From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753660AbdKISOA (ORCPT ); Thu, 9 Nov 2017 13:14:00 -0500 Received: from mga06.intel.com ([134.134.136.31]:2086 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752537AbdKISN7 (ORCPT ); Thu, 9 Nov 2017 13:13:59 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.44,370,1505804400"; d="scan'208";a="148003041" Subject: Re: [RFC][PATCH] x86, sched: allow topolgies where NUMA nodes share an LLC To: Borislav Petkov References: <20171106221500.310295D7@viggo.jf.intel.com> <20171107083019.GG3326@worktop> <20171108093108.GJ3326@worktop> <8d4b2a2c-1044-c657-d73a-5afb96cc47d7@linux.intel.com> <20171109140717.qmdrnbdykjsejkti@pd.tnic> Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, tony.luck@intel.com, tim.c.chen@linux.intel.com, hpa@linux.intel.com, rientjes@google.com, imammedo@redhat.com, prarit@redhat.com, toshi.kani@hp.com, brice.goglin@gmail.com, mingo@kernel.org From: Dave Hansen Message-ID: <0b42d417-9c68-3e48-7736-db33310ba332@linux.intel.com> Date: Thu, 9 Nov 2017 10:13:53 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <20171109140717.qmdrnbdykjsejkti@pd.tnic> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/09/2017 06:07 AM, Borislav Petkov wrote: > On Wed, Nov 08, 2017 at 04:00:38PM -0800, Dave Hansen wrote: >> I'd argue that those two end up looking pretty much the same to an app. >> The only difference is that the slice-local and slice-remote cache hits >> have slightly different access latencies. I don't think it's enough to >> notice. > > So if it is not enough to notice, why do we even bother? I.e., is > there any workload showing any advantages at all from the resources > partitioning? If you want the *absolutely* best latency available, you turn on SNC. You get a small boost to slice-local access and a slight penalty to remote-slice access compared to when Sub-NUMA-Clustering is off. You can measure this for sure, but I'll still say that most folks will never notice. In addition, if you have access interleaved everywhere, the "slice-local boost" and "remote-slice penalty" roughly cancel each-other out.