From owner-freebsd-questions@FreeBSD.ORG Wed Sep 14 18:10:51 2011 Return-Path: Delivered-To: questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 58245106566C for ; Wed, 14 Sep 2011 18:10:51 +0000 (UTC) (envelope-from dan@dan.emsphone.com) Received: from email2.allantgroup.com (email2.emsphone.com [199.67.51.116]) by mx1.freebsd.org (Postfix) with ESMTP id E38C88FC08 for ; Wed, 14 Sep 2011 18:10:50 +0000 (UTC) Received: from dan.emsphone.com (dan.emsphone.com [199.67.51.101]) by email2.allantgroup.com (8.14.4/8.14.4) with ESMTP id p8DFGoBK069932 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) for ; Tue, 13 Sep 2011 10:16:51 -0500 (CDT) (envelope-from dan@dan.emsphone.com) Received: from dan.emsphone.com (smmsp@localhost [127.0.0.1]) by dan.emsphone.com (8.14.5/8.14.5) with ESMTP id p8DFGoYj034298 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) for ; Tue, 13 Sep 2011 10:16:50 -0500 (CDT) (envelope-from dan@dan.emsphone.com) Received: (from dan@localhost) by dan.emsphone.com (8.14.5/8.14.5/Submit) id p8DFGorG034297; Tue, 13 Sep 2011 10:16:50 -0500 (CDT) (envelope-from dan) Date: Tue, 13 Sep 2011 10:16:50 -0500 From: Dan Nelson To: Brett Glass Message-ID: <20110913151650.GF9801@dan.emsphone.com> References: <201109122350.RAA21916@lariat.net> <201109130443.WAA23650@lariat.net> <201109131021.EAA26090@lariat.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201109131021.EAA26090@lariat.net> X-OS: FreeBSD 8.2-STABLE User-Agent: Mutt/1.5.21 (2010-09-15) X-Virus-Scanned: clamav-milter 0.97.2 at email2.allantgroup.com X-Virus-Status: Clean X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.6 (email2.allantgroup.com [199.67.51.78]); Tue, 13 Sep 2011 10:16:51 -0500 (CDT) X-Scanned-By: MIMEDefang 2.68 on 199.67.51.78 Cc: Adam Vande More , questions@freebsd.org Subject: Re: Negative ping times with FreeBSD 8.1-RELEASE on older Celeron system X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 14 Sep 2011 18:10:51 -0000 In the last episode (Sep 13), Brett Glass said: > Thank you! Since it's tunable at runtime I just tested it, and -- sure > enough -- no negative ping times. > > Ironically, it was the kernel that selected the ACPI timer, scoring it > higher than the timestamp counter as a clock source. Perhaps code should > be added to ensure that the timer is not chosen if it rolls over in less > than a second, since this clearly leads to imprecision and missed > rollovers. > > kern.timecounter.tc.ACPI-safe.mask: 16777215 > kern.timecounter.tc.ACPI-safe.counter: 7967112 > kern.timecounter.tc.ACPI-safe.frequency: 3579545 > kern.timecounter.tc.ACPI-safe.quality: 850 It doesn't roll over in less than a second; it rolls over in 16777215 / 3579545 = 4.6 seconds. Your negative time delta problem isn't due to rollover. -- Dan Nelson dnelson@allantgroup.com