Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755075Ab3EHMBI (ORCPT ); Wed, 8 May 2013 08:01:08 -0400 Received: from mail-la0-f45.google.com ([209.85.215.45]:46147 "EHLO mail-la0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752317Ab3EHMBG (ORCPT ); Wed, 8 May 2013 08:01:06 -0400 MIME-Version: 1.0 In-Reply-To: <20130508113442.GB6803@dyad.programming.kicks-ass.net> References: <1367804711-30308-1-git-send-email-alex.shi@intel.com> <1367804711-30308-4-git-send-email-alex.shi@intel.com> <5187760D.8060900@intel.com> <51886460.3020009@intel.com> <20130507095715.GE4068@e103034-lin> <5188DFEF.6010403@intel.com> <20130508113442.GB6803@dyad.programming.kicks-ass.net> From: Paul Turner Date: Wed, 8 May 2013 05:00:34 -0700 Message-ID: Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task To: Peter Zijlstra Cc: Alex Shi , Morten Rasmussen , Ingo Molnar , Thomas Gleixner , Andrew Morton , Borislav Petkov , Namhyung Kim , Mike Galbraith , Vincent Guittot , Preeti U Murthy , Viresh Kumar , LKML , Mel Gorman , Rik van Riel , Michael Wang Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2721 Lines: 60 On Wed, May 8, 2013 at 4:34 AM, Peter Zijlstra wrote: > On Tue, May 07, 2013 at 04:20:55AM -0700, Paul Turner wrote: >> Yes, 1024 was only intended as a starting point. We could also >> arbitrarily pick something larger, the key is that we pick >> _something_. >> >> If we wanted to be more exacting about it we could just give them a >> sched_slice() worth; this would have a few obvious "nice" properties >> (pun intended). > > Oh I see I misunderstood again :/ Its not about the effective load but weight > of the initial effective load wrt adjustment. > > Previous schedulers didn't have this aspect at all, so no experience from me > here. Paul would be the one, since he's ran longest with this stuff. > > That said, I would tend to keep it shorter rather than longer so that it would > adjust quicker to whatever it really wanted to be. > > Morten says the load is unstable specifically on loaded systems. Here, Morten was (I believe) referring to the stability at task startup. To be clear: Because we have such a small runnable period denominator at this point a single changed observation (for an equivalently behaving thread) could have a very large effect. e.g. fork/exec -- happen to take a major #pf, observe a "relatively" long initial block. By associating an initial period (along with our full load_contrib) here, we're making the denominator larger so that these effects are less pronounced; achieving better convergence towards what our load contribution should actually be. Also: We do this conservatively, by converging down, not up. > I would think > this is because we'd experience scheduling latency, we're runnable more pushing > things up. But if we're really an idle task at heart we'd not run again for a > long while, pushing things down again. Exactly, this is why we must be careful to use instaneous weights about wake-up decisions. Interactive and background tasks are largely idle. While this is exactly how we want them to be perceived from a load-balance perspective it's important to keep in mind that while wake-up placement has a very important role in the overall balance of a system, it is not playing quite the same game as the load-balancer. > > So on that point Paul's suggestion of maybe starting with __sched_slice() might > make sense because it increases the weight of the initial avg with nr_running. > Not sure really, we'll have to play and see what works best for a number of > workloads. -- 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/