From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751125AbVH2BT6 (ORCPT ); Sun, 28 Aug 2005 21:19:58 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751166AbVH2BT6 (ORCPT ); Sun, 28 Aug 2005 21:19:58 -0400 Received: from stat16.steeleye.com ([209.192.50.48]:12687 "EHLO hancock.sc.steeleye.com") by vger.kernel.org with ESMTP id S1751125AbVH2BT6 (ORCPT ); Sun, 28 Aug 2005 21:19:58 -0400 Subject: Re: [PATCH] make radix tree gang lookup faster by using a bitmap search From: James Bottomley To: Andrew Morton Cc: Linux Kernel In-Reply-To: <20050828175233.61cada23.akpm@osdl.org> References: <1125159996.5159.8.camel@mulgrave> <20050827105355.360bd26a.akpm@osdl.org> <1125276312.5048.22.camel@mulgrave> <20050828175233.61cada23.akpm@osdl.org> Content-Type: text/plain Date: Sun, 28 Aug 2005 20:19:49 -0500 Message-Id: <1125278389.5048.30.camel@mulgrave> Mime-Version: 1.0 X-Mailer: Evolution 2.0.4 (2.0.4-6) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 2005-08-28 at 17:52 -0700, Andrew Morton wrote: > > +#if BITS_PER_LONG == 32 > > +#define RADIX_TREE_MAP_SHIFT 5 > > +#elif BITS_PER_LONG == 64 > > ... > > struct radix_tree_node { > > - unsigned int count; > > void *slots[RADIX_TREE_MAP_SIZE]; > > - unsigned long tags[RADIX_TREE_TAGS][RADIX_TREE_TAG_LONGS]; > > + unsigned long tags[RADIX_TREE_TAGS]; > > }; > > I don't see why the above change was necessary? Why not stick with the > current more flexible sizing option? > > Note that the RADIX_TREE_MAP_FULL trick can still be used. It just has to > be put inside a `for (idx = 0; idx < RADIX_TREE_TAG_LONGS; idx++)' loop, > which will vanish if RADIX_TREE_TAG_LONGS==1. Well ... It's my opinion (and purely unsubstantiated, I suppose) that it's more efficient on 32 bit platforms to do bit operations on 32 bit quantities, which is why I changed the radix tree map shift to 5 for that case. It also makes much cleaner code than having to open code checks on variable sized bitmaps. James