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

       time - overview of time and timers

   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

   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).

       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)

       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

Linux                              2020-04-11                            TIME(7)