Date: Sat, 25 Nov 2017 09:20:55 -0700 From: Warner Losh <imp@bsdimp.com> To: Bruce Evans <brde@optusnet.com.au> Cc: Ian Lepore <ian@freebsd.org>, Devin Teske <devin@shxd.cx>, "Rodney W. Grimes" <rgrimes@freebsd.org>, "Conrad E. Meyer" <cem@freebsd.org>, Emmanuel Vadot <manu@bidouilliste.com>, src-committers <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r326095 - head/usr.sbin/bsdinstall/scripts Message-ID: <CANCZdfoGOAUDUCM7j3fU4FViUdNu1qAB3N-mt2UX_OnQaR5ZUg@mail.gmail.com> In-Reply-To: <20171125201623.J1236@besplex.bde.org> References: <201711231729.vANHTVmo092083@pdx.rh.CN85.dnsmgr.net> <F94B65A7-D2AA-47FB-90C4-439DDFDD1AC7@shxd.cx> <20171124201621.K980@besplex.bde.org> <1511539000.94268.17.camel@freebsd.org> <20171125201623.J1236@besplex.bde.org>
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On Sat, Nov 25, 2017 at 4:09 AM, Bruce Evans <brde@optusnet.com.au> wrote: > Is there any use for ntp as a client if you have an atomic clock? Just to > validate both it and ntpd? NTP is a peer to peer time exchange. You run the same daemon either way. However, in the reference clock case, which is what you're talking about, the system still steers its time using ntp. ntp measures the differences between the atomic clock and the system time and adjusts the frequency offset to steer our any frequency error as well as introducing slight frequency errors to steer out any phase difference. gettimeofday() can't ask the atomic clock directly what time it is. So the ntp daemon is doing a time exchange not with a remote clock, but with a local one to get the right time. Some reference clock know the phase and frequency of the time (say a GPS receiver), others just know the frequency (an atomic clock just produces a PPS frequency standard, for example, since the time it's set to isn't from an internal to the clock source of truth). ntpd copes with all these issues to create a "paper clock" that it then steers the system to. Warner
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