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 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

       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

       This page is part of release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.