time(7) Miscellaneous Information 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 cal-
endar 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 fur-
ther details, see rtc(4) and hwclock(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
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 sec-
onds. Since Linux 2.6.13, the HZ value is a kernel configuration para-
meter 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 applica-
tions 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 de-
scribed 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), op-
tionally configurable via CONFIG_HIGH_RES_TIMERS. On a system that sup-
ports 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 "res-
olution" 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 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 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 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 ex-
ecution) 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).
SEE ALSO
date(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), time_namespaces(7), hwclock(8)
Linux man-pages 6.9.1 2024-05-02 time(7)
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