From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753959Ab2C0LxR (ORCPT ); Tue, 27 Mar 2012 07:53:17 -0400 Received: from mailout-de.gmx.net ([213.165.64.23]:47782 "HELO mailout-de.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752566Ab2C0LxQ (ORCPT ); Tue, 27 Mar 2012 07:53:16 -0400 X-Authenticated: #14349625 X-Provags-ID: V01U2FsdGVkX1/asaE7kU/aYY+7dqYXj/i0R1fs4Vc6SFhNq2Q7+b BjqZJ0Xpm+Fj6v Message-ID: <1332849183.7679.21.camel@marge.simpson.net> Subject: Re: [PATCH 07/32] cpuset: Set up interface for nohz flag From: Mike Galbraith To: Frederic Weisbecker Cc: Christoph Lameter , 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 , Steven Rostedt , Sven-Thorsten Dietrich , Thomas Gleixner , Zen Lin Date: Tue, 27 Mar 2012 13:53:03 +0200 In-Reply-To: <20120327112254.GC13196@somewhere> References: <1332338318-5958-1-git-send-email-fweisbec@gmail.com> <1332338318-5958-9-git-send-email-fweisbec@gmail.com> <1332389033.5759.52.camel@marge.simpson.net> <20120327112254.GC13196@somewhere> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.1 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 X-Y-GMX-Trusted: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2012-03-27 at 13:22 +0200, Frederic Weisbecker wrote: > On Thu, Mar 22, 2012 at 05:03:53AM +0100, Mike Galbraith wrote: > > On Wed, 2012-03-21 at 09:50 -0500, Christoph Lameter wrote: > > > On Wed, 21 Mar 2012, Frederic Weisbecker wrote: > > > > > > > Prepare the interface to implement the nohz cpuset flag. > > > > This flag, once set, will tell the system to try to > > > > shutdown the periodic timer tick when possible. > > > > > > > > We use here a per cpu refcounter. As long as a CPU > > > > is contained into at least one cpuset that has the > > > > nohz flag set, it is part of the set of CPUs that > > > > run into adaptive nohz mode. > > > > > > What are the drawbacks for nohz? > > > > For nohz in general, latency. To make it at all usable for rt loads, I > > had to make isolated cores immune from playing load balancer. Even so, > > to achieve target latency, I had to hack up cpusets to let the user > > dynamically switch nohz off for specified sets (and the tick has to be > > skewed in both cases or you can just forget it). With nohz, I can't > > quite achieve 30us jitter target, turn it off, I get single digit. Out > > of the current box, triple digit for simple synchronized frame timers + > > compute worker-bees load on 64 cores. Patch 4 probably helps that, but > > don't _think_ it'll fix it. If you (currently) ever become balancer, > > you're latency target is smoking wreckage. > > But this is because of waking up from CPU low power mode, right? If so > then busy tickless shouldn't be concerned. We can certainly have > configurations where the tick is not stopped in idle but can be elsewhere. Boxen are restricted to C1 (even at that Q6600 _sucks rocks_, but more modern CPUs don't). ATM, ticked is cheaper, I can't get there from here with nohz. -Mike