Bogomips mini-Howto - Directory UMM

3. The frequently asked questions about BogoMips
Several authors have contributed to my knowledge of BogoMips. In this place, I would like to thank them
highly.
3.1. What are BogoMips
Quoted from the Internet, origin unknown but brought to the attention by Eric S Raymond esr@thyrsus.com,
and Geoff Mackenzie freon@dialstart.net, there is an humourously illustrative definition of BogoMips as ''the
number of million times per second a processor can do absolutely nothing.''
On a more precise basis, from Lars Wirzenius' wirzeniu@kruuna.Helsinki.FI mail of 9 September 1993,
explaining Bogomips, with additional detailed information by Alessandro Rubini,
rubini@morgana.systemy.it, and by howto−author Wim van Dorst:
`MIPS is short for Millions of Instructions Per Second. It is a measure for the computation
speed of a program. Like most such measures, it is more often abused than used properly (it is
very difficult to justly compare MIPS for different kinds of computers).
BogoMips are Linus's invention. The kernel (or was it a device driver?) needs a timing loop
(the time is too short and/or needs to be too exact for a non−busy−loop method of waiting),
which must be calibrated to the processor speed of the machine. Hence, the kernel measures
at boot time how fast a certain kind of busy loop runs on a computer. "Bogo" comes from
"bogus", i.e, something which is a fake. Hence, the BogoMips value gives some indication of
the processor speed, but it is way too unscientific to be called anything but BogoMips.
The reasons (there are two) it is printed during boot−up is that a) it is slightly useful for
debugging and for checking that the computers caches and turbo button work, and b) Linus
loves to chuckle when he sees confused people on the news.'
BogoMips are being determined in /usr/src/linux/init/main.c (simple C algorithm, with a nice
example of floating point arithmetic within the fully integer kernel), and the pertaining kernel variable
loops_per_sec is used in several drivers for more serious purpose. The actual delay function udelay() is in
assembler, and therefore each port has its own definition in /include/asm/delay.h. The loops_per_sec
variable and the udelay() function are used in numerous drivers, see:
cd /usr/src/linux #or where else source is located
find . −name '*.[hcS]' −exec fgrep loops_per_sec {} /dev/null \;
find . −name '*.[hcS]' −exec fgrep udelay {} /dev/null \;
The BogoMips calculation loop for the non Intel CPUs is similar but not the same, because it is programmed
in another assembler language. BogoMips is however the only portable way over the various CPUs
(Intel−type and non Intel−type) for getting an indication of the CPU speed. Even CPU clock speed is not
available on all CPUs.
3. The frequently asked questions about BogoMips 3