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)