7 ms·

Mark Twain has a quote for everything! An ancient adage warns, "Never go to sea with two chronometers; take one or three." — Fred Brooks

nix23 5y ago
And for NTP, take 5 because two could tell the same lie ;)
- karmakaze 5y ago
  And how do you use them--only use the median value?
  - nix23 5y ago
    2 vs 2 you don't know who to trust, but if you have 3 vs 2 you know who you ~can trust (more), that's why 5 and not 4. But if you have a atomic clock don't trust those 5 ntp's at all. Maybe buy a 2nd clock or even better 5 of them ;)
    - karmakaze 5y ago
      I would assume that all 5 give slightly differing estimates of the current time [a, b, c, d, e] with duplicates being rare and not treated specially than values that are close. You can either choose 'c' to ignore the outliers, or average [b, c, d] or some weighted average of them or more. Maybe the weighted average of the middle 3 might be more stable than the single median.
      - nix23 5y ago
        https://en.wikipedia.org/wiki/Precision_Time_Protocol https://en.wikipedia.org/wiki/Precision_Time_Protocol
        karmakaze 5y ago
        > The best master clock (BMC) algorithm performs a distributed selection of the best candidate clock based on the following clock properties: (IEEE 1588-2008 uses a hierarchical selection algorithm based on the following properties, in the indicated order:[8]:Figure 27) > Priority 1 – the user can assign a specific static-designed priority to each clock, preemptively defining a priority among them. Smaller numeric values indicate higher priority. > Class – each clock is a member of a given class, each class getting its own priority. Accuracy – precision between clock and UTC, in nanoseconds (ns) > Variance – variability of the clock > Priority 2 – final-defined priority, defining backup order in case the other criteria were not sufficient. Smaller numeric values indicate higher priority. > Unique identifier – MAC address-based selection is used as a tiebreaker when all other properties are equal. Only useful if you trust the data: Class, Accuracy, and Variance (which seems to be a self-measured estimate).

nix23 5y ago

And for NTP, take 5 because two could tell the same lie ;)

karmakaze 5y ago
And how do you use them--only use the median value?
- nix23 5y ago
  2 vs 2 you don't know who to trust, but if you have 3 vs 2 you know who you ~can trust (more), that's why 5 and not 4. But if you have a atomic clock don't trust those 5 ntp's at all. Maybe buy a 2nd clock or even better 5 of them ;)
  - karmakaze 5y ago
    I would assume that all 5 give slightly differing estimates of the current time [a, b, c, d, e] with duplicates being rare and not treated specially than values that are close. You can either choose 'c' to ignore the outliers, or average [b, c, d] or some weighted average of them or more. Maybe the weighted average of the middle 3 might be more stable than the single median.
    - nix23 5y ago
      https://en.wikipedia.org/wiki/Precision_Time_Protocol https://en.wikipedia.org/wiki/Precision_Time_Protocol
      - karmakaze 5y ago
        > The best master clock (BMC) algorithm performs a distributed selection of the best candidate clock based on the following clock properties: (IEEE 1588-2008 uses a hierarchical selection algorithm based on the following properties, in the indicated order:[8]:Figure 27) > Priority 1 – the user can assign a specific static-designed priority to each clock, preemptively defining a priority among them. Smaller numeric values indicate higher priority. > Class – each clock is a member of a given class, each class getting its own priority. Accuracy – precision between clock and UTC, in nanoseconds (ns) > Variance – variability of the clock > Priority 2 – final-defined priority, defining backup order in case the other criteria were not sufficient. Smaller numeric values indicate higher priority. > Unique identifier – MAC address-based selection is used as a tiebreaker when all other properties are equal. Only useful if you trust the data: Class, Accuracy, and Variance (which seems to be a self-measured estimate).