[Bug 234733] Setting CPU frequency with sysctl dev.cpu.0.fr slows a Ryzen 2700X down

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234733

            Bug ID: 234733
           Summary: Setting CPU frequency with sysctl dev.cpu.0.fr slows a
                    Ryzen 2700X down
           Product: Base System
           Version: 12.0-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: freebsd.ed.lists at sumeritec.com

Issuing

sysctl dev.cpu.0.fr=3700

or any other value for frequency slows the system down by nearly 50%.

System has a Ryzen 2700X as a CPU.

How to repeat:

Reboot the system with all power saving tools disabled.
Compile the world, it will take around 2h30min on the system mentioned above if
that system call is never used.

Now issue the command

sysctl dev.cpu.0.fr=3700

which would bring a Ryzen 2700X to its nominal operating frequency.

Compile the world, it will take now around 4h30min.

I could repeat this error several times thinking originally that it is a powerd
problem.

An e-mail by Andrea Venturoli (21 Dec 2018 17:40:49) made me look a bit into
powerd.

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