Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756130Ab3DORWF (ORCPT ); Mon, 15 Apr 2013 13:22:05 -0400 Received: from mga02.intel.com ([134.134.136.20]:57003 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751135Ab3DORWC (ORCPT ); Mon, 15 Apr 2013 13:22:02 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.87,476,1363158000"; d="scan'208";a="295208596" Message-ID: <516C371E.70402@linux.intel.com> Date: Mon, 15 Apr 2013 10:21:34 -0700 From: Arjan van de Ven User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20130328 Thunderbird/17.0.5 MIME-Version: 1.0 To: Christoph Lameter CC: paulmck@linux.vnet.ibm.com, Peter Zijlstra , Borislav Petkov , linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, Kevin Hilman Subject: Re: [PATCH documentation 1/2] nohz1: Add documentation. References: <20130411160524.GA30384@linux.vnet.ibm.com> <1365696359-30958-1-git-send-email-paulmck@linux.vnet.ibm.com> <20130411182502.GA31684@pd.tnic> <20130411191355.GO29861@linux.vnet.ibm.com> <1365753904.17140.22.camel@laptop> <20130412175419.GG29861@linux.vnet.ibm.com> <51684AD3.7090305@linux.intel.com> <0000013e0e6cacbb-b7e44c6f-d551-49b9-aa07-62a4ee280ae7-000000@email.amazonses.com> <516C2DAF.6010908@linux.intel.com> <0000013e0e9e0ca6-12c738ba-6939-4b31-8a40-186019c5d16a-000000@email.amazonses.com> In-Reply-To: <0000013e0e9e0ca6-12c738ba-6939-4b31-8a40-186019c5d16a-000000@email.amazonses.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1141 Lines: 28 On 4/15/2013 9:53 AM, Christoph Lameter wrote: > On Mon, 15 Apr 2013, Arjan van de Ven wrote: > >> to put the "cost" into perspective; programming a timer in one-shot mode >> is some math on the cpu (to go from kernel time to hardware time), >> which is a multiply and a shift (or a divide), and then actually >> programming the hardware, which is at the cost of (approximately) a cachemiss >> or two >> (so give or take in the "hundreds" of cycles) >> at least on moderately modern hardware (e.g. last few years) > > Well these are PCI transactions eh no not on anything modern they're touching the local apic which is core-local > Ok then maybe go dynticks if we can save at least one timer tick? switching between periodic versus not is actually non-trivial and much more expensive (and complex) so not something you want to do all the time. once during early boot is hard enough already -- 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/