[NTP measurements] Minimum offset if negative
Measurement #74488034 says on <https://atlas.ripe.net/measurements/74488034>: "Min Offset: -1 173,799". Since offsets can be positive or negative, I suggest it is not a good idea to take the minimum without first removing the sign. Here, the real minimum offset was -0.000345 (the closest from the server) and the maximum -1.173799 (the farthest from the server). (There is also a strange formatting on the Web, and a lack of units.)
Measurement #74488034 says on <https://atlas.ripe.net/measurements/74488034>: "Min Offset: -1173,799". Since offsets can be positive or negative, I suggest it is not a good idea to take the minimum without first removing the sign. Here, the real minimum offset was -0.000345 (the closest from the server) and the maximum -1.173799 (the farthest from the server).
In math. min(-2, -1) = -2 Why would that be different for offset?
On Fri, Jun 28, 2024 at 12:14:45PM +0200, Philip Homburg <pch-atlas-ml@u-1.phicoh.com> wrote a message of 11 lines which said:
In math. min(-2, -1) = -2
Why would that be different for offset?
Because this is not Math 101 but NTP. You typically don't care about the sign of the offset, only about its absolute value.
In your letter dated Fri, 28 Jun 2024 13:24:21 +0200 you wrote:
On Fri, Jun 28, 2024 at 12:14:45PM +0200, Philip Homburg <pch-atlas-ml@u-1.phicoh.com> wrote a message of 11 lines which said:
In math. min(-2, -1) = -2
Why would that be different for offset?
Because this is not Math 101 but NTP. You typically don't care about the sign of the offset, only about its absolute value.
So you want the smallest and largest absolute offset. You already get the minimum and maximum offset. So the maximum absolute offset should be easy to spot. Showing only the maximum absolute offset is not great. It is useful to know if on average probes are behind or ahead of the time source. Only showing absolute offsets doesn't show that. Why would you want to know the smallest absolute offset? That's just measuring noise.
On Fri, 28 Jun 2024, Stephane Bortzmeyer wrote:
Measurement #74488034 says on <https://atlas.ripe.net/measurements/74488034>: "Min Offset: -1 173,799". Since offsets can be positive or negative, I suggest it is not a good idea to take the minimum without first removing the sign. Here, the real minimum offset was -0.000345 (the closest from the server) and the maximum -1.173799 (the farthest from the server).
min( abs( value_1 ) , abs( value_2 ) ) would be the minimum difference, not the minimum offset. Both minimum difference and minimum offset can be useful, but they're not the same thing. -- Povl Ole
On Fri, Jun 28, 2024 at 12:22:45PM +0200, Povl Ole Haarlev Olsen <ripe-atlas@stderr.dk> wrote a message of 26 lines which said:
min( abs( value_1 ) , abs( value_2 ) ) would be the minimum difference, not the minimum offset.
I'm afraid I don't understand. There is no minus sign so no difference computed. IMHO, min(map(abs, values)) *is* the minimum offset.
participants (3)
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Philip Homburg -
Povl Ole Haarlev Olsen -
Stephane Bortzmeyer