time

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



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).

   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), hwclock(8)

COLOPHON
       This page is part of release 5.02 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                             2018-04-30                           TIME(7)