Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753085Ab0AZK5A (ORCPT ); Tue, 26 Jan 2010 05:57:00 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752366Ab0AZK47 (ORCPT ); Tue, 26 Jan 2010 05:56:59 -0500 Received: from ey-out-2122.google.com ([74.125.78.26]:28845 "EHLO ey-out-2122.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751845Ab0AZK46 (ORCPT ); Tue, 26 Jan 2010 05:56:58 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:date:message-id:subject:from:to:content-type; b=CtOkaOzMzHF9WK0TQpG4PW3zTFopCtOAaOugVE8EjpGSCAPHpRG3YMyNBVHs8ewdXb j1BrnW+Lw2yYkhZZ3a+ebxVuBf+Rm9GkX32EIRsGYw581Vi7cFFuqLKjcADg4jpFGfx6 aD12xFpnnquBrmfBL7YBfu2FR1uPI/PsE6h48= MIME-Version: 1.0 Date: Tue, 26 Jan 2010 10:56:57 +0000 Message-ID: <6278d2221001260256q5f35457fye8baabcc333d40@mail.gmail.com> Subject: Re: Hyperthreading on Core i7s: To use or not to use? From: Daniel J Blueman To: Linux Kernel , Justin Piszcz Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2456 Lines: 60 On Jan 26, 10:10 am, Justin Piszcz wrote: > Hello, > > Should the 'correct' kernel [CPU] configuration for a core i7 860/870..? > > - Multi-core support > - Cores: 8 > - SMT: Enabled/ON > > From CONFIG_SCHED_SMT: > > . SMT scheduler support improves the CPU scheduler's decision making . > . when dealing with Intel Pentium 4 chips with HyperThreading at a . > . cost of slightly increased overhead in some places. If unsure say . > . N here. . > > Does this also 'help' and/or 'apply' as much when dealing with Core i7s? > > -- > > Quick little benchmark (pbzip2 -9 linux kernel source), the benchmark is > really within the noise (8 on/off) > - Multicore(8)/HT(Off) = 73.72user 0.33system 0:09.50elapsed 779%CPU (0avgtext+0avgdata 458528maxresident > - Multicore(8)/HT(On) = 74.28user 0.40system 0:09.67elapsed 772%CPU (0avgtext+0avgdata 428304maxresident > - Multicore(4)/HT(On) = 68.76user 0.30system 0:17.44elapsed 396%CPU (0avgtext+0avgdata 213616maxresident)k > > -- > > Has anyone done any in-depth benchmarking for the core i7s that have multiple > cores and HT disabled/enabled? With my Dell Studio 15 (model 1557) laptop, there is no option to disable HT in the current BIOS, so booting with maxcpus=4 (since the kernel enumerates non-sibling cores first) gave me a 5-15% speedup on some large image processing (convolution, FFTs, conversion) on all available cores, presumably due to better cache efficiency. Booting with maxcpus=4 prevents any of the cores sitting in C6, needed for turbo-boost and a lower thermal profile, though I did find scheduling latency and responsiveness better under load booting with maxcpus=4, so favour this when plugged in. Clearly, having the BIOS option allows benefit to certain applications - Dell should give their users the choice! Perhaps the 'noht' boot option should be reintroduced to initialise all cores, but only expose non-sibling cores to the OS (thus allowing C6)? Daniel tip: modprobe msr and use turbostat to monitor turbo-boost and C-state residency: http://bugzilla.kernel.org/attachment.cgi?id=24673 -- Daniel J Blueman -- 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/