ERTS timeouts don't account for computer suspension

[Rickard Green]

On Sun, May 10, 2020 at 11:05 PM Rickard Green <rickard@REDACTED> wrote:

> 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
>


I missed this part:

> I'd be thankful for your thoughts on this matter and whether you think
the default behaviour should be changed.

I do *not* think the default should be changed. I would be very surprised
if it is more than a very small minority of users that are interested in
elapsed time during OS suspend. More or less everyone would have to take
this performance penalty without any benefit. I don't remember any figures
since this was 5 years ago, but I remember that the performance penalty was
quite substantial.

Regards,
Rickard
-- 
Rickard Green, Erlang/OTP, Ericsson AB
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20200511/a3e0b7f7/attachment.htm>