Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262295AbVERTYC (ORCPT ); Wed, 18 May 2005 15:24:02 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262299AbVERTYC (ORCPT ); Wed, 18 May 2005 15:24:02 -0400 Received: from fire.osdl.org ([65.172.181.4]:37084 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S262295AbVERTXx (ORCPT ); Wed, 18 May 2005 15:23:53 -0400 Date: Wed, 18 May 2005 12:25:07 -0700 (PDT) From: Linus Torvalds To: Pavel Machek cc: Christoph Lameter , randy_dunlap , akpm@osdl.org, linux-kernel@vger.kernel.org, shai@scalex86.org, ak@suse.de Subject: Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt. In-Reply-To: <20050518185016.GD1952@elf.ucw.cz> Message-ID: References: <20050516150907.6fde04d3.akpm@osdl.org> <20050516194651.1debabfd.rdunlap@xenotime.net> <20050518185016.GD1952@elf.ucw.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1773 Lines: 42 On Wed, 18 May 2005, Pavel Machek wrote: > > Please don't do this, CONFIG_NO_IDLE_HZ patches are better solution, > and they worked okay last time I tried them. .. and they have nothing to do with this. A number of people who want lower tick frequency are apparently _server_ people. Not because it makes any difference to idle time, but because it can lessen the impact of the timer interrupt under load. I don't know why, but I've actually gotten most of the complaints about the 1kHz timer from ia64 people, who use a 1024Hz timer. Somebody from Intel claimed a several percent reduction in performance between 1kHz and 100Hz under some load, apparently because of bad cache interaction. At the same time, 100Hz really is too low for some desktop-like soft-RT stuff, where you want to delay until the next frame (and humans notice jitter at some fraction of a tenth of a second). With the 100Hz granularity, and the uncertainty on where the jiffy tick ends up being, you effectively have a ~50Hz clock you can depend on, which together with worries about synchronizing with the video refresh rate etc seems to make people unhappy. So this thing has nothing to do with "idle". And the truly-variable-HZ stuff just makes me nervous, but regardless of that, you actually do want a "limit HZ to some value" configuration option anyway. Even with fully variable HZ, you need a limit just to say "this is the highest precision we'll ever use", because otherwise you'll just be wasting a lot of time on timers. Linus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/