Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755353AbZJ1Rrh (ORCPT ); Wed, 28 Oct 2009 13:47:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755281AbZJ1Rrg (ORCPT ); Wed, 28 Oct 2009 13:47:36 -0400 Received: from claw.goop.org ([74.207.240.146]:40593 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755267AbZJ1Rrg (ORCPT ); Wed, 28 Oct 2009 13:47:36 -0400 Message-ID: <4AE883B9.4050409@goop.org> Date: Wed, 28 Oct 2009 10:47:37 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.4pre) Gecko/20091014 Fedora/3.0-2.8.b4.fc11 Lightning/1.0pre Thunderbird/3.0b4 MIME-Version: 1.0 To: Avi Kivity CC: Glauber Costa , Dan Magenheimer , Jeremy Fitzhardinge , kurt.hackel@oracle.com, the arch/x86 maintainers , Linux Kernel Mailing List , Glauber de Oliveira Costa , Xen-devel , Keir Fraser , zach.brown@oracle.com, chris.mason@oracle.com, Ingo Molnar Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation References: <4f079cd6-0872-4cb5-832b-ee6a46841192@default> <4AE739DA.20601@goop.org> <4AE7DC04.7080706@redhat.com> <20091028092900.GG3627@mothafucka.localdomain> <4AE81011.4040904@redhat.com> In-Reply-To: <4AE81011.4040904@redhat.com> X-Enigmail-Version: 0.97a Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1130 Lines: 27 On 10/28/09 02:34, Avi Kivity wrote: > On 10/28/2009 11:29 AM, Glauber Costa wrote: >>> How can you reliably measure performance in a virtualized environment? >>> >> If we loop gettimeofday(), I would expect the vsyscall-based version >> not to show >> up in strace, right? >> > > Much better to have an API for this. Life is hacky enough already. My point is that if an app cares about property X then it should just measure property X. The fact that gettimeofday is a vsyscall is just an implementation detail that apps don't really care about. What they care about is whether gettimeofday is fast or not. If the environment has such unstable timing that the effect can't be measured, then it is moot whether its a vsyscall or not (but in that case its almost certainly better to use the standard API rather than trying to roll your own timesource with rdtsc). J -- 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/