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TIME(7)                    Linux Programmer's ManualTIME(7)NAME       time - overview of time and timersDESCRIPTION   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).  Thetime(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 usingtimes(2),getrusage(2), orclock(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 fur-       ther details, seertc(4) andhwclock(8).   The software clock, HZ, and jiffies       The  accuracy  of  various  system  calls that set timeouts, (e.g., se-lect(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  de-       termined 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, respec-       tively,  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  com-       mon video frame rates (PAL, 25 HZ; NTSC, 30 HZ).       Thetimes(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), op-       tionally 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 toclock_getres(2) or look-       ing at the "resolution" entries in /proc/timer_list.       HRTs are not supported on all hardware architectures.  (Support is pro-       vided 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  theclock_gettime(2)       CLOCK_REALTIME  clock,  which returns time (in seconds and nanoseconds)       that have elapsed since the Epoch;time(2)  provides  similar  informa-       tion,  but  only  with accuracy to the nearest second.  The system time       can be changed usingclock_settime(2).   Broken-down time       Certain library functions use a structure of type tm to represent  bro-       ken-down time, which stores time value separated out into distinct com-       ponents (year, month, day, hour, minute, second, etc.).  This structure       is  described  inctime(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 inctime(3),strftime(3), andstrptime(3).   Sleeping and setting timers       Various system calls and functions allow a program  to  sleep  (suspend       execution)   for   a   specified  period  of  time;  seenanosleep(2),clock_nanosleep(2), andsleep(3).       Various system calls allow a process to set a  timer  that  expires  at       some  point  in  the  future, and optionally at repeated intervals; seealarm(2),getitimer(2),timerfd_create(2), andtimer_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 ker-       nel 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 inprctl(2).SEE ALSOdate(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),hwclock(8)COLOPHON       This page is part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs, and the       latest version of this page, can be found at       https://www.kernel.org/doc/man-pages/.Linux                             2018-04-30TIME(7)