Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933813Ab3DORIY (ORCPT ); Mon, 15 Apr 2013 13:08:24 -0400 Received: from a9-50.smtp-out.amazonses.com ([54.240.9.50]:44322 "EHLO a9-50.smtp-out.amazonses.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932081Ab3DORIX (ORCPT ); Mon, 15 Apr 2013 13:08:23 -0400 X-Greylist: delayed 861 seconds by postgrey-1.27 at vger.kernel.org; Mon, 15 Apr 2013 13:08:23 EDT Date: Mon, 15 Apr 2013 16:53:59 +0000 From: Christoph Lameter X-X-Sender: cl@gentwo.org To: Arjan van de Ven 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. In-Reply-To: <516C2DAF.6010908@linux.intel.com> Message-ID: <0000013e0e9e0ca6-12c738ba-6939-4b31-8a40-186019c5d16a-000000@email.amazonses.com> 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> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-SES-Outgoing: 2013.04.15-54.240.9.50 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1091 Lines: 24 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 which are bound to be high latency reaching may be more than microscond in total. A timer interrupt may last 2-4 microsecond at best without PCI transactions. > not cheap. But also not INSANE expensive... and it breaks-even already if you > only > save one or two cache misses elsewhere. Ok then maybe go dynticks if we can save at least one timer tick? -- 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/