diff options
Diffstat (limited to '')
| -rw-r--r-- | man7/time.7 | 218 |
1 files changed, 218 insertions, 0 deletions
diff --git a/man7/time.7 b/man7/time.7 new file mode 100644 index 0000000..ee0db5d --- /dev/null +++ b/man7/time.7 @@ -0,0 +1,218 @@ +.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.\" 2008-06-24, mtk: added some details about where jiffies come into +.\" play; added section on high-resolution timers. +.\" +.TH time 7 2023-01-22 "Linux man-pages 6.05.01" +.SH NAME +time \- overview of time and timers +.SH DESCRIPTION +.SS Real time and process time +.I "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 +.RI ( "elapsed time" ). +.PP +.I "Process time" +is defined as the amount of CPU time used by a process. +This is sometimes divided into +.I user +and +.I 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 +.BR 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 +.BR times (2), +.BR getrusage (2), +or +.BR clock (3). +.SS 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 +.BR rtc (4) +and +.BR hwclock (8). +.SS The software clock, HZ, and jiffies +The accuracy of various system calls that set timeouts, +(e.g., +.BR select (2), +.BR sigtimedwait (2)) +.\" semtimedop(), mq_timedwait(), io_getevents(), poll() are the same +.\" futexes and thus sem_timedwait() seem to use high-res timers. +and measure CPU time (e.g., +.BR getrusage (2)) +is limited by the resolution of the +.IR "software clock" , +a clock maintained by the kernel which measures time in +.IR jiffies . +The size of a jiffy is determined by the value of the kernel constant +.IR HZ . +.PP +The value of +.I 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). +.PP +The +.BR times (2) +system call is a special case. +It reports times with a granularity defined by the kernel constant +.IR USER_HZ . +User-space applications can determine the value of this constant using +.IR sysconf(_SC_CLK_TCK) . +.\" glibc gets this info with a little help from the ELF loader; +.\" see glibc elf/dl-support.c and kernel fs/binfmt_elf.c. +.\" +.SS System and process clocks; time namespaces +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 +.BR clock_gettime (2). +A few of the clocks are settable using +.BR clock_settime (2). +The values of certain clocks are virtualized by time namespaces; see +.BR time_namespaces (7). +.\" +.SS 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. +.PP +Since Linux 2.6.21, Linux supports high-resolution timers (HRTs), +optionally configurable via +.BR 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 +.BR clock_getres (2) +or looking at the "resolution" entries in +.IR /proc/timer_list . +.PP +HRTs are not supported on all hardware architectures. +(Support is provided on x86, ARM, and PowerPC, among others.) +.SS The Epoch +UNIX systems represent time in seconds since the +.IR Epoch , +1970-01-01 00:00:00 +0000 (UTC). +.PP +A program can determine the +.I "calendar time" +via the +.BR clock_gettime (2) +.B CLOCK_REALTIME +clock, +which returns time (in seconds and nanoseconds) that have +elapsed since the Epoch; +.BR time (2) +provides similar information, but only with accuracy to the +nearest second. +The system time can be changed using +.BR clock_settime (2). +.\" +.SS Broken-down time +Certain library functions use a structure of +type +.I tm +to represent +.IR "broken-down time" , +which stores time value separated out into distinct components +(year, month, day, hour, minute, second, etc.). +This structure is described in +.BR 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 +.BR ctime (3), +.BR strftime (3), +and +.BR strptime (3). +.SS Sleeping and setting timers +Various system calls and functions allow a program to sleep +(suspend execution) for a specified period of time; see +.BR nanosleep (2), +.BR clock_nanosleep (2), +and +.BR sleep (3). +.PP +Various system calls allow a process to set a timer that expires +at some point in the future, and optionally at repeated intervals; +see +.BR alarm (2), +.BR getitimer (2), +.BR timerfd_create (2), +and +.BR timer_create (2). +.SS Timer slack +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 +.B PR_SET_TIMERSLACK +in +.BR prctl (2). +.SH SEE ALSO +.ad l +.nh +.BR date (1), +.BR time (1), +.BR timeout (1), +.BR adjtimex (2), +.BR alarm (2), +.BR clock_gettime (2), +.BR clock_nanosleep (2), +.BR getitimer (2), +.BR getrlimit (2), +.BR getrusage (2), +.BR gettimeofday (2), +.BR nanosleep (2), +.BR stat (2), +.BR time (2), +.BR timer_create (2), +.BR timerfd_create (2), +.BR times (2), +.BR utime (2), +.BR adjtime (3), +.BR clock (3), +.BR clock_getcpuclockid (3), +.BR ctime (3), +.BR ntp_adjtime (3), +.BR ntp_gettime (3), +.BR pthread_getcpuclockid (3), +.BR sleep (3), +.BR strftime (3), +.BR strptime (3), +.BR timeradd (3), +.BR usleep (3), +.BR rtc (4), +.BR time_namespaces (7), +.BR hwclock (8) |