From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751119AbVLNIGt (ORCPT ); Wed, 14 Dec 2005 03:06:49 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751302AbVLNIGt (ORCPT ); Wed, 14 Dec 2005 03:06:49 -0500 Received: from mf01.sitadelle.com ([212.94.174.68]:24265 "EHLO smtp.cegetel.net") by vger.kernel.org with ESMTP id S1751119AbVLNIGt (ORCPT ); Wed, 14 Dec 2005 03:06:49 -0500 Message-ID: <439FD295.7070102@cosmosbay.com> Date: Wed, 14 Dec 2005 09:06:45 +0100 From: Eric Dumazet User-Agent: Mozilla Thunderbird 1.0 (Windows/20041206) X-Accept-Language: fr, en MIME-Version: 1.0 To: Paul Jackson Cc: clameter@engr.sgi.com, akpm@osdl.org, linux-kernel@vger.kernel.org, nickpiggin@yahoo.com.au, Simon.Derr@bull.net, ak@suse.de Subject: Re: [PATCH] Cpuset: rcu optimization of page alloc hook References: <20051211233130.18000.2748.sendpatchset@jackhammer.engr.sgi.com> <439D39A8.1020806@cosmosbay.com> <20051212020211.1394bc17.pj@sgi.com> <20051212021247.388385da.akpm@osdl.org> <20051213075345.c39f335d.pj@sgi.com> <439EF75D.50206@cosmosbay.com> <439F0B43.4080500@cosmosbay.com> <20051213130350.464a3054.pj@sgi.com> <439F3F6E.6010701@cosmosbay.com> <20051213142346.ccd3081a.pj@sgi.com> In-Reply-To: <20051213142346.ccd3081a.pj@sgi.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Paul Jackson a écrit : > Eric wrote: > >>struct kmem_cache itself will be about 512*8 + some bytes >>then for each cpu a 'struct array_cache' will be allocated (count 128 bytes > > > Hmmm ... 'struct array_cache' looks to be about 6 integer words, > so if that is the main per-CPU cost, the minimal cost of a slab > cache (once created, before use) is about 24 bytes per cpu. Nope, because struct array_cache includes a variable length table of pointers to hold a cache of available objects per cpu. The 'limit' (the number of pointer in this cache) depends on the object size. See enable_cpucache in mm/slab.c for 'limit' determination : if (cachep->objsize > 131072) limit = 1; else if (cachep->objsize > PAGE_SIZE) limit = 8; else if (cachep->objsize > 1024) limit = 24; else if (cachep->objsize > 256) limit = 54; else limit = 120; Let's take an example : grep dentry /proc/slabinfo dentry_cache 157113 425289 224 17 1 : tunables 120 60 8 : slabdata 25017 25017 0 'limit' is the number following 'tunable' : 120 On a 64 bits machines, 120*sizeof(void*) = 120*8 = 960 So for small objects (<= 256 bytes), you end with a sizeof(arracy_cache) = 1024 bytes per cpu If 512 CPUS : 512*1024 = 512 Kbytes + all other kmem_cache structures : (If you have a lot of Memory Nodes, then it can be very big too) If you know that no more than 100 objects are used in 99% of setups, then a dedicated cache is overkill. > > But whether its 24 or 128 bytes per cpu, that's a heavier weight > hammer than is needed here. > > Time for me to learn more about rcu. > > Thanks for raising this issue. > You are welcome. Eric