Saturday, January 26, 2013

BogoMIPS rating changed on RPi kernel?

BogoMIPS is unpromisingly described as "an unscientific measurement of CPU speed".
It's easily readable from the file /proc/cpuinfo and I've used it in the Conky display that I use in all of the OSes that I've played with on my Pi (Rasbian, Debian, Arch, Fedora, Gentoo).

Up until a couple of weeks ago, the value reported by BogoMIPS was extremely close to the cpu_freq value I had chosen for my /boot/config.txt.
In every single case, day after day, the BogoMIPS number was 99.5% of the selected cpu_freq.
So, if that's unscientific, it's still an awful lot closer to the assumed reality than many supposedly scientific measurements.

Note that in the link I provided that the "rating" for the RPi CPU is stated as being 1.0 meaning that the true CPU speed is equal to the BogoMIPS measure which is what I was seeing.

That is until the rpi-update (firmware update) I did on 9/01/2013.
Thereafter, the BogoMIPS rating seemed to change from 1.0 to 0.67
I saw this in all of my OSes and it's continued through 3-4 subsequent rpi-updates.

Strangely, it's not been mentioned or seemingly known about in the Raspberry community.
I've seen the phenomenon mentioned in only two recent threads in the RPi forum (here and here) without any explanation given or concern expressed.

I posted to the forum myself a few days ago, but once again nobody either corroborated  or explained my observation.

Admittedly, the observed change is a matter of very little consequence unless you use BogoMIPS to tell you what your cpu speed is.

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