Date: Wed, 23 May 2001 19:43:32 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Greg Lehey <grog@FreeBSD.ORG> Cc: arch@FreeBSD.ORG Subject: Re: Re: http://uptime.netcraft.com/up/accuracy.html#cycle Message-ID: <Pine.BSF.4.21.0105231936220.67212-100000@besplex.bde.org> In-Reply-To: <20010523083342.E41189@wantadilla.lemis.com>
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On Wed, 23 May 2001, Greg Lehey wrote: > Comments? > > Greg > > ----- Forwarded message from Richard Wendland <richard@starburst.demon.co.uk> ----- > > > Date: Tue, 22 May 2001 21:51:02 +0100 (BST) > > From: Richard Wendland <richard@starburst.demon.co.uk> > > To: grog@FreeBSD.org (Greg Lehey) > > Cc: webmaster@netcraft.com > > Subject: Re: http://uptime.netcraft.com/up/accuracy.html#cycle > > X-Mailer: ELM [version 2.5 PL2] > > > >> At this link, you claim: > >> > >> Additionally HP-UX, Linux, Solaris and recent releases of FreeBSD > >> cycle back to zero after 497 days, exactly as if the machine had > >> been rebooted at that precise point. Thus it is not possible to see > >> a HP-UX, Linux or Solaris system with an uptime measurement above > >> 497 days. > >> > >> FreeBSD does not suffer from this problem. You'll notice that you > >> have a large number of FreeBSD systems with uptimes of over 497 days. > >> I'd appreciate if you would correct this statement. > > > > Hi Greg, > > > > I think that statement is accurate. Note that we're not talking about > > the FreeBSD 'uptime' command, but our ability to ascertain uptime remotely > > by decoding the TCP timestamp option. > > > > Prior to FreeBSD 3 the TCP timestamp option was incremented every 500ms, > > as is traditional with BSD. From FreeBSD 3 it was incremented every > > 10ms, presumably to improve RTT measurement. But it does have the > > consequence that the 32-bit TCP timestamp wraps around at 497.1 days. > > Hence, with our current method at least, we don't detect uptimes above > > this for FreeBSD 3 and later. > > > > So the FreeBSD systems listed > 497 days are running FreeBSD 2. > > Once everyone has upgraded from FreeBSD 2, FreeBSD will no longer get > > in that top uptimes list! The TCP timestamp is actually incremented every 1/hz seconds, so it overflows after every 48.5 days on alphas (and on i386's with "options HZ=1024"). Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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