From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753511AbbAGOr6 (ORCPT ); Wed, 7 Jan 2015 09:47:58 -0500 Received: from mail-wi0-f179.google.com ([209.85.212.179]:35055 "EHLO mail-wi0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753172AbbAGOrx (ORCPT ); Wed, 7 Jan 2015 09:47:53 -0500 Date: Wed, 7 Jan 2015 15:47:50 +0100 From: Michal Hocko To: Mel Gorman Cc: Vlastimil Babka , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Zhang Yanfei , Minchan Kim , David Rientjes , Rik van Riel , "Aneesh Kumar K.V" , "Kirill A. Shutemov" , Johannes Weiner , Joonsoo Kim Subject: Re: [PATCH V4 4/4] mm: microoptimize zonelist operations Message-ID: <20150107144750.GF16553@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> <20150107105749.GC16553@dhcp22.suse.cz> <20150107111706.GC2395@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150107111706.GC2395@suse.de> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 07-01-15 11:17:07, Mel Gorman wrote: > On Wed, Jan 07, 2015 at 11:57:49AM +0100, Michal Hocko wrote: > > 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. > > > > IIRC, the difference was a few percent on instruction profiles and cache > profiles when driven from a systemtap microbenchmark but I no longer have > the data and besides it would have been based on an ancient machine by > todays standards. When zeroing of pages is taken into account it's going > to be marginal so a userspace test would probably show nothing. Still, > I see little motivation to replace a single deference with multiple > dereferences and pointer arithmetic when zonelist_zone_idx() is called. OK, fair enough. I have tried to convert back to simple zone * and it turned out we wouldn't save too much code so this is really not worth time and possible complications. -- Michal Hocko SUSE Labs