Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757228Ab2HTPsB (ORCPT ); Mon, 20 Aug 2012 11:48:01 -0400 Received: from a194-183.smtp-out.amazonses.com ([199.255.194.183]:53771 "EHLO a194-183.smtp-out.amazonses.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753266Ab2HTPr4 (ORCPT ); Mon, 20 Aug 2012 11:47:56 -0400 Date: Mon, 20 Aug 2012 15:47:54 +0000 From: Christoph Lameter X-X-Sender: cl@gentwo.org To: Arjan van de Ven cc: Alex Shi , Matthew Garrett , Peter Zijlstra , Suresh Siddha , vincent.guittot@linaro.org, svaidy@linux.vnet.ibm.com, Ingo Molnar , Andrew Morton , Linus Torvalds , "linux-kernel@vger.kernel.org" Subject: Re: [discussion]sched: a rough proposal to enable power saving in scheduler In-Reply-To: <502CFC41.1030501@linux.intel.com> Message-ID: <0000013944b80714-c594cc1e-de63-4901-9aa7-0b5a3d8629e5-000000@email.amazonses.com> References: <5028F12C.7080405@intel.com> <20120815161911.GA14534@srcf.ucam.org> <502C7F24.6020900@intel.com> <502CFC41.1030501@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: 199.255.194.183 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1227 Lines: 22 One issue that is often forgotten is that there are users who want lowest latency and not highest performance. Our systems sit idle for most of the time but when a specific event occurs (typically a packet is received) they must react in the fastest way possible. On every new generation of hardware and software we keep on running into various mechanisms that automatically power down when idle for a long time (to save power...). And its pretty hard to figure these things out given the complexity of modern hardware. F.e. for the Sandybridges we found that the memory channel powers down after 2 milliseconds idle time and that was unaffected by any of the bios config options. Similar mechanisms exist in the kernel but those are easier discover since there is source. So please make sure that there are obvious and easy ways to switch this stuff off or provide "low latency" know that keeps the system from assuming that idle time means that full performance is not needed. -- 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/