[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

--- Comment #3 from Conrad Meyer <cem at freebsd.org> ---
I tried the following test:

$ cpuset -l 0 time sha1 <~200MBfile>
(burn one result for caching, then repeat 3 trials)
I got average 0.24 real, 0.22 user.  You might pick a slightly larger file for
bigger and more obvious differences at different steppings.

Then I set CPBDis: cpucontrol -m '0xc0010015|=0x02000000' /dev/cpuctl0
and repeated the tests; I got average 0.29 real, 0.26 user.

I ran "cpucontrol -m '0xc0010015&=~0x02000000' /dev/cpuctl0" and got 0.24/0.22
again.

I also tried sysctl dev.cpu.0.freq=3400 (on my system, dev.cpu.0.freq_levels:
3400/3825 2800/2765 2200/1952).

cpucontrol -m '0xc0010015' did not show CPBDis.  Also, SHA1 timings remained
~0.24/0.22.

I tried dev.cpu.0.freq=2800, waited a few seconds, and restored
dev.cpu.0.freq=3400.

cpucontrol -m '0xc0010015' still did not show CPBDis.  SHA1 timings seemed to
show the expected boosted performance.

My CPU is a Zen1 Threadripper 1950x; perhaps this behavior is different on Zen+
(Ryzen 2xxx), or there is some difference due to BIOSes.  But you might be
interested in running the same tests and seeing what you find.

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