2001-08-29 14:35:15 +00:00
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<title>Kernel Model for Precision Timekeeping</title>
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<h3>Kernel Model for Precision Timekeeping</h3>
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<img align="left" src="pic/alice61.gif" alt="gif"> <a href=
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"http://www.eecis.udel.edu/~mills/pictures.htm">from <i>Pogo</i>,
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Walt Kelly</a>
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<p>Exploding kernel<br clear="left">
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</p>
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<hr>
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<p>The technical report [2], which is a major revision and update
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of an earlier report [3], describes an engineering model for a
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precision time-of-day function for a generic operating system. The
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model is based on the principles of disciplined oscillators using
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phase-lock loops (PLL) and frequency-lock loops (FLL) often found
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in the engineering literature. The model uses a hybrid PLL/FLL
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discipline algorithm implemented in the kernel. The hybrid loop
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provides automatic time and frequency steering with update
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intervals from a few seconds to over one day.</p>
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<p>The hybrid PLL/FLL has been implemented in the Unix kernels for
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several operating systems, including FreeBSD and Linux and those
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made by Sun Microsystems, Digital/Compaq and Hewlett Packard. The
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modifications are currently included in the licensed kernels for
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Digital Unix 4.0 (aka Tru64) and Sun Solaris 2.8. Since the
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modifications involve proprietary kernel interface code, they
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cannot be provided for other licensed kernels directly. Inquiries
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should be directed to the manufacturer's representatives. The
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software and documentation, including a simulator with code
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segments almost identical to the implementations, but not involving
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licensed code, is called <tt>nanokernel.tar.gz</tt> and available
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via the web at <a href="http://www.ntp.org">www.ntp.org</a> or by
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anonymous FTP from ftp.udel.edu in the <tt>pub/ntp/software</tt>
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directory.</p>
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<p>Recently [1], the model has been re-implemented to support a
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nanosecond system clock. The <tt>/usr/include/sys/timex.h</tt>
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header file defines the applications programming interface (API)
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routines and data structures. Implementations are available for
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Linux, FreeBSD, SunOS and Tru64; however, only the Linux and
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FreeBSD implementations, which are included in recent system
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versions, are directly available. The software and documentation,
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including a simulator with code segments almost identical to the
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implementations, but not involving licensed code, is called <tt>
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nanokernel.tar.gz</tt> and available via the web at <a href=
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"http://www.ntp.org">www.ntp.org</a> or by anonymous FTP from
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ftp.udel.edu in the <tt>pub/ntp/software</tt> directory.</p>
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<p>The model changes the way the system clock is adjusted in time
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and frequency, as well as provides mechanisms to discipline its
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time and frequency to an external precision timing source, such as
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described in the <a href="pps.htm">Pulse-per-second (PPS) Signal
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Interfacing</a> page. The model incorporates a generic system call
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interface for use with the NTP or similar time synchronization
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protocol. The NTP software daemons for Version 3 <tt>xntpd</tt> and
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Version 4 <tt>ntpd</tt> use this API to provide synchronization
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limited in principle only by the accuracy and stability of the
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external timing source. There are two new system calls defined in
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<tt>timex.h</tt>, <tt>ntp_gettime()</tt>, which returns a structure
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1999-12-09 13:01:21 +00:00
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including the current time, estimated error and maximum error, and
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2001-08-29 14:35:15 +00:00
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<tt>ntp_adjtime()</tt>, which provides a means to adjust kernel
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variables, including the current time and frequency offsets.</p>
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<p>These kernel modifications are normally used in conjunction with
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a kernel hardware interface such as described in the <a href=
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"kernpps.htm">Kernel Programming Interface for Precision Time
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Signals</a> page.</p>
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<h4>References</h4>
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<ol>
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<li><p>Mills, D.L., and P.-H. Kamp. The nanokernel. <i>Proc. Precision
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Time and Time Interval (PTTI) Applications and Planning Meeting</i>
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(Reston VA, November 2000). Paper: <a href=
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"database/papers/nano/nano2.ps">PostScript</a> | <a href=
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"http://www.eecis.udel.edu/~mills/database/papers/nano/nano2.pdf">
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PDF</a>, Slides: <a href=
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"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.htm">
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HTML</a> | <a href=
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"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.ps">
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PostScript</a> | <a href=
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"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.pdf">
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PDF</a> | <a href=
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"http://www.eecis.udel.edu/~mills/database/brief/nano/nano.ppt">
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PowerPoint</a></p></li>
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<li><p>Mills, D.L. Unix kernel modifications for precision time
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synchronization. Electrical Engineering Department Report 94-10-1,
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University of Delaware, October 1994, 24 pp. Abstract: <a href=
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"http://www.eecis.udel.edu/~mills/database/reports/kern/kerna.ps">
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PostScript</a> | <a href="database/reports/kern/kerna.pdf">PDF</a>,
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Body: <a href=
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"http://www.eecis.udel.edu/~mills/database/reports/kern/kernb.ps">
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PostScript</a> | <a href=
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"http://www.eecis.udel.edu/~mills/database/reports/kern/kernb.pdf">
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PDF</a></p></li>
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<li><p>Mills, D.L. A kernel model for precision timekeeping. Network
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Working Group Report RFC-1589, University of Delaware, March 1994.
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31 pp. <a href=
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"http://www.eecis.udel.edu/~mills/database/rfc/rfc1589.txt">
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ASCII</a></p></li>
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</ol>
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<a href="index.htm"><img align="left" src="pic/home.gif" alt=
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"gif"></a>
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<address><a href="mailto:mills@udel.edu">David L. Mills
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<mills@udel.edu></a></address>
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