From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754080AbcHXIG7 (ORCPT ); Wed, 24 Aug 2016 04:06:59 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:34629 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751268AbcHXIG3 (ORCPT ); Wed, 24 Aug 2016 04:06:29 -0400 Date: Wed, 24 Aug 2016 10:05:36 +0200 From: Michal Hocko To: Joonsoo Kim Cc: Aruna Ramakrishna , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mike Kravetz , Christoph Lameter , Pekka Enberg , David Rientjes , Andrew Morton , Mel Gorman , Jiri Slaby Subject: Re: what is the purpose of SLAB and SLUB (was: Re: [PATCH v3] mm/slab: Improve performance of gathering slabinfo) stats Message-ID: <20160824080536.GD31179@dhcp22.suse.cz> References: <1471458050-29622-1-git-send-email-aruna.ramakrishna@oracle.com> <20160818115218.GJ30162@dhcp22.suse.cz> <20160823021303.GB17039@js1304-P5Q-DELUXE> <20160823153807.GN23577@dhcp22.suse.cz> <20160824011501.GA21997@js1304-P5Q-DELUXE> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160824011501.GA21997@js1304-P5Q-DELUXE> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 24-08-16 10:15:02, Joonsoo Kim wrote: > On Tue, Aug 23, 2016 at 05:38:08PM +0200, Michal Hocko wrote: > > On Tue 23-08-16 11:13:03, Joonsoo Kim wrote: > > > On Thu, Aug 18, 2016 at 01:52:19PM +0200, Michal Hocko wrote: > > [...] > > > > I am not opposing the patch (to be honest it is quite neat) but this > > > > is buggering me for quite some time. Sorry for hijacking this email > > > > thread but I couldn't resist. Why are we trying to optimize SLAB and > > > > slowly converge it to SLUB feature-wise. I always thought that SLAB > > > > should remain stable and time challenged solution which works reasonably > > > > well for many/most workloads, while SLUB is an optimized implementation > > > > which experiment with slightly different concepts that might boost the > > > > performance considerably but might also surprise from time to time. If > > > > this is not the case then why do we have both of them in the kernel. It > > > > is a lot of code and some features need tweaking both while only one > > > > gets testing coverage. So this is mainly a question for maintainers. Why > > > > do we maintain both and what is the purpose of them. > > > > > > I don't know full history about it since I joined kernel communitiy > > > recently(?). Christoph would be a better candidate for this topic. > > > Anyway, > > > > > > SLAB if SLUB beats SLAB completely. But, there are fundamental > > > differences in implementation detail so they cannot beat each other > > > for all the workloads. It is similar with filesystem case that various > > > filesystems exist for it's own workload. > > > > Do we have any documentation/study about which particular workloads > > benefit from which allocator? It seems that most users will use whatever > > the default or what their distribution uses. E.g. SLES kernel use SLAB > > because this is what we used to have for ages and there was no strong > > reason to change that default. From such a perspective having a stable > > allocator with minimum changes - just bug fixes - makes a lot of sense. > > It doesn't make sense to me. Even if someone uses SLAB due to > conventional reason, they would want to use shiny new feature and get > performance improvement. > > And, it is not only reason to use SLAB. There would be many different > reasons to use SLAB. Could you be more specific please? Are there any inherent problems that would make one allocator unsuitable for specific workloads? > > I remember Mel doing some benchmarks when "why opensuse kernels do not > > use the default SLUB allocator" came the last time and he didn't see any > > large winner there > > https://lists.opensuse.org/opensuse-kernel/2015-08/msg00098.html > > This set of workloads is of course not comprehensive to rule one or > > other but I am wondering whether there are still any pathological > > workloads where we really want to keep SLAB or add new features to it. > > AFAIK, some network benchmark still shows regression in SLUB. > > http://lkml.kernel.org/r/20150907113026.5bb28ca3@redhat.com That suggests that this is not an inherent problem of SLUB though. -- Michal Hocko SUSE Labs