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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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time(7) Miscellaneous Information Manual time(7)
time - overview of time and timers
Real time and process time
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 (elapsed time).
Process time is defined as the amount of CPU time used by a
process. This is sometimes divided into user and 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 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 times(2), getrusage(2), or clock(3).
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 rtc(4) and hwclock(8).
The software clock, HZ, and jiffies
The accuracy of various system calls that set timeouts, (e.g.,
select(2), sigtimedwait(2)) and measure CPU time (e.g.,
getrusage(2)) is limited by the resolution of the software clock,
a clock maintained by the kernel which measures time in jiffies.
The size of a jiffy is determined by the value of the kernel
constant HZ.
The value of 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).
The times(2) system call is a special case. It reports times
with a granularity defined by the kernel constant USER_HZ. User-
space applications can determine the value of this constant using
sysconf(_SC_CLK_TCK).
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 clock_gettime(2). A few of the clocks are
settable using clock_settime(2). The values of certain clocks
are virtualized by time namespaces; see time_namespaces(7).
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 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 clock_getres(2) or looking at the "resolution" entries in
/proc/timer_list.
HRTs are not supported on all hardware architectures. (Support
is provided on x86, ARM, and PowerPC, among others.)
The Epoch
UNIX systems represent time in seconds since the Epoch,
1970-01-01 00:00:00 +0000 (UTC).
A program can determine the calendar time via the
clock_gettime(2) CLOCK_REALTIME clock, which returns time (in
seconds and nanoseconds) that have elapsed since the Epoch;
time(2) provides similar information, but only with accuracy to
the nearest second. The system time can be changed using
clock_settime(2).
Broken-down time
Certain library functions use a structure of type tm to represent
broken-down time, which stores time value separated out into
distinct components (year, month, day, hour, minute, second,
etc.). This structure is described in 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 ctime(3), strftime(3), and strptime(3).
Sleeping and setting timers
Various system calls and functions allow a program to sleep
(suspend execution) for a specified period of time; see
nanosleep(2), clock_nanosleep(2), and sleep(3).
Various system calls allow a process to set a timer that expires
at some point in the future, and optionally at repeated
intervals; see alarm(2), getitimer(2), timerfd_create(2), and
timer_create(2).
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 PR_SET_TIMERSLACK in prctl(2).
date(1), time(1), timeout(1), adjtimex(2), alarm(2),
clock_gettime(2), clock_nanosleep(2), getitimer(2), getrlimit(2),
getrusage(2), gettimeofday(2), nanosleep(2), stat(2), time(2),
timer_create(2), timerfd_create(2), times(2), utime(2),
adjtime(3), clock(3), clock_getcpuclockid(3), ctime(3),
ntp_adjtime(3), ntp_gettime(3), pthread_getcpuclockid(3),
sleep(3), strftime(3), strptime(3), timeradd(3), usleep(3),
rtc(4), time_namespaces(7), hwclock(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 time(7)
Pages that refer to this page: adjtimex(2), alarm(2), clock_getres(2), clock_nanosleep(2), getitimer(2), gettimeofday(2), intro(2), io_getevents(2), nanosleep(2), poll(2), select(2), semop(2), sigaction(2), sigwaitinfo(2), time(2), timer_create(2), timer_delete(2), timerfd_create(2), timer_getoverrun(2), timer_settime(2), times(2), adjtime(3), aio_suspend(3), clock_getcpuclockid(3), ctime(3), mq_receive(3), mq_send(3), ntp_gettime(3), pthread_getcpuclockid(3), sem_wait(3), timeradd(3), tm(3type), ualarm(3), usleep(3), rtc(4), proc(5), arp(7), cpuset(7), inode(7), time_namespaces(7), tc-etf(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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