From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx159.postini.com [74.125.245.159]) by kanga.kvack.org (Postfix) with SMTP id E20136B0036 for ; Fri, 28 Jun 2013 11:13:03 -0400 (EDT) Date: Fri, 28 Jun 2013 17:12:56 +0200 From: Peter Zijlstra Subject: Re: [PATCH 7/8] sched: Split accounting of NUMA hinting faults that pass two-stage filter Message-ID: <20130628151256.GC6626@twins.programming.kicks-ass.net> References: <1372257487-9749-1-git-send-email-mgorman@suse.de> <1372257487-9749-8-git-send-email-mgorman@suse.de> <20130628070027.GD17195@linux.vnet.ibm.com> <20130628093625.GF29209@dyad.programming.kicks-ass.net> <20130628101245.GD8362@linux.vnet.ibm.com> <20130628103304.GF28407@twins.programming.kicks-ass.net> <20130628142925.GB1875@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130628142925.GB1875@suse.de> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML On Fri, Jun 28, 2013 at 03:29:25PM +0100, Mel Gorman wrote: > > Oh duh indeed. I totally missed it did that. Changelog also isn't giving > > rationale for this. Mel? > > > > There were a few reasons > > First, if there are many tasks sharing the page then they'll all move towards > the same node. The node will be compute overloaded and then scheduled away > later only to bounce back again. Alternatively the shared tasks would > just bounce around nodes because the fault information is effectively > noise. Either way I felt that accounting for shared faults with private > faults would be slower overall. > > The second reason was based on a hypothetical workload that had a small > number of very important, heavily accessed private pages but a large shared > array. The shared array would dominate the number of faults and be selected > as a preferred node even though it's the wrong decision. > > The third reason was because multiple threads in a process will race > each other to fault the shared page making the information unreliable. > > It is important that *something* be done with shared faults but I haven't > thought of what exactly yet. One possibility would be to give them a > different weight, maybe based on the number of active NUMA nodes, but I had > not tested anything yet. Peter suggested privately that if shared faults > dominate the workload that the shared pages would be migrated based on an > interleave policy which has some potential. > It would be good to put something like this in the Changelog, or even as a comment near how we select the preferred node. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org