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) 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.,
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, respec-
tively, 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 com-
mon 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 appli-
cations can determine the value of this constant using
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 look-
ing at the "resolution" entries in /proc/timer_list.
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 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(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 ker-
nel 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
date(1), time(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), pthread_getcpuclockid(3), sleep(3),
strftime(3), strptime(3), timeradd(3), usleep(3), rtc(4), hwclock(8)
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