linux man page for time

TIME(7) 						     Linux Programmer's Manual							   TIME(7)

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 execu-
       tion 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 appli-
       cations 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 set-
       timeofday(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_TIMER-
       SLACK 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 3.55 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/.

Linux								    2012-10-28								   TIME(7)