2. 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.

2.1 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@norgana.systemy.it, and by 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.

2.2 How to estimate what the proper BogoMips rating should be

From a initiative by Ian Jackson, ijackson@nyx.cs.du.edu, and Przemek Klosowski, much updated and expanded by Wim van Dorst for current data, as listed below:

As a very approximate guide, the BogoMips can be calculated by:

System                      BogoMips           Comparison 
Intel 8088                  clock * 0.004         0.02 
Intel/AMD 386SX             clock * 0.14          0.8 
Intel/AMD 386DX             clock * 0.18          1 (definition) 
Motorola 68030              clock * 0.25          1.4 
Cyrix/IBM 486               clock * 0.34          1.8 
Intel Pentium               clock * 0.40          2.2 
Intel 486                   clock * 0.50          2.8
AMD 5x86                    clock * 0.50          2.8 
Mips R4000/R4400            clock * 0.50          2.8
Motorola 68040              clock * 0.67          3.7
Nexgen Nx586                clock * 0.75          4.2 
PowerPC 601                 clock * 0.84          4.7

Alpha 21064/21064A          clock * 0.99          5.5
Alpha 21066/21066A          clock * 0.99          5.5 
Alpha 21164/21164A          clock * 0.99          5.5 
Intel Pentium Pro           clock * 0.99          5.5 
Cyrix 5x86/6x86             clock * 1.00          5.6 
Intel Pentium II/III        clock * 1.00          5.6
AMD K7/Athlon               clock * 1.00          5.6
Intel Celeron               clock * 1.00          5.6
Mips R4600                  clock * 1.00          5.6 

Alpha 21264                 clock * 1.99         11.1
AMD K5/K6/K6-2/K6-III       clock * 2.00         11.1
AMD Duron                   clock * 2.00         11.1
UltraSparc II               clock * 2.00         11.1
Pentium MMX                 clock * 2.00         11.1 
Pentium 4                   clock * 2.00         11.1
Centaur C6-2                clock * 2.00         11.1
PowerPC 604/604e/750        clock * 2.00         11.1
Motorola 68060              clock * 2.01         11.2 

IBM S390                    not enough data (yet)

Note that the BogoMips calculation loop does not take full advantage of the parallelism of various processors, such as the Intel Pentium and the Alpha 21164.

2.3 How to determine what the current BogoMips rating is

There are three methods to determine the current BogoMips, viz.

1. looking in /proc/cpuinfo, e.g., with `cat /proc/cpuinfo`. This method is highly preferred above the alternatives.
2. looking in the syslog output to see what was printed there during booting (if necessary retrieving the information explicitly with dmesg or syslogk). This alternative gives accurate information but is more work.
3. using the standalone bogomips program. This is only recommended for non-Linux system, for reasons pointed out below.

A non-determinative alternative, which is also applicable for non-Linux systems such as Crays and so, may be the standalone BogoMips program. From the readme file by Jeff Tranter, jeff_tranter@mitel.com:

`Tired of rebooting your system so you can see how many BogoMips it's running at today? [...] "Bogomips" is a standalone program that displays your system performance using one of the world's most recognized benchmarks. It uses the same code that is used in the Linux kernel while booting, but runs as a user program. [...] Version 1.3 of BogoMips is now portable and should run on any system that supports an ANSI C compiler and library.'

Note that due to system load values calculated with the standalone program on Linux systems may be lower than registered in the list below. Intrinsically the standalone cannot give precisely similar information to the boot sequence BogoMips, since system load will compete with this program run by an ordinary user. Therefore only boot sequence BogoMips ratings are listed below.

Be aware that the file sunsite.unc.edu:/pub/Linux/system/status/bogo-1.2.tar.gz/ contains the latest version 1.3 (sic), which is yet rather outdated. Nonetheless, due to its good portability, it should compile and run on nearly every system with a C compiler.

2.4 Variations in BogoMips rating

From Linus Torvalds, torvalds@cc.helsinki.fi, explaining about the variation one may see in the BogoMips rating, in c.o.l.development, at 28 April 1994

`The BogoMips calculation loop is "quantizised", so you're most likely to get the exact same number all the time. You usually will get different numbers only if the speed is just on the "edge", when small variations (different time for interrupt ticks etc) will make it jump from one value to the other.'

If a kernel is not compiled specifically for the pertaining CPU, also some (even large) variations of the BogoMips ratings can occur, mainly due to erroneous alignment. This problem apparently only occurs on the various x86 CPUs (Intel and clones). Fortunately it can easily be solved: recompile the kernel specifically for your CPU.

2.5 New BogoMips algorithm?

No, the BogoMips algorithm, contrarily to popular believe, did NOT change over the various kernel versions. Furthermore, it is intrinsically the same for all CPUs of all makes.

What did change in kernel version 2.2.14 is the CPU state setting just before the BogoMips calculation. This affects the BogoMips rating for all Intel and AMD Pentium variations, resulting in approximately 2*clock, where these were not 2*clock.

The entries based on these upgraded kernel versions for CPUs where this change was actually relevant for (Pentium II/III, Celeron, Athlon) are marked in the list below with an initial * (star symbol).

2.6 BogoMips ... failed

Suggested by various questions on the net and private mail, e.g., by Lily, lbliao@alumni.caltech.edu, and by Pierre Frenkiel, frenkiel@cdfap2.in2p3.fr. In March 1995 they asked:

`When I boot Linux I get the message:

      Calibrating delay loop.. ok - 23.96 BogoMips
      failed

Where/why has the calibration delay loop failed?'

It didn't fail. If it had failed the text would have been

      Calibrating delay loop.. failed

What likely did fail was a driver for some gadget which you may not have in your machine. Just after calculating the BogoMips rating all device drivers are initiated. First the SCSI devices, then Net devices, etc. Any failure is duly reported. Noteworthy is the AHA152x driver. Other effects of failing drivers (and not of failing BogoMips calculations) are systems crashes, long waits, and complete system locks, somewhere close to (just before or just after) outputting the ok - xx.xx BogoMips text.

Since Linux 1.2 many error messages have improved, so upgrade to at least that version to find out which particular driver it is that is failing. Also recompile the kernel to only include the drivers that are actually needed for your hardware configuration.

2.7 What about clone CPUs (Cyrix, NexGen, AMD, etc)

Cyrix 486-like CPUs need cache enabling software, sometimes referred to as BogoBoost software. Cyrix 5x86 and 6x86 CPUs may have their BogoMips improved drastically by branch-prediction (BIOS option). Note that the performance improvement may be marginal. There are several packages available for adjusting Cyrix CPUs, such as the BogoBoost patch, cx5x86mod, and set6x86, all from the normal archives, in obvious places. It is reported the Cyrix 6x86 CPUs may give better performance when the kernel is compiled with 486-optimization, instead Pentium-optimization.

NexGen 386-enhanced CPUs, marked as Nx586, are listed as 386-like, since the fact that they are performing like Pentium machines is not relevant to BogoMips.

AMD 5x86, also denoted as AMD 486DX5, are quadrupled 486/33 machines. They are fully in line with other 486 CPUs. The AMD K5 and the K6 are Pentium-like CPUs, with their own BogoMips multipliers.

2.8 Why to pay attention to BogoMips

Let me add that there are only two reasons for paying attention to the BogoMips rating that is presented on booting Linux:

1. To see whether it is in the proper range for the particular processor, its clock frequency, and the potentially present cache. Many CPUs are prone to faulty setups of
   • memory cache setting (write-back is wrong for BogoMips, often reported lower than 5; write-through is ok)
   • turbo-buttons (should be ON)
   • BIOS-software emulated fake cache (change it for real cache)
   • similar cache and clock related things, sometimes also BIOS-software related
2. To see whether your system is faster than mine. Of course this is completely wrong, unreliable, ill-founded, and utterly useless, but all benchmarks suffer from this same problem. So why not use it? This inherent stupidity has never before stopped people from using benchmarks, has it? :-)

Note that more serious uses for real benchmarking are addressed in the Linux Benchmarking Howto by André D. Balsa.