From owner-freebsd-current Sun Sep 6 23:41:56 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id XAA27236 for freebsd-current-outgoing; Sun, 6 Sep 1998 23:41:56 -0700 (PDT) (envelope-from owner-freebsd-current@FreeBSD.ORG) Received: from critter.freebsd.dk (critter.freebsd.dk [212.242.40.131]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id XAA27226 for ; Sun, 6 Sep 1998 23:41:52 -0700 (PDT) (envelope-from phk@critter.freebsd.dk) Received: from critter.freebsd.dk (localhost [127.0.0.1]) by critter.freebsd.dk (8.9.1/8.8.5) with ESMTP id IAA21874; Mon, 7 Sep 1998 08:37:07 +0200 (CEST) To: Mike Smith cc: Thomas Dean , current@FreeBSD.ORG Subject: Re: Dmesg Timecounter Varies In-reply-to: Your message of "Sun, 06 Sep 1998 14:37:47 PDT." <199809062137.OAA10133@word.smith.net.au> Date: Mon, 07 Sep 1998 08:37:07 +0200 Message-ID: <21872.905150227@critter.freebsd.dk> From: Poul-Henning Kamp Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG In message <199809062137.OAA10133@word.smith.net.au>, Mike Smith writes: >> I have noticed that the Timecounter value varies greatly in DMESG. >> >> I am running an SMP kernel. I extracted the values from >> /var/log/messages and passed them thru a stat process: >> >> Timecounter Cost Statistics >> Processing /var/log/messages and /var/log/messages.?.gz >> >> Frequency: 1193182 Hz - Always this value, unless stated. >> 1/freq Time: 838 nsec >> No. Cycles: 3 - floor( mean/(1/freqTime) ). >> Cycle Time: 948 nsec - mean/(no. cycles). >> Overhead: 332 nsec - Total overhead in Timecounter cost. >> >> Entries: 125 - Number of Timecounter entries in messages. >> Minimum: 2517 nsec >> Maximum: 41603 nsec - Wow! Flyer! >> Mean: 2846 nsec >> Std. Dev.: 3494.46 >> >> Notice the maximum flyer. What causes this variability? > >Unexpected execution discontinuities; most often SMI activity. We are >going to have to discontinue our (ab)use of the TSC simply because it's >not reliable enough. PC timekeeping *sucks*. Ahh, but mind you mike, this is >not< the TSC, this is the i8254 :-( -- Poul-Henning Kamp FreeBSD coreteam member phk@FreeBSD.ORG "Real hackers run -current on their laptop." "ttyv0" -- What UNIX calls a $20K state-of-the-art, 3D, hi-res color terminal To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message