[erlang-questions] Get Erlang node uptime and way to test it is working

[Rickard Green]

Hmm, looked closer at statistics(wall_clock) and found that it is buggy 
on 32-bit architectures. This due to an unfortunate cast which causes it 
to wrap after approximately 50 days. This will be fixed in an upcoming 
patches for OTP 19 and 20.

Internally erlang:statistics(wall_clock) use Erlang monotonic time, so 
it will be as accurate as using erlang monotonic time directly. It is 
however not as efficient as using erlang monotonic time directly due to 
the locking that is needs. That is, the recommended way to get this info is:
  erlang:convert_time_unit(erlang:monotonic_time() - 
erlang:system_info(start_time), native, TheTimeUnitYouWant).

Regards,
Rickard, Erlang/OTP, Ericsson AB

On 06/22/2017 10:45 AM, Richard Carlsson wrote:
> Would be good to patch the uptime function in c.erl, in that case, since 
> that uses wall_clock.
> 
> 
>          /Richard
> 
> 2017-06-20 12:56 GMT+02:00 Rickard Green <rickard@REDACTED 
> <mailto:rickard@REDACTED>>:
> 
> 
>     tis 20 juni 2017 kl. 09:21 skrev Mithun B <mithunb@REDACTED
>     <mailto:mithunb@REDACTED>>:
> 
>         Hi all,
> 
>         We are using "erlang:statistics(wall_clock)" to get the Erlang
>         node up
>         time in millisecond.
> 
>         It works fine till 49 days, but on 49th day it will role back to
>         start
>         from 0 millisecond.
> 
>         I need to know two things here, First of all, a different method
>         for how
>         to test it, I don't want to wait for 49+ days to see it works or
>         not.
> 
>         Secondly, I need to know a method which gives proper up time
>         even after
>         49+ days. How about following:
> 
>         "(erlang:monotonic_time() - erlang:system_info(start_time)) div
>         1000000000"
> 
> 
>     Apart from the assumption of nanosecond native time unit this is the
>     way to do it. Use erlang:convert_time_unit() instead. There are
>     Erlang systems not using nanoseconds as native time unit.
> 
>     Regards,
>     Rickard Green, Erlang/OTP
> 
> 
>         If any one having more insight on this, please shed some light
>         on these
>         topics.
> 
>         Thanks and regards,
> 
>         Mithun B
> 
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>     -- 
>     Rickard Green, Erlang/OTP, Ericsson AB
> 
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