time(7) - Linux manual page (original) (raw)

time(7) Miscellaneous Information Manual time(7)

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   time - overview of time and timers

DESCRIPTION top

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)](../man1/time.1.html) 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)](../man2/times.2.html), [getrusage(2)](../man2/getrusage.2.html), or [clock(3)](../man3/clock.3.html).

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)](../man2/times.2.html) system call is a special case.  It reports times with
   a granularity defined by the kernel constant _USERHZ_.  User-space
   applications can determine the value of this constant using
   _sysconf(SCCLKTCK)_.

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)](../man2/clock%5Fgetres.2.html) or looking at the "resolution" entries in
   _/proc/timerlist_.

   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)](../man2/clock%5Fgettime.2.html)
   **CLOCK_REALTIME** clock, which returns time (in seconds and
   nanoseconds) that have elapsed since the Epoch; [time(2)](../man2/time.2.html) provides
   similar information, but only with accuracy to the nearest second.
   The system time can be changed using [clock_settime(2)](../man2/clock%5Fsettime.2.html).

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)](../man2/alarm.2.html), [getitimer(2)](../man2/getitimer.2.html), [timerfd_create(2)](../man2/timerfd%5Fcreate.2.html), and
   [timer_create(2)](../man2/timer%5Fcreate.2.html).

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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Linux man-pages 6.10 2024-05-02 time(7)

Pages that refer to this page:adjtimex(2), alarm(2), clock_getres(2), clock_nanosleep(2), getitimer(2), gettimeofday(2), intro(2), io_getevents(2), nanosleep(2), poll(2), select(2), semop(2), sigaction(2), sigwaitinfo(2), time(2), timer_create(2), timer_delete(2), timerfd_create(2), timer_getoverrun(2), timer_settime(2), times(2), adjtime(3), aio_suspend(3), clock_getcpuclockid(3), ctime(3), mq_receive(3), mq_send(3), ntp_gettime(3), pthread_getcpuclockid(3), sem_wait(3), timeradd(3), tm(3type), ualarm(3), usleep(3), rtc(4), proc(5), arp(7), cpuset(7), inode(7), time_namespaces(7), tc-etf(8)

time(7) - Linux manual page (original) (raw)

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DESCRIPTION top

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