Ubuntu Manpage: time - overview of time and timers

NAME

       time - overview of time and timers

DESCRIPTION

   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 Linux 2.4.x, HZ was 100, giving a jiffy value  of  0.01  seconds;
       starting  with  Linux 2.6.0, HZ was raised to 1000, giving a jiffy of 0.001 seconds.  Since Linux 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 Linux 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 tm(3type), 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).

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DESCRIPTION

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