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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 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.    Userspace
       applications   can   determine   the   value  of  this  constant  using

   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

   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

   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).


       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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