NAME | DESCRIPTION | SEE ALSO | COLOPHON
TIME(7) Linux Programmer's Manual TIME(7)
time - overview of time and timers
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).
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 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. Userspace applications
can determine the value of this constant using sysconf(_SC_CLK_TCK).
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(3) 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.)
Unix systems represent time in seconds since the Epoch, which is defined as
0:00:00 UTC on the morning of 1 January 1970.
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).
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).
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(3).
date(1), time(1), adjtimex(2), alarm(2), clock_nanosleep(2), getitimer(2),
getrlimit(2), getrusage(2), gettimeofday(2), nanosleep(2), stat(2), time(2),
timerfd_create(2), times(2), utime(2), adjtime(3), clock(3),
clock_getcpuclockid(3), ctime(3), sleep(3), strftime(3), strptime(3),
timeradd(3), usleep(3), rtc(4), hwclock(8)
This page is part of release 3.11 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-08-11 TIME(7)