Is it safe to change system date/time during execution

asked 2011-07-26 20:14:21 -0500

Victor Lopez
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updated 2011-07-30 12:45:32 -0500

tfoote

58457 ●128 ●550 ●526 http://www.ros.org/

Reading on how ros Time is implemented, it looks like it uses system wall-time. http://www.ros.org/wiki/Clock

Would it be safe to change system time when ros nodes are running?

We plan to synchronize the time between robots using NTP on startup using Wi-Fi.

But if a robot was powered on out of Wi-Fi range, started all it's nodes, and then (once in Wi-Fi range) changed it's system time using NTP, we are afraid that those nodes would start acting weird.

This question is both for C++ and python implementation of Time.

answered 2011-07-26 23:11:21 -0500

dornhege

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It is not safe as you already suspected. Timestamps might jump and this might lead to inconsistencies. If you sync with NTP before, there shouldn't be much difference, though.

It is recommended to use chrony for your application, although there still is the problem that the times might get out of sync. In a comparably short time that should not matter unless you are synching high-frequency sensor data between different machines.

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I was thinking about using a Clock Server that publishes time based on monotonic time, therefore a change on system time wouldn't cause timestamp jumps, I'll have to test it which harmful effects this has.

Victor Lopez ( 2011-07-26 23:18:21 -0500 )edit

I think that wouldn't solve the problem. A clock server still has to run on one of the machines and once you have a disconnect the problem is again the sync. You can see one systems clock as a clock server.

dornhege ( 2011-07-27 03:40:04 -0500 )edit

This is why chrony is recommended. It doesn't jump the clock it skews the rate to synchronize instead of jumping time.

tfoote ( 2011-07-30 12:44:55 -0500 )edit

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