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aa12cea2cc
Although groff_mdoc(7) gives another impression, this is the ordering most widely used and also required by mdocml/mandoc. Reviewed by: ru Approved by: philip, ed (mentors)
179 lines
4.9 KiB
Groff
179 lines
4.9 KiB
Groff
.\"
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.\" Copyright (c) 1996 Joerg Wunsch
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.\"
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.\" All rights reserved.
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.\"
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.\" This program is free software.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\" "
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.Dd January 18, 2008
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.Dt CLOCKS 7
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.Os
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.Sh NAME
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.Nm clocks
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.Nd various system timers
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.Sh SYNOPSIS
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.In time.h
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.Sh DESCRIPTION
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.Dv HZ
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is not part of the application interface in
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.Bx .
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.Pp
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There are many different real and virtual (timekeeping) clocks with
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different frequencies:
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.Bl -bullet
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.It
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The scheduling clock.
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This is a real clock with frequency that happens to be 100.
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It is not available to applications.
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.It
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The statistics clock.
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This is a real clock with frequency that happens to be 128.
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It is not directly available to applications.
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.It
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The clock reported by
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.Xr clock 3 .
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This is a virtual clock with a frequency that happens to be 128.
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Its actual frequency is given by the macro
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.Dv CLOCKS_PER_SEC .
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Note that
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.Dv CLOCKS_PER_SEC
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may be floating point.
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Do not use
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.Xr clock 3
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in new programs under
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.Fx .
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It is feeble compared with
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.Xr getrusage 2 .
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It is provided for
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.Tn ANSI
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conformance.
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It is implemented by calling
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.Xr getrusage 2
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and throwing away information and resolution.
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.It
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The clock reported by
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.Xr times 3 .
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This is a virtual clock with a frequency that happens to be 128.
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Its actual frequency is given by the macro
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.Dv CLK_TCK
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(deprecated; do not use) and by
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.Fn sysconf _SC_CLK_TCK
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and by
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.Xr sysctl 3 .
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Note that its frequency may be different from
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.Dv CLOCKS_PER_SEC .
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Do not use
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.Xr times 3
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in new programs under
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.Fx .
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It is feeble compared with
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.Xr gettimeofday 2
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together with
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.Xr getrusage 2 .
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It is provided for
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.Tn POSIX
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conformance.
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It is implemented by calling
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.Xr gettimeofday 2
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and
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.Xr getrusage 2
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and throwing away information and resolution.
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.It
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The profiling clock.
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This is a real clock with frequency 1024.
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It is used mainly by
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.Xr moncontrol 3 ,
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.Xr kgmon 8
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and
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.Xr gprof 1 .
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Applications should determine its actual frequency using
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.Xr sysctl 3
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or by reading it from the header in the profiling data file.
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.It
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The mc146818a clock.
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This is a real clock with a nominal frequency of 32768.
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It is divided down to give the statistic clock and the profiling clock.
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It is not available to applications.
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.It
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The microseconds clock.
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This is a virtual clock with frequency 1000000.
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It is used for most timekeeping in
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.Bx
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and is exported to applications in
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.Xr getrusage 2 ,
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.Xr gettimeofday 2 ,
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.Xr select 2 ,
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.Xr getitimer 2 ,
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etc.
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This is the clock that should normally be used by
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.Bx
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applications.
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.It
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The i8254 clock.
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This is a real clock/timer with a nominal frequency of 1193182.
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It has three independent time counters to be used.
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It is divided down to give the scheduling clock.
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It is not available to applications.
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.It
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The TSC clock (64-bit register) on fifth-generation or later x86 systems.
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This is a real clock with a frequency that is equivalent to the number of
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cycles per second of the CPU(s).
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Its frequency can be found using the
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.Va machdep.tsc_freq
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sysctl, if it is available.
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It is used to interpolate between values of the scheduling clock.
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It can be accessed using the
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.Dv PMIOTSTAMP
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request of
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.Xr perfmon 4 .
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.It
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The ACPI clock.
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This is a real clock/timer with a nominal frequency of 3579545.
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It is accessed via a 24 or 32 bit register.
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Unlike the TSC clock, it maintains a constant tick rate even when the CPU
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sleeps or its clock rate changes.
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It is not available to applications.
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.El
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.Pp
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Summary: if
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.Dv HZ
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is not 1000000 then the application is probably using the wrong clock.
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.Sh SEE ALSO
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.Xr gprof 1 ,
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.Xr clock_gettime 2 ,
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.Xr getitimer 2 ,
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.Xr getrusage 2 ,
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.Xr gettimeofday 2 ,
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.Xr select 2 ,
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.Xr clock 3 ,
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.Xr moncontrol 3 ,
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.Xr times 3
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.Sh AUTHORS
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.An -nosplit
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This manual page was written by
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.An J\(:org Wunsch
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after a description posted by
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.An Bruce Evans .
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