From mboxrd@z Thu Jan 1 00:00:00 1970 From: George Dunlap Subject: Re: Virt overehead with HT [was: Re: Xen 4.5 development update] Date: Mon, 14 Jul 2014 17:55:45 +0100 Message-ID: <53C40B91.7080006@eu.citrix.com> References: <20140701164347.61662A7843@laptop.dumpdata.com> <1405354372.29306.687.camel@Solace> <53C4062A.3040403@bobich.net> <1405356283.7341.5.camel@Abyss> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1X6jXv-00007Q-Q7 for xen-devel@lists.xenproject.org; Mon, 14 Jul 2014 16:56:11 +0000 In-Reply-To: <1405356283.7341.5.camel@Abyss> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Dario Faggioli , Gordan Bobic Cc: Lars Kurth , George Dunlap , Ross Lagerwall , "stefano.stabellini@citrix.com" , "xen-devel@lists.xenproject.org" List-Id: xen-devel@lists.xenproject.org On 07/14/2014 05:44 PM, Dario Faggioli wrote: > On Mon, 2014-07-14 at 17:32 +0100, Gordan Bobic wrote: >> On 07/14/2014 05:12 PM, Dario Faggioli wrote: >>> Elapsed(stddev) BAREMETAL HVM >>> kernbench -j4 31.604 (0.0963328) 34.078 (0.168582) >>> kernbench -j8 26.586 (0.145705) 26.672 (0.0432435) >>> kernbench -j 27.358 (0.440307) 27.49 (0.364897) >>> >>> With HT disabled in BIOS (which means only 4 CPUs for both): >>> Elapsed(stddev) BAREMETAL HVM >>> kernbench -j4 57.754 (0.0642651) 56.46 (0.0578792) >>> kernbench -j8 31.228 (0.0775887) 31.362 (0.210998) >>> kernbench -j 32.316 (0.0270185) 33.084 (0.600442) > BTW, there's a mistake here. The three runs, in the no-HT case are as > follows: > kernbench -j2 > kernbench -j4 > kernbench -j > > I.e., half the number of VCPUs, as much as there are VCPUs and > unlimited, exactly as for the HT case. Ah -- that's a pretty critical piece of information. So actually, on native, HT enabled and disabled effectively produce the same exact thing if HT is not actually being used: 31 seconds in both cases. But on Xen, enabling HT when it's not being used (i.e., when in theory each core should have exactly one process running), performance goes from 31 seconds to 34 seconds -- roughly a 10% degradation. -George