NAME
time - overview of time and timers
DESCRIPTION
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 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. User-space 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(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.) The Epoch UNIX systems represent time in seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). 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(2). 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 PR_SET_TIMERSLACK in prctl(2).
SEE ALSO
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), hwclock(8)
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
More Linux Commands
manpages/git-remote-ext.1.html
git-remote-ext(1) - Bridge smart transport to external comma
This remote helper uses the specified <command> to connect to a remote Git server. Data written to stdin of the specified <command> is assumed to be sent to a g
manpages/XSetSubwindowMode.3.html
XSetSubwindowMode(3) - GC convenience routines (Man Page)...
The XSetArcMode function sets the arc mode in the specified GC. XSetArcMode can generate BadAlloc, BadGC, and BadValue errors. The XSetSubwindowMode function se
manpages/squid_ldap_auth.8.html
squid_ldap_auth(8) - Squid LDAP authentication helper.......
This helper allows Squid to connect to a LDAP directory to validate the user name and password of Basic HTTP authentication. LDAP options are specified as param
manpages/Encode::Guess.3pm.html
Encode::Guess(3pm) - Guesses encoding from data (Man Page)
By default, it checks only ascii, utf8 and UTF-16/32 with BOM . use Encode::Guess; # ascii/utf8/BOMed UTF To use it more practically, you have to give the names
manpages/hp2xx.1.html
hp2xx(1) A HPGL converter into some vector- and raster forma
hp2xx reads HPGL ASCII source files, interprets them, and converts them into either another vector-oriented format or one of several rasterfile formats. Current
manpages/Tcl_LimitSetGranularity.3.html
Tcl_LimitSetGranularity(3) - manage and check resource limit
Tcls interpreter resource limit subsystem allows for close control over how much computation time a script may use, and is useful for cases where a program is d
manpages/Tcl_CreateMathFunc.3.html
Tcl_CreateMathFunc(3) - Define, query and enumerate math fun
Tcl allows a number of mathematical functions to be used in expressions, such as sin, cos, and hypot. These functions are represented by commands in the namespa
manpages/fgets_unlocked.3.html
fgets_unlocked(3) - nonlocking stdio functions (Man Page)...
Each of these functions has the same behavior as its counterpart without the _unlocked suffix, except that they do not use locking (they do not set locks themse
manpages/vgs.8.html
vgs(8) - report information about volume groups (Man Page)
vgs produces formatted output about volume groups. OPTIONS See lvm(8) for common options. --all List all volume groups. Equivalent to not specifying any volume
manpages/mouse-test.1.html
mouse-test(1) - a tool for determining mouse type and device
This experimental and incomplete application tries to help in detecting which protocol does your mouse speak. It is able to detect MouseMan devices, and to choo
manpages/valgrind-listener.1.html
valgrind-listener(1) listens on a socket for Valgrind commen
valgrind-listener accepts (multiple) connections from valgrind processes that use the --log-socket option on the specified port and copies the commentary it is
manpages/Tcl_FSFileSystemInfo.3.html
Tcl_FSFileSystemInfo(3) - procedures to interact with any fi
There are several reasons for calling the Tcl_FS API functions (e.g. Tcl_FSAccess and Tcl_FSStat) rather than calling system level functions like access and sta
