From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-x231.google.com (mail-wm0-x231.google.com [IPv6:2a00:1450:400c:c09::231]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3qpX3P3WbzzDq60 for ; Tue, 19 Apr 2016 01:23:28 +1000 (AEST) Received: by mail-wm0-x231.google.com with SMTP id l6so22556455wml.1 for ; Mon, 18 Apr 2016 08:23:28 -0700 (PDT) Subject: Re: clock_gettime.2: _COARSE clocks are not always faster... To: Rasmus Villemoes References: <87k2k67ml4.fsf@rasmusvillemoes.dk> <5714F972.6080603@gmail.com> Cc: mtk.manpages@gmail.com, linux-man@vger.kernel.org, linuxppc-dev@lists.ozlabs.org From: "Michael Kerrisk (man-pages)" Message-ID: <5714FBEC.80209@gmail.com> Date: Mon, 18 Apr 2016 16:23:24 +0100 MIME-Version: 1.0 In-Reply-To: <5714F972.6080603@gmail.com> Content-Type: text/plain; charset=UTF-8 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 04/18/2016 04:12 PM, Michael Kerrisk (man-pages) wrote: > Hello Rasmus, > > On 04/09/2016 05:50 PM, Rasmus Villemoes wrote: >> Hi Michael >> >> The other day, I was curious how the vdso was implemented on ppc, and I >> noted that neither ppc32 or ppc64 handle the _COARSE versions of >> CLOCK_{REALTIME,MONOTONIC} in the vdso, so they fall back to an actual >> syscall. And sure enough, measuring CLOCK_MONOTONIC >> vs. CLOCK_MONOTONIC_COARSE shows that the latter has three times as much >> overhead as the former. >> >> Whether it's worth adding a note to the man page is up to you. > > I think it's useful to do so. For both *COARSE flags, I added: > > [[ > Requires per-architecture support, and probably also architecture support > for this flag in the vdso(7). > ]] And in the PowerPC sections of vdso(7), I added: The CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE clocks are not supported by the __kernel_clock_getres and __ker‐ nel_clock_gettime interfaces; the kernel falls back to the real system call. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/