From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-we0-f179.google.com (mail-we0-f179.google.com [74.125.82.179]) by kanga.kvack.org (Postfix) with ESMTP id 9223A6B0071 for ; Wed, 7 Jan 2015 05:57:52 -0500 (EST) Received: by mail-we0-f179.google.com with SMTP id q59so913104wes.38 for ; Wed, 07 Jan 2015 02:57:52 -0800 (PST) Received: from mail-we0-x22c.google.com (mail-we0-x22c.google.com. [2a00:1450:400c:c03::22c]) by mx.google.com with ESMTPS id ee5si4030487wic.103.2015.01.07.02.57.51 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 07 Jan 2015 02:57:51 -0800 (PST) Received: by mail-we0-f172.google.com with SMTP id k11so927449wes.31 for ; Wed, 07 Jan 2015 02:57:51 -0800 (PST) Date: Wed, 7 Jan 2015 11:57:49 +0100 From: Michal Hocko Subject: Re: [PATCH V4 4/4] mm: microoptimize zonelist operations Message-ID: <20150107105749.GC16553@dhcp22.suse.cz> References: <1420478263-25207-1-git-send-email-vbabka@suse.cz> <1420478263-25207-5-git-send-email-vbabka@suse.cz> <20150106150920.GE20860@dhcp22.suse.cz> <54ACF93B.3060801@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <54ACF93B.3060801@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mel Gorman , Zhang Yanfei , Minchan Kim , David Rientjes , Rik van Riel , "Aneesh Kumar K.V" , "Kirill A. Shutemov" , Johannes Weiner , Joonsoo Kim On Wed 07-01-15 10:15:39, Vlastimil Babka wrote: > On 01/06/2015 04:09 PM, Michal Hocko wrote: > > On Mon 05-01-15 18:17:43, Vlastimil Babka wrote: > >> The function next_zones_zonelist() returns zoneref pointer, as well as zone > >> pointer via extra parameter. Since the latter can be trivially obtained by > >> dereferencing the former, the overhead of the extra parameter is unjustified. > >> > >> This patch thus removes the zone parameter from next_zones_zonelist(). Both > >> callers happen to be in the same header file, so it's simple to add the > >> zoneref dereference inline. We save some bytes of code size. > > > > Dunno. It makes first_zones_zonelist and next_zones_zonelist look > > different which might be a bit confusing. It's not a big deal but > > I am not sure it is worth it. > > Yeah I thought that nobody uses them directly anyway thanks to > for_each_zone_zonelist* so it's not a big deal. OK, I have checked why we need the whole struct zoneref when it only caches zone_idx. dd1a239f6f2d (mm: have zonelist contains structs with both a zone pointer and zone_idx) claims this will reduce cache contention by reducing pointer chasing because we do not have to dereference pgdat so often in hot paths. Fair enough but I do not see any numbers in the changelog nor in the original discussion (https://lkml.org/lkml/2007/11/20/547 resp. https://lkml.org/lkml/2007/9/28/170). Maybe Mel remembers what was the benchmark which has shown the difference so that we can check whether this is still relevant and caching the index is still worth it. -- Michal Hocko SUSE Labs -- 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