Date: Wed, 22 Aug 2001 20:42:41 -0600 From: Warner Losh <imp@harmony.village.org> To: Thierry Herbelot <thierry@herbelot.com> Cc: hackers@FreeBSD.ORG Subject: Re: clock synchronization quality via NTP ? Message-ID: <200108230242.f7N2gfW80510@harmony.village.org> In-Reply-To: Your message of "Wed, 22 Aug 2001 21:47:34 %2B0200." <3B840C56.BDB7424D@herbelot.com> References: <3B840C56.BDB7424D@herbelot.com>
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In message <3B840C56.BDB7424D@herbelot.com> Thierry Herbelot writes: : I know FreeBSD can be used with great success for timing solutions (at : least two core members do it ?). Well, there's one core member, and a second committer. Or was until a few days ago... : has someone some performance data of the quality of system clock : synchronization, while using NTPd with a GPS reveiver and a hard 1PPS : signal ? : : More precisely : is it reasonable to hope having a system clock not : farther from the GPS clock by more than 50 micro-seconds ? It depends on the machine. We had troubles with 486-class hardware getting below a tens microseconds for extended periods of time. Part of this was due resolution of the timer used in the sytem. Part of it was due to thermal variance in the temperature. Part of it was due to the extremely crappy oscillator that was on the board. And we also had to hack the parallel port driver to use fast interrupts. Otherwise the interrupt latency variance caused too much jitter. Or at least enough jitter to measure and be concerned about. We've not repated the experiment now that pentium class hardware is available, which we think will yield much better results. PC hardware really sucks for timing. Warner To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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