# time

## 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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