From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758570Ab3APHbv (ORCPT ); Wed, 16 Jan 2013 02:31:51 -0500 Received: from mga03.intel.com ([143.182.124.21]:32978 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752676Ab3APHbu (ORCPT ); Wed, 16 Jan 2013 02:31:50 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.84,476,1355126400"; d="scan'208";a="191670034" Message-ID: <50F657A1.1080102@intel.com> Date: Wed, 16 Jan 2013 15:32:49 +0800 From: Alex Shi User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120912 Thunderbird/15.0.1 MIME-Version: 1.0 To: Morten Rasmussen CC: "mingo@redhat.com" , "peterz@infradead.org" , "tglx@linutronix.de" , "akpm@linux-foundation.org" , "arjan@linux.intel.com" , "bp@alien8.de" , "pjt@google.com" , "namhyung@kernel.org" , "efault@gmx.de" , "vincent.guittot@linaro.org" , "gregkh@linuxfoundation.org" , "preeti@linux.vnet.ibm.com" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v3 17/22] sched: packing small tasks in wake/exec balancing References: <1357375071-11793-1-git-send-email-alex.shi@intel.com> <1357375071-11793-18-git-send-email-alex.shi@intel.com> <20130110171728.GG2046@e103034-lin> <50EF8B37.7050404@intel.com> <20130114170048.GB8528@e103034-lin> In-Reply-To: <20130114170048.GB8528@e103034-lin> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/15/2013 01:00 AM, Morten Rasmussen wrote: >>> Why multiply rq->util by nr_running? >>> > > >>> > > Let's take an example where rq->util = 50, nr_running = 2, and putil = >>> > > 10. In this case the value of putil doesn't really matter as vacancy >>> > > would be negative anyway since FULL_UTIL - rq->util * nr_running is -1. >>> > > However, with rq->util = 50 there should be plenty of spare cpu time to >>> > > take another task. >> > >> > for this example, the util is not full maybe due to it was just wake up, >> > it still is possible like to run full time. So, I try to give it the >> > large guess load. > I don't see why rq->util should be treated different depending on the > number of tasks causing the load. rq->util = 50 means that the cpu is > busy about 50% of the time no matter how many tasks contibute to that > load. > > If nr_running = 1 instead in my example, you would consider the cpu > vacant if putil = 6, but if nr_running > 1 you would not. Why should the > two scenarios be treated differently? > >>> > > >>> > > Also, why multiply putil by 8? rq->util must be very close to 0 for >>> > > vacancy to be positive if putil is close to 12 (12.5%). >> > >> > just want to pack small util tasks, since packing is possible to hurt >> > performance. > I agree that packing may affect performance. But why don't you reduce > FULL_UTIL instead of multiplying by 8? With current expression you will > not pack a 10% task if rq->util = 20 and nr_running = 1, but you would > pack a 6% task even if rq->util = 50 and the resulting cpu load is much > higher. > Yes, the threshold has no strong theory or experiment support. I had tried cyclitest which Vicent used, the case's load avg is too small to be caught. so just use half of Vicent value as 12.5%. If you has more reasonable value, let me know. As to nr_running engaged as multiple mode. it's base on 2 reasons. 1, load avg/util need 345ms to accumulate as 100%. so, if a tasks is cost full cpu time, it still has 345ms with rq->util < 1. 2, if there are more tasks, like 2 tasks running on one cpu, it's possible to has capacity to burn 200% cpu time, while the biggest rq->util is still 100%. Consider to figure out precise utils is complicate and cost much. I do this simple calculation. It is not very precise, but it is efficient and more bias toward performance. -- Thanks Alex