From owner-freebsd-current@FreeBSD.ORG  Fri Aug 13 17:28:57 2004
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP
	id 1D01016A4CE; Fri, 13 Aug 2004 17:28:57 +0000 (GMT)
Received: from duke.cs.duke.edu (duke.cs.duke.edu [152.3.140.1])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 9812243D49; Fri, 13 Aug 2004 17:28:56 +0000 (GMT)
	(envelope-from gallatin@cs.duke.edu)
Received: from grasshopper.cs.duke.edu (grasshopper.cs.duke.edu
	[152.3.145.30])
	by duke.cs.duke.edu (8.12.10/8.12.10) with ESMTP id i7DHSuJt004764
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO);
	Fri, 13 Aug 2004 13:28:56 -0400 (EDT)
Received: (from gallatin@localhost)
	by grasshopper.cs.duke.edu (8.12.9p2/8.12.9/Submit) id i7DHSpdM090769;
	Fri, 13 Aug 2004 13:28:51 -0400 (EDT)
	(envelope-from gallatin)
From: Andrew Gallatin <gallatin@cs.duke.edu>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <16668.64083.212658.727644@grasshopper.cs.duke.edu>
Date: Fri, 13 Aug 2004 13:28:51 -0400 (EDT)
To: John Baldwin <jhb@FreeBSD.org>
In-Reply-To: <200408131326.16412.jhb@FreeBSD.org>
References: <16668.61707.474283.639200@grasshopper.cs.duke.edu>
	<200408131326.16412.jhb@FreeBSD.org>
X-Mailer: VM 6.75 under 21.1 (patch 12) "Channel Islands" XEmacs Lucid
cc: freebsd-current@FreeBSD.org
Subject: Re: Is the TSC timecounter safe on SMP system?
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 13 Aug 2004 17:28:57 -0000


John Baldwin writes:
 > On Friday 13 August 2004 12:49 pm, Andrew Gallatin wrote:
 > > I have a system where the TSC timecounter is quite a bit more accurate
 > > (or perhaps its just much cheaper) than the ACPI timecounter.  This is a
 > > single CPU, HTT system running an SMP kernel.
 > >
 > > A simple program which calls gettimeofday() in a tight loop, looking
 > > for the microseconds to change sees ~998,000 microsecond updates/sec
 > > with kern.timecounter.hardware=TSC, and 28,500 updates/sec with ACPI-safe.
 > >
 > > 1) Is it safe to switch to TSC?
 > >
 > > 2) If yes, would it be safe to switch to TSC if this was a real
 > >    SMP system with multiple physical cpus?
 > 
 > Probably not.  The problem is that the TSC is not necessarily in sync between 
 > the CPUs so time would "jump around" as you migrated between CPUs.  If you 
 > can get the TSC's synchronized between the CPUs and keep them that way then 
 > you can use the TSC (Linux does this FWIW).
 > 

But on a single CPU HTT machine, does each HTT core reads the same TSC?


Drew