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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
TIME(7) Linux Programmer's 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
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 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).
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).
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 using gettimeofday(2),
which returns time (in seconds and microseconds) 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
settimeofday(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 ctime(3), 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), hwclock(8)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-03-15 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), ualarm(3), usleep(3), rtc(4), proc(5), arp(7), cpuset(7)