From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761727AbZEFWw5 (ORCPT ); Wed, 6 May 2009 18:52:57 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758488AbZEFWvS (ORCPT ); Wed, 6 May 2009 18:51:18 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:55986 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758144AbZEFWvQ (ORCPT ); Wed, 6 May 2009 18:51:16 -0400 Date: Wed, 6 May 2009 15:51:15 -0700 From: "Paul E. McKenney" To: David Howells Cc: Andrew Morton , Ingo Molnar , linux-kernel@vger.kernel.org, niv@us.ibm.com, dvhltc@us.ibm.com, lethal@linux-sh.org, kernel@wantstofly.org, matthew@wil.cx Subject: Re: [PATCH] v4 RCU: the bloatwatch edition Message-ID: <20090506225115.GM6771@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090506112459.5edd0902.akpm@linux-foundation.org> <20090502163423.GA1633@linux.vnet.ibm.com> <12234.1241558327@redhat.com> <20090506120909.GR25203@elte.hu> <11979.1241648574@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <11979.1241648574@redhat.com> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 06, 2009 at 11:22:54PM +0100, David Howells wrote: > Andrew Morton wrote: > > > More like "concerns". It's unobvious to me that the modest .text > > savings justify the costs of an additional RCU implementation. Where > > those costs include > > It may be more than just modest .text savings. Being optimised to be as > simple as possible, and to only support one CPU, it may be quicker too. I'm > not sure how best to benchmark it though. The read side is unchanged, but the update side is another story, given that synchronize_rcu()'s latency decreases from multiple milliseconds to the sub-microsecond range: void synchronize_rcu(void) { unsigned long flags; local_irq_save(flags); rcu_ctrlblk.completed++; local_irq_restore(flags); } So boot speed is one possible metric, depending on how many synchronize_rcu() invocations are in your arch's boot path. It appears that x86 has a fair number. ;-) Thanx, Paul