From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759156AbZEGRT1 (ORCPT ); Thu, 7 May 2009 13:19:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754292AbZEGRTS (ORCPT ); Thu, 7 May 2009 13:19:18 -0400 Received: from casper.infradead.org ([85.118.1.10]:49855 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753990AbZEGRTR (ORCPT ); Thu, 7 May 2009 13:19:17 -0400 Subject: Re: [PATCH] x86: Reduce the default HZ value From: Peter Zijlstra To: Christoph Lameter Cc: Alok Kataria , "H. Peter Anvin" , Ingo Molnar , Thomas Gleixner , the arch/x86 maintainers , LKML , "alan@lxorguk.ukuu.org.uk" , "Paul E. McKenney" In-Reply-To: <1241716422.6311.1524.camel@laptop> 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> Content-Type: text/plain Date: Thu, 07 May 2009 19:18:38 +0200 Message-Id: <1241716718.6311.1531.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.