time - overview of time and timers
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).
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 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).
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).
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.)
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).
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).
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).
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).
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)