1 .\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
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25 .\" 2008-06-24, mtk: added some details about where jiffies come into
26 .\" play; added section on high-resolution timers.
28 .TH TIME 7 2016-03-15 "Linux" "Linux Programmer's Manual"
30 time \- overview of time and timers
32 .SS Real time and process time
34 is defined as time measured from some fixed point,
35 either from a standard point in the past
36 (see the description of the Epoch and calendar time below),
37 or from some point (e.g., the start) in the life of a process
38 .RI ( "elapsed time" ).
41 is defined as the amount of CPU time used by a process.
42 This is sometimes divided into
47 User CPU time is the time spent executing code in user mode.
48 System CPU time is the time spent by the kernel executing
49 in system mode on behalf of the process (e.g., executing system calls).
52 command can be used to determine the amount of CPU time consumed
53 during the execution of a program.
54 A program can determine the amount of CPU time it has consumed using
59 .SS The hardware clock
60 Most computers have a (battery-powered) hardware clock which the kernel
61 reads at boot time in order to initialize the software clock.
62 For further details, see
66 .SS The software clock, HZ, and jiffies
67 The accuracy of various system calls that set timeouts,
71 .\" semtimedop(), mq_timedwait(), io_getevents(), poll() are the same
72 .\" futexes and thus sem_timedwait() seem to use high-res timers.
73 and measure CPU time (e.g.,
75 is limited by the resolution of the
76 .IR "software clock" ,
77 a clock maintained by the kernel which measures time in
79 The size of a jiffy is determined by the value of the kernel constant
84 varies across kernel versions and hardware platforms.
85 On i386 the situation is as follows:
86 on kernels up to and including 2.4.x, HZ was 100,
87 giving a jiffy value of 0.01 seconds;
88 starting with 2.6.0, HZ was raised to 1000, giving a jiffy of
90 Since kernel 2.6.13, the HZ value is a kernel
91 configuration parameter and can be 100, 250 (the default) or 1000,
92 yielding a jiffies value of, respectively, 0.01, 0.004, or 0.001 seconds.
93 Since kernel 2.6.20, a further frequency is available:
94 300, a number that divides evenly for the common video
95 frame rates (PAL, 25 HZ; NTSC, 30 HZ).
99 system call is a special case.
100 It reports times with a granularity defined by the kernel constant
102 User-space applications can determine the value of this constant using
103 .IR sysconf(_SC_CLK_TCK) .
104 .\" glibc gets this info with a little help from the ELF loader;
105 .\" see glibc elf/dl-support.c and kernel fs/binfmt_elf.c.
107 .SS High-resolution timers
108 Before Linux 2.6.21, the accuracy of timer and sleep system calls
109 (see below) was also limited by the size of the jiffy.
111 Since Linux 2.6.21, Linux supports high-resolution timers (HRTs),
112 optionally configurable via
113 .BR CONFIG_HIGH_RES_TIMERS .
114 On a system that supports HRTs, the accuracy of sleep and timer
115 system calls is no longer constrained by the jiffy,
116 but instead can be as accurate as the hardware allows
117 (microsecond accuracy is typical of modern hardware).
118 You can determine whether high-resolution timers are supported by
119 checking the resolution returned by a call to
121 or looking at the "resolution" entries in
122 .IR /proc/timer_list .
124 HRTs are not supported on all hardware architectures.
125 (Support is provided on x86, arm, and powerpc, among others.)
127 UNIX systems represent time in seconds since the
129 1970-01-01 00:00:00 +0000 (UTC).
131 A program can determine the
134 .BR gettimeofday (2),
135 which returns time (in seconds and microseconds) that have
136 elapsed since the Epoch;
138 provides similar information, but only with accuracy to the
140 The system time can be changed using
141 .BR settimeofday (2).
143 Certain library functions use a structure of
147 .IR "broken-down time" ,
148 which stores time value separated out into distinct components
149 (year, month, day, hour, minute, second, etc.).
150 This structure is described in
152 which also describes functions that convert between calendar time and
154 Functions for converting between broken-down time and printable
155 string representations of the time are described in
160 .SS Sleeping and setting timers
161 Various system calls and functions allow a program to sleep
162 (suspend execution) for a specified period of time; see
164 .BR clock_nanosleep (2),
168 Various system calls allow a process to set a timer that expires
169 at some point in the future, and optionally at repeated intervals;
173 .BR timerfd_create (2),
175 .BR timer_create (2).
177 Since Linux 2.6.28, it is possible to control the "timer slack"
179 The timer slack is the length of time by
180 which the kernel may delay the wake-up of certain
181 system calls that block with a timeout.
182 Permitting this delay allows the kernel to coalesce wake-up events,
183 thus possibly reducing the number of system wake-ups and saving power.
184 For more details, see the description of
196 .BR clock_gettime (2),
197 .BR clock_nanosleep (2),
201 .BR gettimeofday (2),
205 .BR timer_create (2),
206 .BR timerfd_create (2),
211 .BR clock_getcpuclockid (3),
215 .BR pthread_getcpuclockid (3),