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       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

   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

   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).


       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)


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