.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\"
-.TH TIME 7 2006-04-28 "Linux" "Linux Programmer's Manual"
+.\" 2008-06-24, mtk: added some details about where jiffies come into
+.\" play; added section on high-resolution timers.
+.\"
+.TH TIME 7 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
-time \- overview of time
+time \- overview of time and timers
.SH DESCRIPTION
-.SS "Real time and process time"
+.SS Real time and process time
.I "Real time"
is defined as time measured from some fixed point,
either from a standard point in the past
.BR getrusage (2),
or
.BR clock (3).
-.SS "The Hardware Clock"
+.SS The hardware clock
Most computers have a (battery-powered) hardware clock which the kernel
reads at boot time in order to initialize the software clock.
For further details, see
.BR rtc (4)
and
.BR hwclock (8).
-.SS "The Software Clock, HZ, and Jiffies"
-The accuracy of many system calls and timestamps is limited by
-the resolution of the
+.SS The software clock, HZ, and jiffies
+The accuracy of various system calls that set timeouts,
+(e.g.,
+.BR select (2),
+.BR sigtimedwait (2))
+.\" semtimedop(), mq_timedwait(), io_getevents(), poll() are the same
+.\" futexes and thus sem_timedwait() seem to use high-res timers.
+and measure CPU time (e.g.,
+.BR getrusage (2))
+is limited by the resolution of the
.IR "software clock" ,
a clock maintained by the kernel which measures time in
.IR jiffies .
The size of a jiffy is determined by the value of the kernel constant
.IR HZ .
+
The value of
.I HZ
varies across kernel versions and hardware platforms.
on kernels up to and including 2.4.x, HZ was 100,
giving a jiffy value of 0.01 seconds;
starting with 2.6.0, HZ was raised to 1000, giving a jiffy of
-0.001 seconds; since kernel 2.6.13, the HZ value is a kernel
+0.001 seconds.
+Since kernel 2.6.13, the HZ value is a kernel
configuration parameter and can be 100, 250 (the default) or 1000,
yielding a jiffies value of, respectively, 0.01, 0.004, or 0.001 seconds.
Since kernel 2.6.20, a further frequency is available:
300, a number that divides evenly for the common video
frame rates (PAL, 25 HZ; NTSC, 30 HZ).
-.SS "The Epoch"
-Unix systems represent time in seconds since the
+
+The
+.BR times (2)
+system call is a special case.
+It reports times with a granularity defined by the kernel constant
+.IR USER_HZ .
+User-space applications can determine the value of this constant using
+.IR sysconf(_SC_CLK_TCK) .
+.\" glibc gets this info with a little help from the ELF loader;
+.\" see glibc elf/dl-support.c and kernel fs/binfmt_elf.c.
+.\"
+.SS High-resolution timers
+Before Linux 2.6.21, the accuracy of timer and sleep system calls
+(see below) was also limited by the size of the jiffy.
+
+Since Linux 2.6.21, Linux supports high-resolution timers (HRTs),
+optionally configurable via
+.BR CONFIG_HIGH_RES_TIMERS .
+On a system that supports HRTs, the accuracy of sleep and timer
+system calls is no longer constrained by the jiffy,
+but instead can be as accurate as the hardware allows
+(microsecond accuracy is typical of modern hardware).
+You can determine whether high-resolution timers are supported by
+checking the resolution returned by a call to
+.BR clock_getres (2)
+or looking at the "resolution" entries in
+.IR /proc/timer_list .
+
+HRTs are not supported on all hardware architectures.
+(Support is provided on x86, arm, and powerpc, among others.)
+.SS The Epoch
+UNIX systems represent time in seconds since the
.IR Epoch ,
-which is defined as 0:00:00 UTC on the morning of 1 January 1970.
+1970-01-01 00:00:00 +0000 (UTC).
A program can determine the
.I "calendar time"
nearest second.
The system time can be changed using
.BR settimeofday (2).
-.SS "Broken-down time"
+.SS Broken-down time
Certain library functions use a structure of
type
.I tm
.BR strftime (3),
and
.BR strptime (3).
-.SS "Sleeping and Setting Timers"
+.SS Sleeping and setting timers
Various system calls and functions allow a program to sleep
(suspend execution) for a specified period of time; see
-.BR nanosleep (2)
+.BR nanosleep (2),
+.BR clock_nanosleep (2),
and
.BR sleep (3).
see
.BR alarm (2),
.BR getitimer (2),
+.BR timerfd_create (2),
and
-.BR timer_create (3).
-.\" FIXME . timer_create() and friends are not yet in man-pages
-.SH "SEE ALSO"
+.BR timer_create (2).
+.SS Timer slack
+Since Linux 2.6.28, it is possible to control the "timer slack"
+value for a thread.
+The timer slack is the length of time by
+which the kernel may delay the wake-up of certain
+system calls that block with a timeout.
+Permitting this delay allows the kernel to coalesce wake-up events,
+thus possibly reducing the number of system wake-ups and saving power.
+For more details, see the description of
+.B PR_SET_TIMERSLACK
+in
+.BR prctl (2).
+.SH SEE ALSO
+.ad l
+.nh
.BR date (1),
.BR time (1),
+.BR timeout (1),
.BR adjtimex (2),
.BR alarm (2),
+.BR clock_gettime (2),
+.BR clock_nanosleep (2),
.BR getitimer (2),
.BR getrlimit (2),
.BR getrusage (2),
.BR nanosleep (2),
.BR stat (2),
.BR time (2),
-.\" FIXME . timer_create (2),
+.BR timer_create (2),
.BR timerfd_create (2),
.BR times (2),
.BR utime (2),
.BR adjtime (3),
.BR clock (3),
-.BR sleep (3),
-.BR timeradd (3),
+.BR clock_getcpuclockid (3),
.BR ctime (3),
+.BR ntp_adjtime (3),
+.BR ntp_gettime (3),
+.BR pthread_getcpuclockid (3),
+.BR sleep (3),
.BR strftime (3),
.BR strptime (3),
+.BR timeradd (3),
.BR usleep (3),
.BR rtc (4),
.BR hwclock (8)
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