From owner-svn-src-all@FreeBSD.ORG Wed Jul 25 17:27:50 2012 Return-Path: Delivered-To: svn-src-all@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id F0F391065693; Wed, 25 Jul 2012 17:27:49 +0000 (UTC) (envelope-from jkim@FreeBSD.org) Received: from hammer.pct.niksun.com (freefall.freebsd.org [IPv6:2001:4f8:fff6::28]) by mx1.freebsd.org (Postfix) with ESMTP id 35C678FC08; Wed, 25 Jul 2012 17:27:49 +0000 (UTC) Message-ID: <50102C94.9030706@FreeBSD.org> Date: Wed, 25 Jul 2012 13:27:48 -0400 From: Jung-uk Kim User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:13.0) Gecko/20120626 Thunderbird/13.0.1 MIME-Version: 1.0 To: Bruce Evans References: <201207242210.q6OMACqV079603@svn.freebsd.org> <500F9E22.4080608@FreeBSD.org> <20120725102130.GH2676@deviant.kiev.zoral.com.ua> <500FE6AE.8070706@FreeBSD.org> <20120726001659.M5406@besplex.bde.org> In-Reply-To: <20120726001659.M5406@besplex.bde.org> X-Enigmail-Version: 1.4.2 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Jim Harris , src-committers@freebsd.org, svn-src-all@freebsd.org, Andriy Gapon , svn-src-head@freebsd.org, Konstantin Belousov Subject: Re: svn commit: r238755 - head/sys/x86/x86 X-BeenThere: svn-src-all@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: "SVN commit messages for the entire src tree \(except for " user" and " projects" \)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 25 Jul 2012 17:27:50 -0000 -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 2012-07-25 10:44:04 -0400, Bruce Evans wrote: > On Wed, 25 Jul 2012, Andriy Gapon wrote: > >> on 25/07/2012 13:21 Konstantin Belousov said the following: >>> ... diff --git a/sys/x86/x86/tsc.c b/sys/x86/x86/tsc.c index >>> 085c339..229b351 100644 --- a/sys/x86/x86/tsc.c +++ >>> b/sys/x86/x86/tsc.c @@ -594,6 +594,7 @@ static u_int >>> tsc_get_timecount(struct timecounter *tc __unused) { >>> >>> + rmb(); return (rdtsc32()); } >> >> This makes sense to me. We probably want correctness over >> performance here. [BTW, I originally thought that the change was >> here; brain malfunction] > > And I liked the original change because it wasn't here :-). > >>> @@ -602,8 +603,9 @@ tsc_get_timecount_low(struct timecounter >>> *tc) { uint32_t rv; >>> >>> + rmb(); __asm __volatile("rdtsc; shrd %%cl, %%edx, %0" - >>> : "=a" (rv) : "c" ((int)(intptr_t)tc->tc_priv) : "edx"); + >>> : "=a" (rv) : "c" ((int)(intptr_t)tc->tc_priv) : "edx"); return >>> (rv); } >>> >> >> It would correct here too, but not sure if it would make any >> difference given that some lower bits are discarded anyway. >> Probably depends on exact CPU. > > It is needed to pessimize this too. :-) > > As I have complained before, the loss of resolution from the shift > is easy to see by reading the time from userland, even with syscall > overhead taking 10-20 times longer than the read. On core2 with > TSC-low, a clock- checking utility gives: > > % min 481, max 12031, mean 530.589452, std 51.633626 % 1th: 550 > (1296487 observations) % 2th: 481 (448425 observations) % 3th: 482 > (142650 observations) % 4th: 549 (61945 observations) % 5th: 551 > (47619 observations) > > The numbers are diffences in nanoseconds measured by > clock_gettime(). The jump from 481 to 549 is 68. From this I can > tell that the clock frequency is 1.86 Ghz and the shift is 128, or > the clock frequency is 3.72 Ghz and the shift is 256. > > On AthlonXP with TSC: > > % min 273, max 29075, mean 274.412811, std 80.425963 % 1th: 273 > (853962 observations) % 2th: 274 (745606 observations) % 3th: 275 > (400212 observations) % 4th: 276 (20 observations) % 5th: 280 (10 > observations) > > Now the numbers cluster about the mean. Although syscalls take > much longer than the loss of resolution with TSC-low, and even the > core2 TSC takes almost as long to read as the loss, it is still > possible to see things happening at the limits of the resolution > (~0.5 nsec). > >> And, oh hmm, I read AMD Software Optimization Guide for AMD >> Family 10h Processors and they suggest using cpuid (with a note >> that it may be intercepted in virtualized environments) or >> _mfence_ in the discussed role (Appendix F of the document). >> Googling for 'rdtsc mfence lfence' yields some interesting >> results. > > The second hit was for the shrd pessimization/loss of resolution > and a memory access hack in lkml in 2011. I now seem to remember > jkim mentioning the memory access hack. rmb() on i386 has a > related memory access hack, but now with a lock prefix that defeats > the point of the 2011 hack (it wanted to save 5 nsec by removing > fences). rmb() on amd64 uses lfence. I believe I mentioned this thread at the time: https://patchwork.kernel.org/patch/691712/ FYI, r238755 is essentially this commit for Linux: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=93ce99e849433ede4ce8b410b749dc0cad1100b2 > Some of the other hits are a bit old. The 8th one was by me in > the thread about kib@ implementing gettimeofday() in userland. Since we have gettimeofday() in userland, the above Linux thread is more relevant now, I guess. Jung-uk Kim -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (FreeBSD) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAlAQLJQACgkQmlay1b9qnVMR8ACglzKrNWGeYJeqRhHQmna5stQQ qM4AoKn4xdey8nglvdVm7UiQ1NZRr81E =15v+ -----END PGP SIGNATURE-----