<div dir="ltr"><div dir="ltr"><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sun, May 10, 2020 at 9:20 PM Guilherme Andrade <<a href="mailto:g@gandrade.net">g@gandrade.net</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Hello list,</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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: "<span>--enable-prefer-elapsed-monotonic-time-during-suspend" and "<span>--disable-prefer-elapsed-monotonic-time-during-suspend", introduced[4] in OTP 18.0.2.<br></span></span></div><div><span><span><br></span></span></div><div><span><span>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.<br><br>I would like to know the rationale for this. Is it for:</span></span></div><div><span><span>- performance reasons? (say, because it's faster and server hardware rarely suspends, if at all)<br></span></span></div><div><span><span>- intended behaviour? (contrary to my expectations)</span></span></div><div><span><span><br></span></span></div><div><span><span>I'd be thankful for your thoughts on this matter and whether you think the default behaviour should be changed.<br></span></span></div><div><br></div><div>[1]: <a href="https://github.com/erlware/erlcron" target="_blank">https://github.com/erlware/erlcron</a></div><div>[2]: <a href="https://gist.github.com/g-andrade/508c779a931dde14c22c6e96319caa24" target="_blank">https://gist.github.com/g-andrade/508c779a931dde14c22c6e96319caa24</a><br>[3]: <a href="http://erlang.org/documentation/doc-10.5/doc/installation_guide/INSTALL.html#configuring" target="_blank">http://erlang.org/documentation/doc-10.5/doc/installation_guide/INSTALL.html#configuring</a></div><div>[4]: <a href="https://github.com/erlang/otp/commit/4a864c1cbe16a42f3f5190881187e3c9849e985f" target="_blank">https://github.com/erlang/otp/commit/4a864c1cbe16a42f3f5190881187e3c9849e985f</a></div><div>[5]: <a href="https://github.com/erlang/otp/blob/OTP-22.3.4/erts/aclocal.m4#L2355-L2358" target="_blank">https://github.com/erlang/otp/blob/OTP-22.3.4/erts/aclocal.m4#L2355-L2358<br></a></div><div><a href="https://github.com/erlang/otp/blob/OTP-22.3.4/erts/aclocal.m4#L2355-L2358" target="_blank"></a></div><div><div><br>-- <br><div dir="ltr"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr">Guilherme<br></div></div></div></div></div></div></div></div></div>
</blockquote></div><br clear="all"><div><div>Yes, for performance reasons. See release note from OTP 18.0.2 <<a href="http://erlang.org/download/OTP-18.0.2.README">http://erlang.org/download/OTP-18.0.2.README</a>>:<br><pre> 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.
</pre><br><br></div>Regards,<br></div>Rickard<br>-- <br><div dir="ltr" class="gmail_signature">Rickard Green, Erlang/OTP, Ericsson AB</div></div></div>