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).
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/.
