Yo ASSI! On Sun, 24 Nov 2019 07:37:13 +0100 ASSI via devel <devel@ntpsec.org> wrote:
> Gary E. Miller via devel writes:
> > There is a gpsd program in the contrib/ directory. It tests your
> > CPU granularity. On a Raspberry Pi that is about 52 ns. Worse
> > on an Intel chip.
>
> The actual granularity on RasPi can't be better than 52ns (the clock
> it's based on is 19.2MHz) and you can determine it rather precisely
> since you can read quite a bit faster than the clock actually ticks.
Yes, exactly what clock_test does. And clock_test does more!
Also check out attic/clocks in NTPsec.
> On Intel it is typically around 17ns if you use TSC, but the clock
> it's based on is 200MHz, so you read slower than the clock ticks. If
> you use HPET as the clock source (14.318MHz typically, although only
> specified as
> >=10MHz), you'll see around 72ns granularity.
Or just use clock_test and see. Theory is great, but I trust data.
The granularity is important, but the fun part is the spread of results.
An interrupt here, a CPU swap there, and it gets real bad real fast.
ntpd could do a better job of tossing out wild measurements. Maybe someday.
RGDS
GARY
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"If you can't measure it, you can't improve it." - Lord Kelvin
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