Hal Murray wrote:
Right, I would assume the oscillator on a typical motherboard will
have too much variance to get down into the nanosecond range? When
you say system has special hardare, do you mean like an Oregano PCIe
card that the system could use as a reference or something else.
The crystals in most PCs are good thermometers. If you hold the temperature
stable the frequency will be pretty stable.
Most PCs use spread spectrum clocking to dance around the EMI rules. That's
an interesting monkey wrench. I'll have to work out the numbers sometime.
(If the data sheet tells you the spread and the modulation frequency, you
should be able to work out the max deviation from the long term average.)
Sometimes, the BIOS lets you turn that off.
There are 2 types of special hardware I was thinking of. One is a counter
that gets latched by the PPS signal. The other is a more stable clock.
The Soekris NET4501 allows this to be done.
You can also replace its 33.3333Mhz clock with a more stable one.
Another idea would be an add-on board, probably with a FPGA. Details TBD,
but the general idea is to feed it a good clock and a PPS signal.
[email protected] said:
I'd like to go to the next step. I'd also want to start graphing a
bit some of my ntpd usage (I assume folks basically grok peerstats and
look at the offsets over time?).
Look at loopstats and clockstats too. In particular, the drift in loopstats
is a good indication of system health.
There is another large rathole you can get sucked in. If you have good
clocks at both ends, you can measure one-way network times rather than just
round-trip times. If you want that info, look at rawstats.
Bruce
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