Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752038AbZJ2MOT (ORCPT ); Thu, 29 Oct 2009 08:14:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751670AbZJ2MOS (ORCPT ); Thu, 29 Oct 2009 08:14:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46959 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751566AbZJ2MOS (ORCPT ); Thu, 29 Oct 2009 08:14:18 -0400 Message-ID: <4AE986FE.3040104@redhat.com> Date: Thu, 29 Oct 2009 14:13:50 +0200 From: Avi Kivity 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 Thunderbird/3.0b4 MIME-Version: 1.0 To: Jeremy Fitzhardinge 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> <4AE883B9.4050409@goop.org> In-Reply-To: <4AE883B9.4050409@goop.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1443 Lines: 32 On 10/28/2009 07:47 PM, Jeremy Fitzhardinge wrote: >> 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. > But we can not make a reliable measurement. > 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). > If you're interested in gettimeofday() for a global monotonic counter you can fall back to atomic_fetch_and_add() which will be faster than a syscall even on large systems. Maybe we should provide a vsyscall for global monotonic counters and implement it using a atomics or tsc instead of these hacks (I'm assuming here that the gettimeofday() calls are used to implement an atomic counter - are they?) -- error compiling committee.c: too many arguments to function -- 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/