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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).

   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)


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