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6 .\" "THE BEER-WARE LICENSE" (Revision 42):
9 .\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
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16 .A "Poul-Henning Kamp" "The FreeBSD Project"
18 The FreeBSD timecounters are an architecture-independent implementation
20 for tracking time. The binary timescale converts using simple
21 multiplication to canonical timescales based on micro- or nano-seconds
23 synchronisation. Timecounters are implemented using lock-less
24 stable-storage based primitives which scale efficiently in SMP
36 identify the first computer which knew the time of day.
44 and if systems like the ``SAGE'' [SAGE] did not know what time
47 On the other hand, it took a long time for a real time clock to
51 had neither in hardware or software any notion what time it was.
53 The original ``IBM PC'' did know what time it was, provided you typed
54 it in when you booted it, but it forgot when you turned it off.
57 backed CMOS chip which kept track of time even when the computer
58 was powered off.
60 Today we expect our computers to know the time, and with network
65 track of time.
67 Time and timescale basics
69 Despite the fact that time is the physical quantity (or maybe entity
70 ?) about which we know the least, it is at the same time [sic!] what we
77 with one micrometer precision, in real time.
79 While it is possible to measure time by means other than oscillations,
80 for instance transport or consumption of a substance at a well-known
81 rate, such designs play no practical role in time measurement because
90 sun-dial Earths rotation about its axis.
94 atomic Quantum-state transitions in atoms.
108 with an error less than �2 cdot 10 sup{-15}� [DMK2001] with commercially
109 available products doing better than �1 cdot 10 sup{-14}� [AG2002].
112 longitude for instance, the ephemeral nature of time prevents us
114 there. For measuring time we have to rely on ``dead reckoning'',
122 ``Relative time'' is a time interval between two events, and for this
125 ``Absolute time'' consists of a well defined point in time and the
126 time interval since then, this is a bit more tricky.
129 starts at (from a physics point of view) arbitrary points in time
137 Earth. This resulted in time-intervals being very unwieldy business,
140 Eventually the new leap-second method were introduced in 1972.
149 UTC is defined basically the same way, but every so often a leap-second
161 and measurement company formerly known as ``Hewlett-Packard'') which
165 custom-design units like the PTB2 and NIST7.
172 The perceived wisdom of leap-seconds have been gradually decreasing
173 in recent years, as devices and products with built-in calendar
181 as the count of standard seconds since 00:00:00 01-01-1970 UTC,
182 ignoring the leap-seconds. This definition has never been perceived
187 one second off. Another implication is that the length of a
188 time interval calculated on UNIX time_t variables, can be up to 22
189 (and counting) seconds wrong relative to the same time interval
193 deficiency by transmitting the UTC-TAI offset as part of the protocol.
197 ``timespec''. Both of these formats are two-component structures
206 t3.tv_sec = t1.tv_sec - t2.tv_sec;
207 t3.tv_nsec = t1.tv_nsec -
211 t3.tv_nsec -= 1000000000;
213 t3.tv_sec--;
223 time formats available on UNIX for consumer grade hardware,
238 Resolution in clocks is simply a matter of the step-size of the
264 representation as the diameter of the bullet-hole is not correct,
267 gets too quantum-mechanical-oid to serve the instructional purpose.
275 On the x-axis we have time and on the y-axis how wrong the clock
276 was at a given point in time.
289 devices, once you get into the �10 cdot 10 sup{-15}� territory
295 This particularly becomes an issue with space-based atomic standards
311 The ``uptime'' timescale is convenient for time intervals which are
312 not anchored in UTC time: the run time of processes, the access
313 time of disks and similar.
317 estimate of the POSIX time when the system booted to the uptime
320 .I "using whatever hardware we have available at the time,"
327 .I "while supporting time the NTP PLL/FLL discipline code,"
350 mostly a question of the resolution and steer-ability requirements.
366 chosen hardware and this hardware might be the GHz range CPU-clock.
375 �2 sup{32} / (2 sup{32}-1)� �=� 1.000 Hz
392 �2 sup{64} / (2 sup{64}-1)� �=� 1.000 Hz
401 time_t sec;
411 timespec formats is that it is a binary number, not a pseudo-decimal
427 u = bt1->frac;
428 bt3->frac = bt1->frac + bt2->frac;
429 bt3->sec = bt1->sec + bt2->sec;
430 if (u > bt3->frac)
431 bt3->sec += 1;
446 ts->tv_sec = bt->sec;
447 ts->tv_nsec =
449 (uint32_t)(bt->frac >> 32)) >> 32;
460 bt->sec = ts->tv_sec;
463 bt->frac = ts->tv_nsec *
481 This timestamp is on the ``uptime'' time scale, so if
482 UNIX/UTC time is requested, the estimated time of boot is
516 hardware to determine how much time has elapsed since then.
522 The delta-count operation is straightforward subtraction, but we
523 need to logically AND the result with a bit-mask with the same number
529 Delta Count = (Count sub{now} - Count sub{ref}) ~ BITAND ~ mask
550 Delta Count = (Count sub{now} - Count sub{ref}) ~ BITAND ~ mask
589 This quasi-decimal number is a bit of a square peg in our round binary
600 within an factor of a million of the �10 sup{-15}� performance level
606 In this case the correction may be as large as \(+- 5000 PPM which
607 leaves us room to multiply with about 850 in a multiply-before-divide
610 than 850 and at the same time divide by a power of two, which is
614 A divide-before-multiply approximation necessarily results in a loss
624 of the requested change, or �1.06 cdot 10 sup -14� per nanosecond
688 This would however be a very heavy-handed approach. First of
696 A pseudo-stable-storage with generation count method has been
706 for some amount of time into the future.
712 meta-data.
715 This scheme has an inherent risk that a process may be de-scheduled for
716 so long time that it will not manage to complete the timestamping
731 gen = th->th_generation;
734 gen != th->th_generation);
756 at any time while the system is running.
766 instruction(-sequence).
779 The problem with this device is that it only has 8bit bus-width,
784 Obviously, on multi-CPU systems this cannot be done without some
786 to do the same thing at the same time.
798 fact that the counter does not count down from a power of two and
815 counter'' called ``TSC'' in official data-sheets.
816 This is basically a on-CPU counter, which counts at the rate
819 Unfortunately, the electrical power needed to run a CPU is pretty
824 CPU clock to match computing demand in order to minimise the power
835 items which through inefficient designs waste more power than a
839 Another wiggle for the TSC is that it is not usable on multi-CPU
844 to run syntonously (ie: show the same count at the same time).
857 The reason for this odd-ball frequency has to be sought in the ghastly
867 have failed to provide latching suitable to avoid meta-stability
874 latched by examining the width of a histogram over read delta-values.
884 One example of this is the Loran-C receiver designed by Prof. Dave Mills
887 implements the software-half of the receiver has properly initialised
888 and locked onto a Loran-C signal.
914 a precision of \(+- 10 nanoseconds \(+- one count which in practice
915 averages out to roughly \(+- 15 nanoseconds\**:
921 from the ideal time.
957 The source-code is located almost entirely in the kernel source file
966 open source license or the even more free ``Beer-ware'' license.
972 of NTP as a time synchronization protocol, without rounding
982 At this point in time, no specific plans exist for further
985 Various micro-optimizations, mostly to compensate for inadequate
1003 NTP, for lending out the neglected twin Loran-C receiver and for
1013 The staff at the NELS Loran-C control station in B�, Norway for providing
1014 information about step-changes.
1016 The staff at NELS Loran-C station Ei�e, Faeroe
1037 Poul-Henning Kamp
1041 "A computer-controlled LORAN-C receiver for precision timekeeping."
1043 Electrical Engineering Department Report 92-3-1, University of Delaware, March 1992, 63 pp.
1046 …Time Protocol to transmit International Atomic Time (TAI)". Proc. Precision Time and Time Interval…
1050 Mills, D.L., and P.-H. Kamp.
1051 Proc. Precision Time and Time Interval (PTTI) Applications and Planning Meeting (Reston VA, Novembe…
1063 Time" Penguin USA (Paper); ISBN: 0140258795.
1066 This ``gee-wiz'' kind of article in Dr. Dobbs Journal is a good place to