From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756934Ab2C1BTX (ORCPT ); Tue, 27 Mar 2012 21:19:23 -0400 Received: from smtp104.prem.mail.ac4.yahoo.com ([76.13.13.43]:30675 "HELO smtp104.prem.mail.ac4.yahoo.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1753570Ab2C1BTV (ORCPT ); Tue, 27 Mar 2012 21:19:21 -0400 X-Yahoo-Newman-Property: ymail-3 X-YMail-OSG: lCDbAAIVM1lUmKlF6O2wxzYJMXVIAOHASl_8GOMLKnMdlnJ XHlm0eExy9gbavd2YdtHJqgeeLtMbk84Ks8qi9DG99CfCpOYdgmhTkfa3IMq d.WYgtCpzmt8rBUQ3DyFYrkh0.I3mWBsYTzMdVVr6UzTOakbNWXPzsDneSbW uLsQd__ccEBRg6snqepTbZq6XmSkPHkrhQLpcu4QTAJvKaRBiBk5eKwmkxgP xkleOpY_eA.oiBAZ4xSCMaw6burebgJEnJMe.7qOpvVCZCr4CNoWVYmN239H 1GfbEyB9uSeADnccgb9kq8YjatQc63ugVJpyOx0dX_o1YEXuQxTfWqp9XmSc xabhaprGNbsoDH89Cl86Jev3I9l_zDcMjan.YUp7KsVmPRd4_xXvl9CaETZA o X-Yahoo-SMTP: _Dag8S.swBC1p4FJKLCXbs8NQzyse1SYSgnAbY0- Date: Tue, 27 Mar 2012 20:19:15 -0500 (CDT) From: Christoph Lameter X-X-Sender: cl@router.home To: Steven Rostedt 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 Subject: Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it In-Reply-To: <1332896805.23924.148.camel@gandalf.stny.rr.com> Message-ID: 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> User-Agent: Alpine 2.00 (DEB 1167 2008-08-23) 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 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. > "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.