ConnorRigby
I have a system in a project where an end user schedules something to happen. I use Process.send_after/3 to do the actual scheduling, i hadn’t really considered the accuracy of sending a message to yourself long in the future. (in this case about 24 hours). I noticed this morning that my message was received at 9:40 AM, when it was scheduled to run at 10:05.
I know how to fix this issue pretty easily. I was just wondering if anyone has hit this before, or if anyone knows what causes the skew?
FWIW this is a Nerves app deployed to an RPI3. It has no real time clock, so the time on the device skews by about a minute before ntpd fixes it. i don’t feel as if the timer otp application uses the system time internally tho does it?
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outlog
quite interesting since the RPI is most likely one of very few places the BEAM runs without rtc.
send_after uses monotonic time, and afaik by default the BEAM runs with no time warp mode Time and Time Correction in Erlang — OTP 29.0.2 (erts 17.0.2)
my unqualified guess would be if the time skews in “wrong” direction, monotonic time will still increase (being monotonic, after all) - and you end up with way of accuracy..
I wonder if using(and calculating) the absolute monotonic time, using the :abs option provides better accuracy, or is still prone to this inaccuracy Process — Elixir v1.20.2
I’ve never quite understood how (especially the more modern) RPIs lacks rtc on the board..
so maybe try and check the time warp mode used by nerves, and try and use single time warp in the vm.args..
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ConnorRigby
Very interesting. I will have a read through this.