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.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2008-06-24, mtk: added some details about where jiffies come into
.\" play; added section on high-resolution timers.
.\"
.TH time 7 2023-10-31 "Linux man-pages 6.06"
.SH NAME
time \- overview of time and timers
.SH DESCRIPTION
.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" ).
.P
.I "Process time"
is defined as the amount of CPU time used by a process.
This is sometimes divided into
.I user
and
.I system
components.
User CPU time is the time spent executing code in user mode.
System CPU time is the time spent by the kernel executing
in system mode on behalf of the process (e.g., executing system calls).
The
.BR time (1)
command can be used to determine the amount of CPU time consumed
during the execution of a program.
A program can determine the amount of CPU time it has consumed using
.BR times (2),
.BR getrusage (2),
or
.BR clock (3).
.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 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 .
.P
The value of
.I HZ
varies across kernel versions and hardware platforms.
On i386 the situation is as follows:
on kernels up to and including Linux 2.4.x,
HZ was 100,
giving a jiffy value of 0.01 seconds;
starting with Linux 2.6.0,
HZ was raised to 1000,
giving a jiffy of 0.001 seconds.
Since Linux 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 Linux 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).
.P
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 System and process clocks; time namespaces
The kernel supports a range of clocks that measure various kinds of
elapsed and virtual (i.e., consumed CPU) time.
These clocks are described in
.BR clock_gettime (2).
A few of the clocks are settable using
.BR clock_settime (2).
The values of certain clocks are virtualized by time namespaces; see
.BR time_namespaces (7).
.\"
.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.
.P
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 .
.P
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 ,
1970-01-01 00:00:00 +0000 (UTC).
.P
A program can determine the
.I "calendar time"
via the
.BR clock_gettime (2)
.B 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 clock_settime (2).
.\"
.SS Broken-down time
Certain library functions use a structure of
type
.I tm
to represent
.IR "broken-down time" ,
which stores time value separated out into distinct components
(year, month, day, hour, minute, second, etc.).
This structure is described in
.BR tm (3type),
which also describes functions that convert between calendar time and
broken-down time.
Functions for converting between broken-down time and printable
string representations of the time are described in
.BR ctime (3),
.BR strftime (3),
and
.BR strptime (3).
.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).
.P
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 alarm (2),
.BR getitimer (2),
.BR timerfd_create (2),
and
.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 gettimeofday (2),
.BR nanosleep (2),
.BR stat (2),
.BR time (2),
.BR timer_create (2),
.BR timerfd_create (2),
.BR times (2),
.BR utime (2),
.BR adjtime (3),
.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),
.BR strptime (3),
.BR timeradd (3),
.BR usleep (3),
.BR rtc (4),
.BR time_namespaces (7),
.BR hwclock (8)
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