[time-nuts] Cheap jitter measurements

Gary E. Miller gem at rellim.com
Tue Apr 10 18:58:51 EDT 2018


Yo Hal!

On Tue, 10 Apr 2018 15:41:41 -0700
Hal Murray <hmurray at megapathdsl.net> wrote:

> > Yeah, but the granularity is much worse.  
> 
> Do you have code that demonstrates that?  (or tell me how and I'll
> try it)

Look in gpsd git head.  In the contrib directory here is a program:
clock_test.c 

It tests the time to do two back to back clock_gettime().  Output
looks like this:

# ./clock_test 
samples 101, delay 10000000 ns
min 67 ns, max 302 ns, mean 176 ns, median 197 ns, StdDev 52 ns

You run it a few times and you will see the granularity in the
measurements is 10's of ns, or more, depending on your CPU.

Probably better discussed on the gpsd-users@ or NTPsec devel@ mailing
list.  67 ns not exactly time-nuts precision.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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