[Bug 270785] Performance and power efficiency regression due to pthread_cond_timedwait() changes

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270785

--- Comment #6 from bllcha013@myuct.ac.za ---
Interestingly, there is a bug report for Minecraft that seems related:
https://bugs.mojang.com/browse/MC-183518

It seems that an extremely low timeout is causing massive numbers of calls per
second. This issue wasn't noticeable before, but now is, probably due to the
increased precision of timing allowed with the commit. For Linux, changing the
value in /proc/<pid>/timerslack_ns apparently helps, but it isn't there on
FreeBSD.

I noticed that the difference between "steps" of the tv_nsec value in the
dtrace output was seemingly tied to kern.hz before the commit, so kern.hz=100
would give a 10ms step, and kern.hz=1000 would give a 1ms step, no matter what
kern.eventtimer.periodic is set to, which I confirmed with dtrace. After the
commit, kern.hz doesn't seem to have an impact when kern.eventtimer.periodic=0:
the step is always 0.1ms. However, using kern.eventtimer.periodic=1 changes
things: it becomes 0.5ms with kern.hz=1000 and 2.5ms with kern.hz=100.

From my (limited) understanding, the commit improves the precision of timed
waits, but the old behavior inadvertently helped with CPU utilization and power
efficiency by preventing software from being too abusive with short timeouts
and constantly waking the CPU.

Please let me know if there is any other information I can provide.

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