From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gordan Bobic Subject: Re: Virtualization Performance: Intel vs. AMD Date: Sun, 15 Nov 2009 22:29:22 +0000 Message-ID: <4B0080C2.1010309@bobich.net> References: <200911151656.56318.thomas@scripty.at> <7AEAE1FABF3C4D1DA1F380583F68EAFE@neilhp> <200911151054.57510.tfjellstrom@shaw.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit To: kvm@vger.kernel.org Return-path: Received: from 78-86-195-86.zone2.bethere.co.uk ([78.86.195.86]:49292 "EHLO sentinel1.shatteredsilicon.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751305AbZKOW3S (ORCPT ); Sun, 15 Nov 2009 17:29:18 -0500 Received: from ariia.shatteredsilicon.net (ariia.shatteredsilicon.net [10.2.3.1]) by sentinel1.shatteredsilicon.net (Postfix) with ESMTP id 1A4064C280 for ; Sun, 15 Nov 2009 22:29:23 +0000 (GMT) In-Reply-To: <200911151054.57510.tfjellstrom@shaw.ca> Sender: kvm-owner@vger.kernel.org List-ID: Thomas Fjellstrom wrote: > On Sun November 15 2009, Neil Aggarwal wrote: >>> The Core i7 has hyperthreading, so you see 8 logical CPUs. >> Are you saying the AMD processors do not have hyperthreading? > > Course not. Hyperthreading is dubious at best. That's a rather questionable answer to a rather broad issue. SMT is useful, especially on processors with deep pipelines (think Pentium 4 - and in general, deeper pipelines tend to be required for higher clock speeds), because it reduces the number of context switches. Context switches are certainly one of the most expensive operations if not the most expensive operation you can do on a processor, and typically requires flushing the pipelines. Double the number of hardware threads, and you halve the number of context switches. This typically isn't useful if your CPU is processing one single-threaded application 99% of the time, but on a loaded server it can make a significant difference to throughput. Gordan