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time (7)
  • time (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • time (1) ( FreeBSD man: Команды и прикладные программы пользовательского уровня )
  • time (1) ( Русские man: Команды и прикладные программы пользовательского уровня )
  • time (1) ( Linux man: Команды и прикладные программы пользовательского уровня )
  • time (1) ( POSIX man: Команды и прикладные программы пользовательского уровня )
  • time (2) ( Solaris man: Системные вызовы )
  • time (2) ( Русские man: Системные вызовы )
  • time (2) ( Linux man: Системные вызовы )
  • time (3) ( FreeBSD man: Библиотечные вызовы )
  • time (3) ( POSIX man: Библиотечные вызовы )
  • >> time (7) ( Linux man: Макропакеты и соглашения )
  • time (9) ( FreeBSD man: Ядро )
  • Ключ time обнаружен в базе ключевых слов.
  •  

    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. Userspace 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(3) 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, which is defined as 0:00:00 UTC on the morning of 1 January 1970.

    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(3).  

    SEE ALSO

    date(1), time(1), adjtimex(2), alarm(2), clock_nanosleep(2), getitimer(2), getrlimit(2), getrusage(2), gettimeofday(2), nanosleep(2), stat(2), time(2), timerfd_create(2), times(2), utime(2), adjtime(3), clock(3), clock_getcpuclockid(3), ctime(3), sleep(3), strftime(3), strptime(3), timeradd(3), usleep(3), rtc(4), hwclock(8)  

    COLOPHON

    This page is part of release 3.14 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/.


     

    Index

    NAME
    DESCRIPTION
    Real time and process time
    The Hardware Clock
    The Software Clock, HZ, and Jiffies
    High-Resolution Timers
    The Epoch
    Broken-down time
    Sleeping and Setting Timers
    SEE ALSO
    COLOPHON


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