From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751336Ab2D0EgW (ORCPT ); Fri, 27 Apr 2012 00:36:22 -0400 Received: from mailout-de.gmx.net ([213.165.64.23]:40233 "HELO mailout-de.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1750858Ab2D0EgU (ORCPT ); Fri, 27 Apr 2012 00:36:20 -0400 X-Authenticated: #14349625 X-Provags-ID: V01U2FsdGVkX19gzsFBidhg2p/Meg9NiHxpvcfgyl+dajj2VDNa+a ZI8lb9UGw/BusW Message-ID: <1335501371.10374.30.camel@marge.simpson.net> Subject: Re: [PATCH RFC tip/core/rcu 6/6] rcu: Reduce cache-miss initialization latencies for large systems From: Mike Galbraith To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org Date: Fri, 27 Apr 2012 06:36:11 +0200 In-Reply-To: <1335199347-13926-6-git-send-email-paulmck@linux.vnet.ibm.com> References: <20120423164159.GA13819@linux.vnet.ibm.com> <1335199347-13926-1-git-send-email-paulmck@linux.vnet.ibm.com> <1335199347-13926-6-git-send-email-paulmck@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 X-Y-GMX-Trusted: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2012-04-23 at 09:42 -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > Commit #0209f649 (rcu: limit rcu_node leaf-level fanout) set an upper > limit of 16 on the leaf-level fanout for the rcu_node tree. This was > needed to reduce lock contention that was induced by the synchronization > of scheduling-clock interrupts, which was in turn needed to improve > energy efficiency for moderate-sized lightly loaded servers. > > However, reducing the leaf-level fanout means that there are more > leaf-level rcu_node structures in the tree, which in turn means that > RCU's grace-period initialization incurs more cache misses. This is > not a problem on moderate-sized servers with only a few tens of CPUs, With a distro config (4096 CPUs) interrupt latency is bad even on a quad. Traversing empty nodes taking locks and cache misses hurts. -Mike