time(7) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | COLOPHON

TIME(7)                 Linux Programmer's Manual                TIME(7)

NAME         top

       time - overview of time and timers

DESCRIPTION         top

   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         top

       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         top

       This page is part of release 5.13 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                          2020-04-11                        TIME(7)

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