Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758286AbZFPVBM (ORCPT ); Tue, 16 Jun 2009 17:01:12 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752384AbZFPVA6 (ORCPT ); Tue, 16 Jun 2009 17:00:58 -0400 Received: from e1.ny.us.ibm.com ([32.97.182.141]:33156 "EHLO e1.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751666AbZFPVA6 (ORCPT ); Tue, 16 Jun 2009 17:00:58 -0400 Date: Tue, 16 Jun 2009 14:00:59 -0700 From: "Paul E. McKenney" To: Vaidyanathan Srinivasan Cc: Andrew Morton , Gautham R Shenoy , linux-kernel@vger.kernel.org, Peter Zijlstra , Balbir Singh , Rusty Russel , Nathan Lynch , Ingo Molnar , Venkatesh Pallipadi , Dipankar Sarma , Shoahua Li Subject: Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support. Message-ID: <20090616210059.GL6842@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090616053431.30891.18682.stgit@sofia.in.ibm.com> <20090615232318.74b099a6.akpm@linux-foundation.org> <20090616080715.GB7961@dirshya.in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090616080715.GB7961@dirshya.in.ibm.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 Content-Length: 2243 Lines: 61 On Tue, Jun 16, 2009 at 01:37:15PM +0530, Vaidyanathan Srinivasan wrote: > * 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 Perhaps also reducing boot-up time? If I am correctly interpreting the above numbers, an eight-CPU system would be consuming 175 milliseconds bringing up the seven non-boot CPUs. Reducing this by 150 milliseconds might be of interest to some people. ;-) Thanx, Paul > 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/