Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933168Ab3GVS5y (ORCPT ); Mon, 22 Jul 2013 14:57:54 -0400 Received: from g1t0026.austin.hp.com ([15.216.28.33]:15694 "EHLO g1t0026.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932957Ab3GVS5x (ORCPT ); Mon, 22 Jul 2013 14:57:53 -0400 Message-ID: <1374519467.7608.87.camel@j-VirtualBox> Subject: Re: [RFC PATCH v2] sched: Limit idle_balance() From: Jason Low To: Srikar Dronamraju Cc: Ingo Molnar , Peter Zijlstra , LKML , Mike Galbraith , Thomas Gleixner , Paul Turner , Alex Shi , Preeti U Murthy , Vincent Guittot , Morten Rasmussen , Namhyung Kim , Andrew Morton , Kees Cook , Mel Gorman , Rik van Riel , aswin@hp.com, scott.norton@hp.com, chegu_vinod@hp.com Date: Mon, 22 Jul 2013 11:57:47 -0700 In-Reply-To: <20130722070144.GC5138@linux.vnet.ibm.com> References: <1374220211.5447.9.camel@j-VirtualBox> <20130722070144.GC5138@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2424 Lines: 59 On Mon, 2013-07-22 at 12:31 +0530, Srikar Dronamraju wrote: > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index e8b3350..da2cb3e 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -1348,6 +1348,8 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags) > > else > > update_avg(&rq->avg_idle, delta); > > rq->idle_stamp = 0; > > + > > + rq->idle_duration = (rq->idle_duration + delta) / 2; > > Cant we just use avg_idle instead of introducing idle_duration? A potential issue I have found with avg_idle is that it may sometimes be not quite as accurate for the purposes of this patch, because it is always given a max value (default is 1000000 ns). For example, a CPU could have remained idle for 1 second and avg_idle would be set to 1 millisecond. Another question I have is whether we can update avg_idle at all times without putting a maximum value on avg_idle, or increase the maximum value of avg_idle by a lot. > Should we take the consideration of whether a idle_balance was > successful or not? I recently ran fserver on the 8 socket machine with HT-enabled and found that load balance was succeeding at a higher than average rate, but idle balance was still lowering performance of that workload by a lot. However, it makes sense to allow idle balance to run longer/more often when it has a higher success rate. > I am not sure whats a reasonable value for n can be, but may be we could > try with n=3. Based on some of the data I collected, n = 10 to 20 provides much better performance increases. > Also have we checked the performance after adjusting the > sched_migration_cost tunable? > > I guess, if we increase the sched_migration_cost, we should have lesser > newly idle balance requests. Yes, I have done quite a bit of testing with sched_migration_cost and adjusting it does help performance when idle balance overhead is high. But I have found that a higher value may decrease the performance during situations where the cost of idle_balance is not high. Additionally, when to modify this tunable and by how much to modify it by can sometimes be unpredictable. Thanks, Jason -- 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/