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NAME
time - overview of time and timers
DESCRIPTION
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).
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 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).
SEE ALSO
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)
COLOPHON
This page is part of release 5.10 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/.