From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755455AbZEGRgU (ORCPT ); Thu, 7 May 2009 13:36:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752278AbZEGRgL (ORCPT ); Thu, 7 May 2009 13:36:11 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:47282 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752498AbZEGRgK (ORCPT ); Thu, 7 May 2009 13:36:10 -0400 Date: Thu, 7 May 2009 10:36:09 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Christoph Lameter , Alok Kataria , "H. Peter Anvin" , Ingo Molnar , Thomas Gleixner , the arch/x86 maintainers , LKML , "alan@lxorguk.ukuu.org.uk" Subject: Re: [PATCH] x86: Reduce the default HZ value Message-ID: <20090507173608.GC6693@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1241462661.412.8.camel@alok-dev1> <4A00ADDE.9000908@zytor.com> <1241560625.8665.17.camel@alok-dev1> <1241716053.6311.1514.camel@laptop> <1241716422.6311.1524.camel@laptop> <1241716718.6311.1531.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1241716718.6311.1531.camel@laptop> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 07, 2009 at 07:18:38PM +0200, Peter Zijlstra wrote: > On Thu, 2009-05-07 at 19:13 +0200, Peter Zijlstra wrote: > > On Thu, 2009-05-07 at 19:09 +0200, Peter Zijlstra wrote: > > > On Thu, 2009-05-07 at 10:13 -0400, Christoph Lameter wrote: > > > > I think we need to reduce the general tick frequency to be as low as > > > > possible. With high resolution timers the tick frequency is just the > > > > frequency with which the timer interrupt disturbs a running application. > > > > > > > > Are there any benefits remaining from frequent timer interrupts? I would > > > > think that 60 HZ would be sufficient. > > > > > > > > It would be good if the kernel would be truly tickless. Scheduler events > > > > would be driven by the scheduling intervals and not the invokations of the > > > > scheduler softirq. > > > > > > The only thing that's driven by the softirq is load-balancing, there's > > > way more to the scheduler-tick than kicking that thing awake every so > > > often. > > > > > > The problem is that running the scheduler of off hrtimers is too > > > expensive. We have the code, we tried it, people complained. > > > > Therefore, decreasing the HZ value to say 50, we'd get a minimum > > involuntary preemption granularity of 20ms, something on the high end of > > barely usable. > > Another user is RCU, the grace period is tick driven, growing these > ticks by a factor 50 or so might require some tinkering with forced > grace periods when we notice our batch queues getting too long. One approach would be to enter nohz mode when running a CPU-bound application on a CPU that had nothing else (other than the idle task) on its runqueue and for which rcu_needs_cpu() returns zero. In this mode, RCU would need to be informed on each system call, perhaps with an rcu_kernel_enter() and rcu_kernel_exit() that work like rcu_irq_enter() and rcu_irq_exit() -- and that perhaps replace rcu_irq_enter() and rcu_irq_exit(). Then RCU would ignore any CPU that was executing a CPU-bound application, allowing the HZ to be dialed down as low as you like, or perhaps really entering something like nohz mode. Thanx, Paul