Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751528AbZFPIH7 (ORCPT ); Tue, 16 Jun 2009 04:07:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750915AbZFPIHo (ORCPT ); Tue, 16 Jun 2009 04:07:44 -0400 Received: from e28smtp08.in.ibm.com ([59.145.155.8]:34793 "EHLO e28smtp08.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750830AbZFPIHm (ORCPT ); Tue, 16 Jun 2009 04:07:42 -0400 Date: Tue, 16 Jun 2009 13:37:15 +0530 From: Vaidyanathan Srinivasan To: Andrew Morton Cc: Gautham R Shenoy , linux-kernel@vger.kernel.org, Peter Zijlstra , Balbir Singh , Rusty Russel , Paul E McKenney , Nathan Lynch , Ingo Molnar , Venkatesh Pallipadi , Dipankar Sarma , Shoahua Li Subject: Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support. Message-ID: <20090616080715.GB7961@dirshya.in.ibm.com> Reply-To: svaidy@linux.vnet.ibm.com References: <20090616053431.30891.18682.stgit@sofia.in.ibm.com> <20090615232318.74b099a6.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20090615232318.74b099a6.akpm@linux-foundation.org> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1798 Lines: 53 * Andrew Morton [2009-06-15 23:23:18]: > On Tue, 16 Jun 2009 11:08:39 +0530 Gautham R Shenoy wrote: > > > Currently on a ppc64 box with 16 CPUs, the time taken for > > a individual cpu-hotplug operation is as follows. > > > > # time echo 0 > /sys/devices/system/cpu/cpu2/online > > real 0m0.025s > > user 0m0.000s > > sys 0m0.002s > > > > # time echo 1 > /sys/devices/system/cpu/cpu2/online > > real 0m0.021s > > user 0m0.000s > > sys 0m0.000s > > Surprised. Do people really online and offline CPUs frequently enough > for this to be a problem? Certainly not for hardware faults or hardware replacement, but cpu-hotplug interface is useful for changing system configuration to meet different objectives like * Reduce system capacity to reduce average power and reduce heat * Increasing number of cores and threads in a CPU package is leading to multiple cpu offline/online operations for any perceivable effect * Dynamically change CPU configurations in virtualized environments Ref: [1] Saving power by cpu evacuation sched_max_capacity_pct=n http://lkml.org/lkml/2009/5/13/173 [2] Make offline cpus to go to deepest idle state using http://lkml.org/lkml/2009/5/22/431 [3] cpuset: add new API to change cpuset top group's cpus http://lkml.org/lkml/2009/5/19/54 For getting stuff off a certain CPU, cpu-hotplug framework seems to do the right thing. Identifying bottlenecks in the framework can significantly help other use cases. --Vaidy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/