From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752553Ab3BUHAp (ORCPT ); Thu, 21 Feb 2013 02:00:45 -0500 Received: from e28smtp03.in.ibm.com ([122.248.162.3]:55485 "EHLO e28smtp03.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751914Ab3BUHAo (ORCPT ); Thu, 21 Feb 2013 02:00:44 -0500 Message-ID: <5125C607.8090909@linux.vnet.ibm.com> Date: Thu, 21 Feb 2013 15:00:23 +0800 From: Michael Wang User-Agent: Mozilla/5.0 (X11; Linux i686; rv:16.0) Gecko/20121011 Thunderbird/16.0.1 MIME-Version: 1.0 To: Mike Galbraith CC: Ingo Molnar , LKML , Peter Zijlstra , Paul Turner , Andrew Morton , alex.shi@intel.com, Ram Pai , "Nikunj A. Dadhania" , Namhyung Kim Subject: Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair() References: <51079178.3070002@linux.vnet.ibm.com> <20130220104958.GA9152@gmail.com> <5125A7C8.8020308@linux.vnet.ibm.com> <1361427108.5861.41.camel@marge.simpson.net> In-Reply-To: <1361427108.5861.41.camel@marge.simpson.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13022106-3864-0000-0000-000006EC2AB1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/21/2013 02:11 PM, Mike Galbraith wrote: > On Thu, 2013-02-21 at 12:51 +0800, Michael Wang wrote: >> On 02/20/2013 06:49 PM, Ingo Molnar wrote: >> [snip] [snip] >> >> if wake_affine() >> new_cpu = select_idle_sibling(curr_cpu) >> else >> new_cpu = select_idle_sibling(prev_cpu) >> >> return new_cpu >> >> Actually that doesn't make sense. >> >> I think wake_affine() is trying to check whether move a task from >> prev_cpu to curr_cpu will break the balance in affine_sd or not, but why >> won't break balance means curr_cpu is better than prev_cpu for searching >> the idle cpu? > > You could argue that it's impossible to break balance by moving any task > to any idle cpu, but that would mean bouncing tasks cross node on every > wakeup is fine, which it isn't. I don't get it... could you please give me more detail on how wake_affine() related with bouncing? > >> So the new logical in this patch set is: >> >> new_cpu = select_idle_sibling(prev_cpu) >> if idle_cpu(new_cpu) >> return new_cpu > > So you tilted the scales in favor of leaving tasks in their current > package, which should benefit large footprint tasks, but should also > penalize light communicating tasks. Yes, I'd prefer to wakeup the task on a cpu which: 1. idle 2. close to prev_cpu So if both curr_cpu and prev_cpu have idle cpu in their topology, which one is better? that depends on how task benefit from cache and the balance situation, whatever, I don't think the benefit worth the high cost of wake_affine() in most cases... Regards, Michael Wang > > I suspect that much of the pgbench improvement comes from the preemption > mitigation from keeping 1:N load maximally spread, which is the perfect > thing to do with such loads. In all the testing I ever did with it in > 1:N mode, preemption dominated performance numbers. Keep server away > from clients, it has fewer fair competition worries, can consume more > CPU preemption free, pushing the load collapse point strongly upward. > > -Mike > > -- > 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/ >