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 5.09 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/.