[time-nuts] x86 CPU Timekeeping and clock generation

[Poul-Henning Kamp]

--------
Tom Van Baak writes:

>If you do your kernel timekeeping in integers and modulus arithmetic you 
>are essentially doing cycle counting and the kernel will keep perfect 
>time relative to the external oscillator. So that should be the goal. 
>Not e-6, not e-9, not e-10, but perfect cycle counting. Consider this a 
>strong plea for someone in both BSD- and Linux- land to pull that off.

But not all of us have masers Tom :-)

Problem is, you still need fractions when NTP tells you that the
X-tal is 7.1PPM off frequency.

When I did timecounters, I tried a couple of things of the sort you
suggest, but when I benchmarked it, a single brutal 64 bit multiply
with a 32bit shift invariably ran faster, had more predictable
latency, worked with any frequency the hardware happened to have
and did NTP's bidding:

	http://phk.freebsd.dk/pubs/timecounter.pdf

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