Technical Notes

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Reading the System Clock

There are two interesting numbers here, the precision of the clock and the time it takes to read the clock. These serve as a floor to the jitter component. This also helps to make sure we don't divide by zero...

DLM says that the next leap forward on lowering the floor will happen when it takes less than 100nsec to read the clock.

NTP's sys_fuzz_nsec is the time it takes to read the clock in nanoseconds (it's an integer type), and sys_tick is the same thing in a floating type but in usec. sys_precision is log2 of sys_tick.

Some systems have clock_getres() and it behaves differently on different OSes. NTP generally uses CLOCK_REALTIME for its operations, but on some OSes that value will not give use the information we want:

OS sys_fuzz_nsec sys_tick CLOCK_REALTIME OtherSorted ascending
FreeSBD7 1676 1.676 280  
Solaris10 916 0.916 1000000 83 CLOCK_HIGHRES

This topic: Dev > WebHome > TechnicalNotes
Topic revision: r2 - 2013-01-01 - 01:26:46 - HarlanStenn
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