From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: 8bf00a529967dafbbb210b377c38a15834d1e979 - performance regression? Date: Mon, 4 Nov 2013 21:30:42 +0200 Message-ID: <20131104193042.GA28102@redhat.com> References: <20131031002146.GA28569@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: gleb@redhat.com To: kvm@vger.kernel.org Return-path: Received: from mx1.redhat.com ([209.132.183.28]:20116 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750719Ab3KDT1t (ORCPT ); Mon, 4 Nov 2013 14:27:49 -0500 Received: from int-mx01.intmail.prod.int.phx2.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id rA4JRmhA014537 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Mon, 4 Nov 2013 14:27:49 -0500 Content-Disposition: inline In-Reply-To: <20131031002146.GA28569@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Oct 31, 2013 at 02:21:46AM +0200, Michael S. Tsirkin wrote: > commit 8bf00a529967dafbbb210b377c38a15834d1e979: > " KVM: VMX: add support for switching of PERF_GLOBAL_CTRL " was > as far as I can tell supposed to bring about performance improvement > on hardware that supports it? > Instead it seems to make the typical case (not running guest > under perf) a bit slower than it used to be. > the cost of VMexit goes up by about 50 cycles > on sandy bridge where the optimization in question > actually is activated. > > Why that's not a large regression, it's a far cry from > actually helping performance. > > So are we better off not using this feature? > What kind of workload is improved by this change? Ping. Let's revert 8bf00a529967dafbbb210b377c38a15834d1e979? I see a small performance gain from reverting it ... -- MST