Date: Wed, 23 May 2001 08:33:42 +0930 From: Greg Lehey <grog@FreeBSD.org> To: arch@FreeBSD.org Subject: Re: Re: http://uptime.netcraft.com/up/accuracy.html#cycle Message-ID: <20010523083342.E41189@wantadilla.lemis.com>
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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! > >> I also have been told that the Linux 2.4 kernel no longer suffers from >> this problem, but I can't confirm this information. > > Yep, as I understand it at one time the Linux 'uptime' command would wrap > at 497.1 days; presumably because the kernel stored uptime in 10ms units > in 32-bits. That was fixed, but our remote uptime detection would still > suffer this problem. It explains why no Linux (nor Solaris or HP-UX) > systems are in that list. > > Richard > -- > Richard Wendland richard@netcraft.com ----- End forwarded message ----- -- See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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