From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756967AbZEGTSt (ORCPT ); Thu, 7 May 2009 15:18:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751970AbZEGTSh (ORCPT ); Thu, 7 May 2009 15:18:37 -0400 Received: from casper.infradead.org ([85.118.1.10]:41473 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751570AbZEGTSg (ORCPT ); Thu, 7 May 2009 15:18:36 -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" In-Reply-To: References: <1241462661.412.8.camel@alok-dev1> <4A00ADDE.9000908@zytor.com> <1241560625.8665.17.camel@alok-dev1> <1241716053.6311.1514.camel@laptop> <1241718302.6311.1573.camel@laptop> Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Thu, 07 May 2009 21:17:34 +0200 Message-Id: <1241723854.11251.166.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2009-05-07 at 13:50 -0400, Christoph Lameter wrote: > On Thu, 7 May 2009, Peter Zijlstra wrote: > > > No it won't, you want fairly decent involuntary preemption rate to keep > > the full service latency at a usable figure. > > > > The problem with scheduling a hrtimer along with tasks is that at high > > context switch rates the timer will never fire but you do pay the > > overhead of programming the hardware each time, something that can be > > about as expensive as the whole context switch itself. > > What are high context switch rates? 1000 HZ? Generally it seems that > context switches are bad for cpu caches and thus to be avoided. We're talking about in excess of 250k switches a second.