ERTS timeouts don't account for computer suspension

[Rickard Green]

On Sun, May 10, 2020 at 9:20 PM Guilherme Andrade <g@REDACTED> wrote:

> Hello list,
>
> I've recently run into an unexpected behaviour of erlcron[1] - it doesn't
> account for any time period the computer spends suspended - scheduled
> events get delayed by equal amounts.
>
> This is because it leverages gen_server timeouts which, in turn, use
> bog-standard VM timeouts, which behave in the way I described. I've
> confirmed[2] their behaviour by comparing time elapsed until timeout
> according to erlang:monotonic_time/1 vs. os:system_time/1.
>
> After delving into the Erlang/OTP source code for a while I realized there
> are two configuration options[3] that appear to control the unexpected
> behaviour: "--enable-prefer-elapsed-monotonic-time-during-suspend" and "--disable-prefer-elapsed-monotonic-time-during-suspend",
> introduced[4] in OTP 18.0.2.
>
> The default value of the setting configured by either of those two options
> is "no"[5] - that is, elapsed monotonic time during suspend is not prefered.
>
> I would like to know the rationale for this. Is it for:
> - performance reasons? (say, because it's faster and server hardware
> rarely suspends, if at all)
> - intended behaviour? (contrary to my expectations)
>
> I'd be thankful for your thoughts on this matter and whether you think the
> default behaviour should be changed.
>
> [1]: https://github.com/erlware/erlcron
> [2]: https://gist.github.com/g-andrade/508c779a931dde14c22c6e96319caa24
> [3]:
> http://erlang.org/documentation/doc-10.5/doc/installation_guide/INSTALL.html#configuring
> [4]:
> https://github.com/erlang/otp/commit/4a864c1cbe16a42f3f5190881187e3c9849e985f
> [5]:
> https://github.com/erlang/otp/blob/OTP-22.3.4/erts/aclocal.m4#L2355-L2358
> <https://github.com/erlang/otp/blob/OTP-22.3.4/erts/aclocal.m4#L2355-L2358>
>
> --
> Guilherme
>

Yes, for performance reasons. See release note from OTP 18.0.2 <
http://erlang.org/download/OTP-18.0.2.README>:

  OTP-12895    Application(s): erts

               *** POTENTIAL INCOMPATIBILITY ***

               Changed default OS monotonic clock source chosen at
               build time. This in order to improve performance. The
               behavior will now on most systems be that (both OS and
               Erlang) monotonic time stops when the system is
               suspended.

               If you prefer that monotonic time elapse during suspend
               of the machine, you can pass the command line argument
               --enable-prefer-elapsed-monotonic-time-during-suspend
               to configure when building Erlang/OTP. The
               configuration stage will try to find such a clock
               source, but might not be able to find it. Note that
               there might be a performance penalty associated with
               such a clock source.



Regards,
Rickard
-- 
Rickard Green, Erlang/OTP, Ericsson AB
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