Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757897AbYCYRtX (ORCPT ); Tue, 25 Mar 2008 13:49:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756614AbYCYRtP (ORCPT ); Tue, 25 Mar 2008 13:49:15 -0400 Received: from wx-out-0506.google.com ([66.249.82.238]:20106 "EHLO wx-out-0506.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756601AbYCYRtN (ORCPT ); Tue, 25 Mar 2008 13:49:13 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=Gl3Xy+wpGRGRH20yNpFKyS1FkMbDlIuT/3aY6noH0nG0wiAa5+Ny0o8rAKFytwLLT6XXMYpHdm5jB5+tcaYBJziliOtFzInmN2qx5XnHh2yhsz/ELbs/Y9PMOW/LaBTPhNvRquYd6Scm+gzUxQsQGRq031v6AfAqvsRn/9JEMZc= Message-ID: <5699f8f00803251049s1823098u86b5431eadad2917@mail.gmail.com> Date: Tue, 25 Mar 2008 18:49:01 +0100 From: "Wander Winkelhorst" To: "Michael Meyer" Subject: Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online" Cc: "Andi Kleen" , linux-kernel@vger.kernel.org In-Reply-To: <57127.43039.qm@web25812.mail.ukl.yahoo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <87abkmhgg0.fsf@basil.nowhere.org> <57127.43039.qm@web25812.mail.ukl.yahoo.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1937 Lines: 69 On Tue, Mar 25, 2008 at 6:23 PM, Michael Meyer wrote: > > --- Andi Kleen schrieb: > > > > Luciano Rocha writes: > > > > > On Tue, Mar 25, 2008 at 02:47:50PM +0100, Michael > > Meyer wrote: > > > > Hi, > > > > > > > > what is the difference between booting a dual > > core > > > > machine with "maxcpus=1" or by deactivating the > > second > > > > core at run time with "echo 0 > > > > > /sys/devices/system/cpu/cpu1/online"? > > > > > > maxcpus=1 should turn off the SMP alternative and > > switch to UP only, > > > optimising some locks and instructions. > > > > CPU hot unplug will do the same. But it is unlikely > > it accounts > > for that much performance difference. > > > > If he used maxcpus=0 it would make sense. maxcpus=0 > > disables > > the IO-APIC which likely makes a large difference. > > But it should > > be actually slower. > > > > There should be actually no difference in theory > > between max_cpus=1 > > and hot unplug to one CPU. Might be some bug. > > I had the following time values: > > maxcpus=1: > real 0m1.642s > user 0m1.528s > sys 0m0.068s > > maxcpus=2 and > echo 1 > /sys/devices/system/cpu/cpu1/online: > real 0m2.579s > user 0m4.096s > sys 0m0.160s > > maxcpus=2 and > echo 0 > /sys/devices/system/cpu/cpu1/online: > real 0m3.757s > user 0m3.632s > sys 0m0.112s > > What kind of CPU are you using? Some Intel CPU's do "funny stuff", like dynamically overclocking itself when working on a single thread, or using all of the 2nd level cache instead of sharing it with the second core. Regards, Wander Winkelhorst. -- 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/