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man7/time.7
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man7/time.7
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.\" 2008-06-24, mtk: added some details about where jiffies come into
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.\" play; added section on high-resolution timers.
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.\"
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.TH TIME 7 2008-06-24 "Linux" "Linux Programmer's Manual"
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.TH TIME 7 2008-06-25 "Linux" "Linux Programmer's Manual"
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.SH NAME
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time \- overview of time and timers
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.SH DESCRIPTION
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@ -91,16 +91,33 @@ yielding a jiffies value of, respectively, 0.01, 0.004, or 0.001 seconds.
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Since kernel 2.6.20, a further frequency is available:
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300, a number that divides evenly for the common video
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frame rates (PAL, 25 HZ; NTSC, 30 HZ).
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The
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.BR times (2)
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system call is a special case.
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It reports times with a granularity defined by the kernel constant
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.IR USER_HZ .
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Userspace applications can determine the value of this constant using
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.IR sysconf(_SC_CLK_TCK) .
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.\" glibc gets this info with a little help from the ELF loader;
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.\" see glibc elf/dl-support.c and kernel fs/binfmt_elf.c.
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.\"
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.SS "High-Resolution Timers"
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Before Linux 2.6.16, the accuracy of timer and sleep system calls
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Before Linux 2.6.21, the accuracy of timer and sleep system calls
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(see below) was also limited by the size of the jiffy.
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Since Linux 2.6.16, Linux supports high-resolution timers (HRTs),
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optionally configurable since kernel 2.6.21 via CONFIG_HIGH_RES_TIMERS.
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Since Linux 2.6.21, Linux supports high-resolution timers (HRTs),
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optionally configurable via
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.BR CONFIG_HIGH_RES_TIMERS .
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On a system that supports HRTs, the accuracy of sleep and timer
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system calls is no longer constrained by the jiffy,
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but instead can be as accurate as the hardware allows
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(microsecond accuracy is typical of modern hardware).
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You can determine whether high-resolution timers are supported by
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checking the resolution returned by a call to
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.BR clock_getres (3)
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or looking at the "resolution" entries in
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.IR /proc/timer_list .
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HRTs are not supported on all hardware architectures.
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(Support is provided on x86, arm, and powerpc, among others.)
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