.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
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.\" 2008-06-24, mtk: added some details about where jiffies come into
.\" play; added section on high-resolution timers.
.\"
-.TH TIME 7 2010-02-25 "Linux" "Linux Programmer's Manual"
+.TH TIME 7 2018-04-30 "Linux" "Linux Programmer's Manual"
.SH NAME
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
(see the description of the Epoch and calendar time below),
or from some point (e.g., the start) in the life of a process
.RI ( "elapsed time" ).
-
+.PP
.I "Process time"
is defined as the amount of CPU time used by a process.
This is sometimes divided into
.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"
+.SS The software clock, HZ, and jiffies
The accuracy of various system calls that set timeouts,
(e.g.,
.BR select (2),
.IR jiffies .
The size of a jiffy is determined by the value of the kernel constant
.IR HZ .
-
+.PP
The value of
.I HZ
varies across kernel versions and hardware platforms.
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).
-
+.PP
The
.BR times (2)
system call is a special case.
It reports times with a granularity defined by the kernel constant
.IR USER_HZ .
-Userspace applications can determine the value of this constant using
+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"
+.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.
-
+.PP
Since Linux 2.6.21, Linux supports high-resolution timers (HRTs),
optionally configurable via
.BR CONFIG_HIGH_RES_TIMERS .
.BR clock_getres (2)
or looking at the "resolution" entries in
.IR /proc/timer_list .
-
+.PP
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
+.SS The Epoch
+UNIX systems represent time in seconds since the
.IR Epoch ,
1970-01-01 00:00:00 +0000 (UTC).
-
+.PP
A program can determine the
.I "calendar time"
-using
-.BR gettimeofday (2),
-which returns time (in seconds and microseconds) that have
+via the
+.BR clock_gettime (2)
+.BR CLOCK_REALTIME
+clock,
+which returns time (in seconds and nanoseconds) that have
elapsed since the Epoch;
.BR time (2)
provides similar information, but only with accuracy to the
nearest second.
The system time can be changed using
-.BR settimeofday (2).
-.SS "Broken-down time"
+.BR clock_settime (2).
+.\"
+.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 clock_nanosleep (2),
and
.BR sleep (3).
-
+.PP
Various system calls allow a process to set a timer that expires
at some point in the future, and optionally at repeated intervals;
see
.BR timerfd_create (2),
and
.BR timer_create (2).
-.SH "SEE ALSO"
+.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 (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),
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