loader: Set twiddle globaldiv to 16 by default
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Authored by cperciva on Sep 27 2021, 10:56 PM.

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allanjude
jrtc27
kevans
manu

Commits

rG45156a946877: loader: Set twiddle globaldiv to 16 by default
rG7457840230c5: loader: Set twiddle globaldiv to 16 by default

Summary

Booting FreeBSD on an EC2 c5.xlarge instance, the loader "twiddles" 810 times
over the course of 510 ms, a rate of 1.59 kHz. Even accepting that many systems
are slower than this particular VM and will take longer to boot (especially if using
spinning-rust disks), this seems like an unhelpfully large amount of twiddling when
compared to the ~60 Hz frame rate of many displays; printing the twiddles also
consumes roughly 10% of the boot time on the aforementioned VM.

Setting the default globaldiv to 16 dramatically reduces the time spent printing
twiddles to the console while still twiddling at roughly 100 Hz; this should be
ample even for systems which take longer to boot and consequently twiddle
slower.

Note that this can adjusted via the twiddle_divisor variable in loader.conf, but
that file is not processed until nearly halfway through the loader's runtime.

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Event Timeline

cperciva created this revision.Sep 27 2021, 10:56 PM

Herald added a subscriber: dab. · View Herald TranscriptSep 27 2021, 10:56 PM

cperciva requested review of this revision.Sep 27 2021, 10:56 PM

stand/defaults/loader.conf, loader(8) and loader_simp(8) all reference this and need to be kept consistent with reality

2nd the documentation updates required, but I agree this change is a good one.

Added documentation.

cperciva added inline comments.Sep 28 2021, 12:34 AM

stand/common/console.c
59	Note that the value here doesn't affect loader behaviour since twiddle's globaldiv is only adjusted when twiddle_divisor changes; but keeping this up to date results in the right value landing in the kernel environment.