4 ms·

…why? My napkin math says a pair of observers, one at sea level and one on the top of a 14,000ft mountain, will drift by about 15 microseconds per year. In oth

…why? My napkin math says a pair of observers, one at sea level and one on the top of a 14,000ft mountain, will drift by about 15 microseconds per year. In other words, the approximation that time is not relative breaks down at this level of accuracy - there’s literally no value in trying to synchronize things this closely.

perfmode 3y ago
See: Spanner
dragontamer 3y ago
My 60-cent 32678 Hz crystal at 12.5pf is specified at +/- 20ppm (aka: 0.002% accuracy). But with a caveat: I need to load the crystal with exactly 12.5pf capacitance. So sure, I can go grab 2x 25pF capacitors and load the crystal to ground (series capacitors half the capacitance, so 2x 25pF in series == 12.5pf). Except... what? Trace-capacitance and on-chip capacitance hasn't been factored in. Hmmmmm... okay. Does anyone have a picoFarad accurate multimeter? Oh snap, those are expensive. No, the easiest way to tune a crystal oscillator to its appropriate accuracy is through the use of a ground truth clock, and comparing that ground-truth clock vs the clock. So I make my guess with 2x 9pF capacitors (5pF internal capacitance on the chip x2 pins + 2pF estimated trace capacitance) and measure the result against the ground truth. Well, to calibrate a 20ppm crystal oscillator means I need a time sync service that has 20ppm accuracy or better. That's ya know: +/- 20 microseconds per second accuracy. ----------- That's my cheap 60-cent crystal by the way. If you go with a temperature-controlled oven crystal oscillator for better accuracy, you'll need more accuracy to tune that.
advisedwang 3y ago
That's even more reason to sync to the microsecond level. Computer systems rarely need an an accurate representation of local time. Instead they benefit from being closely aligned with each other. So unavoidable drift at the microsecond level is a reason to sync to the microsecond level.
sheepshear 3y ago
Correcting drift is exactly what a disciplined clock does. It's the solution to the problem you mentioned.
layer8 3y ago
Most systems don’t care much about their clocks being physically super accurate, but they very much care their clocks being tightly in sync with the rest of the world. That’s what the sync service accomplishes.
mlyle 3y ago
If you have times that are synced to within a few microseconds, then any logs you have will have much less ambiguity to the sequence of events that happened in a system that crosses multiple computers. You also get a more unambiguous (but still imperfect) measure of "what event outside the system happened first."

perfmode 3y ago

See: Spanner

dragontamer 3y ago

My 60-cent 32678 Hz crystal at 12.5pf is specified at +/- 20ppm (aka: 0.002% accuracy). But with a caveat: I need to load the crystal with exactly 12.5pf capacitance. So sure, I can go grab 2x 25pF capacitors and load the crystal to ground (series capacitors half the capacitance, so 2x 25pF in series == 12.5pf). Except... what? Trace-capacitance and on-chip capacitance hasn't been factored in. Hmmmmm... okay. Does anyone have a picoFarad accurate multimeter? Oh snap, those are expensive. No, the easiest way to tune a crystal oscillator to its appropriate accuracy is through the use of a ground truth clock, and comparing that ground-truth clock vs the clock. So I make my guess with 2x 9pF capacitors (5pF internal capacitance on the chip x2 pins + 2pF estimated trace capacitance) and measure the result against the ground truth. Well, to calibrate a 20ppm crystal oscillator means I need a time sync service that has 20ppm accuracy or better. That's ya know: +/- 20 microseconds per second accuracy. ----------- That's my cheap 60-cent crystal by the way. If you go with a temperature-controlled oven crystal oscillator for better accuracy, you'll need more accuracy to tune that.

advisedwang 3y ago

That's even more reason to sync to the microsecond level. Computer systems rarely need an an accurate representation of local time. Instead they benefit from being closely aligned with each other. So unavoidable drift at the microsecond level is a reason to sync to the microsecond level.

sheepshear 3y ago

Correcting drift is exactly what a disciplined clock does. It's the solution to the problem you mentioned.

layer8 3y ago

Most systems don’t care much about their clocks being physically super accurate, but they very much care their clocks being tightly in sync with the rest of the world. That’s what the sync service accomplishes.

mlyle 3y ago

If you have times that are synced to within a few microseconds, then any logs you have will have much less ambiguity to the sequence of events that happened in a system that crosses multiple computers. You also get a more unambiguous (but still imperfect) measure of "what event outside the system happened first."