Re: [Ntp] Antw: Re: Antw: [EXT] Re: NTPv5 draft

[Hal Murray <[e-mail]>]

[e-mail] said:
> (Say your clock has 1ns resolution, but reading it takes 10 to 15 ns, maybe
> even longer if there's high system load).

That 10 or 15 ns is a pipeline delay.  It's not directly related to the clock 
precision.

The typical clock on an Intel PC uses the TSC which ticks at the CPU clock 
frequency.  If your CPU is running at more than 1 GHz, then the clock will be 
good to better than 1 ns.


Here is a histogram of how long it takes to read the clock on a Raspberry Pi.
(Raspberry Pi 3 Model B running Fedora aarch64)
        ns      hits
        52    497520
        53     45070
       104    381437
       105     75962
       156         4
7 samples were bigger than 2551.

So the clock is ticking at somewhere between 2 and 53 ns.

You can read a clock that ticks slowly with (much) more precision that its 
nominal period.  The classic example is the TOY/CMOS battery backed clock that 
only ticks once a second.  If you spin until it ticks, you can get much closer 
than 1 second.  "Spin until" may not be convenient for most applications.


-- 
These are my opinions.  I hate spam.