From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756796Ab0CCVyC (ORCPT ); Wed, 3 Mar 2010 16:54:02 -0500 Received: from ksp.mff.cuni.cz ([195.113.26.206]:37181 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1756707Ab0CCVx4 (ORCPT ); Wed, 3 Mar 2010 16:53:56 -0500 Date: Wed, 3 Mar 2010 22:53:47 +0100 From: Pavel Machek To: Frantisek Rysanek Cc: Mikael Pettersson , linux-kernel@vger.kernel.org Subject: Re: Dynticks on Vortex86SX ? (Cx486SLC + just the standard i8254 PC timer) Message-ID: <20100303215347.GD2579@ucw.cz> References: <19335.44648.623680.272844@pilspetsen.it.uu.se> <4B87FBC3.23033.45BDAB1@Frantisek.Rysanek.post.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4B87FBC3.23033.45BDAB1@Frantisek.Rysanek.post.cz> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi! > > Select HZ=100. > > > right > 1) to make the numbers decadic > 2) seems like the only true way for me to decrease the rate of PC > timer interrupts :-) Try HZ=20 :-). > I am aware that the PC timer has a limited resolution (16 bits for > the single i8254 channel) and thus the maximum time you can possibly > sleep without an interrupt is about 55 ms (18 Hz). Based on all the > PR publicity the dynticks patch used to receive in its time, > I thought that even on my puny hardware, the PC timer would > automatically flex towards that maximum period. Apparently this is > not the case... I guess nohz needs apic timer or something like that. > And perhaps the hunt for dynticks is not really worth the bother, > if the wakeup to serve IRQ0 comes once every 3 million CPU cycles :-) > (This board clocks the CPU at 300 MHz.) How many CPU cycles does it > take before the idle loop calls HLT again? a hundred maybe? So > that's 1/30000 of time running out of HLT? :-) Well, on modern cpus, entering/exiting halt state takes *lomg* time. You are lucky. > There's another power-saving feature of the Vortex SoC: it can > decrease the core clock down to 1/8th of the nominal clock. This can > be configured by a simple IO instruction. This way, you can decrease > consumption of the whole SoC by maybe 6.5% (= down to 93% of full > throttle), at the cost of throttling the core clock by 87%. The > standard PC104 boards don't have a Vcore regulator, so it just > doesn't get any better than that. I can do the underclocking from > userspace using IOperm, I was wondering how nice it would be to have > a "clockmod" driver, but ultimately I guess saving 125 mW is not > worth the bother, as the whole setup including some peripherials eats > over 3 Watts... Yep, hooking it to cpufreq would be nice. -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html