LMA:7 a=PUjeQqilurYA:10 a=ZycB6UtQUfgMyuk2+PxD7w==:117 X-Cloudmark-Score: 0 X-Originating-IP: 74.67.80.29 Message-ID: <1332898535.23924.157.camel@gandalf.stny.rr.com> Subject: Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it From: Steven Rostedt To: Christoph Lameter Cc: Frederic Weisbecker , LKML , linaro-sched-sig@lists.linaro.org, Alessio Igor Bogani , Andrew Morton , Avi Kivity , Chris Metcalf , Daniel Lezcano , Geoff Levand , Gilad Ben Yossef , Ingo Molnar , Max Krasnyansky , "Paul E. McKenney" , Peter Zijlstra , Stephen Hemminger , Sven-Thorsten Dietrich , Thomas Gleixner , Zen Lin Date: Tue, 27 Mar 2012 21:35:35 -0400 In-Reply-To: References: <1332338318-5958-1-git-send-email-fweisbec@gmail.com> <1332338318-5958-13-git-send-email-fweisbec@gmail.com> <20120327121341.GE13196@somewhere> <1332865469.23924.129.camel@gandalf.stny.rr.com> <1332896805.23924.148.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset="ISO-8859-15" X-Mailer: Evolution 3.2.2-1 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2012-03-27 at 20:19 -0500, Christoph Lameter wrote: > On Tue, 27 Mar 2012, Steven Rostedt wrote: > > > > Obviously compiling a kernel with preemptiong introduces additional > > > overhead to guarantee more deterministic behavior. Additional overhead > > > increases latencies generated by the OS in general. Compile a kernel > > > without preemption and it will run faster and thus have lower latencies. > > > > I call that "lower overhead". > > Good marketing but it does not change the facts. I see we are mixing the paint for the bike shed. > > > "Latency is a measure of time delay experienced in a system, the precise > > definition of which depends on the system and the time being measured. > > Latencies may have different meaning in different contexts." > > > > That last sentence is key. So lets avoid the term "latency" as it > > obviously has a different meaning to the both of us. > > > > Instead, lets use "determinism" (what we call latency in the realtime > > world) and "overhead" (what you seem to see as latency caused by the > > kernel). > > I sure wish you would be using the term determinism instead of "latency". > > Overhead causes latency and the definition that you quoted is what I am > talking about. Latencies are the delays in processing experienced by the > application through the speed of system calls and by interruptions of > a user space process by the kernel for various reasons. I could also argue that a non-preempt kernel has a large latency as well. Although it may have good through put for one task, another task may suffer from a large latency waiting for a lower priority task to get out of a system call. You say tomAYto I say tomAHto. Read the article: http://en.wikipedia.org/wiki/Latency_%28engineering%29 Especially the section about: Computer hardware and operating system latency You'll see that it describes latency much closer to my definition than yours. Heck, google "operating system latency" and you'll see a lot of talk about reaction times and how fast the hardware can do its job. I don't see anything about the time a system call takes. -- Steve