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).

   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 using gettimeofday(2),
       which returns time (in seconds and microseconds) 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
       settimeofday(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), 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), pthread_getcpuclockid(3), sleep(3),
       strftime(3), strptime(3), timeradd(3), usleep(3), rtc(4), hwclock(8)

COLOPHON         top

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Linux                            2012-10-28                          TIME(7)