From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756301Ab1KUUl7 (ORCPT ); Mon, 21 Nov 2011 15:41:59 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56101 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752995Ab1KUUl6 (ORCPT ); Mon, 21 Nov 2011 15:41:58 -0500 Date: Mon, 21 Nov 2011 21:41:34 +0100 From: Petr Holasek To: David Rientjes Cc: Andrew Morton , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , linux-kernel@vger.kernel.org, x86@kernel.org, Anton Arapov Subject: Re: NUMA emulation x86_64: numa=fake parameter for custom nodes distance Message-ID: <20111121204133.GA677@dhcp-27-244.brq.redhat.com> References: <1321617308-4998-1-git-send-email-pholasek@redhat.com> <20111118115336.18de45a0.akpm@linux-foundation.org> <20111119003114.GA10179@stainedmachine.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 19 Nov 2011, David Rientjes wrote: > Date: Sat, 19 Nov 2011 18:09:59 -0800 (PST) > From: David Rientjes > To: Petr Holasek > cc: Andrew Morton , Thomas Gleixner > , Ingo Molnar , "H. Peter Anvin" > , linux-kernel@vger.kernel.org, x86@kernel.org, Anton > Arapov > Subject: Re: NUMA emulation x86_64: numa=fake parameter for custom nodes > distance > > On Sat, 19 Nov 2011, Petr Holasek wrote: > > > A lot of developers still have no access to large NUMA machines and > > possibility of NUMA emulation could involve more of them to thinking > > about NUMA awareness of their apps/kernel code. > > > > That's a bogus argument, numa=fake already allows you to construct as > large of a NUMA box as you want in a faked environment. The distances > have nothing to do with that. > > The distances you're adding here are, by definition, incorrect because it > doesn't respect the actual distance between physical nodes that numa=fake > uses already. If you're using numa=fake on an UMA machine, then the > performance of the kernel will be just that, you won't actual see any > introduced latency between fake nodes just by changing the distance. So > you're completely invalidating what internode distances actually mean. > > I'd much rather see an option to fake the SLIT that could do all of this > without limitation and would be possible to debug issues in the future. This patch was designed as nothing more than helper for debugging/testing purposes, e.g. when it is useful to have more values in exports than only LOCAL_DISTANCEs. So that's the reason why it disregards former distances between physical nodes. Faking the SLIT table is a really good point, if this patch would be eventually rejected, I will rework the patch in that manner.