How are BogoMips Calculated (or how can the calculation go wrong)

How are BogoMips Calculated (or how can the calculation go wrong)

Post by Hugh McCur » Sat, 18 May 1996 04:00:00

I've read the mini-howto on bogomips.

The basic question is "what is the calculation method?"  And  more
specifically "how can it fail?"  

Plus "is it reasonable to change loops_per_sec in the kernel to match
what the bogomips should be in the calculation seems flawed?"

The problem is on a KeyData Notebook.  486DX 4/100, Chipset is Pine
from PicoPower.  Phoenix BIOS A486 v 1.03 for PicoPower PT86C 368/388
MB5 V1.10 (the latter related to a custom design for the notebook

A Cyclades card is plugged into the docking station.  The Cyclades
card generally doesn't work.  It will work in another computer.

Upon talking to Cyclades & KeyData, we found that the Bogomips were
being reported at 6.55 when they should be something like 50.08.

The Cyclades driver (and perhaps other drivers) relies on the Bogomips
calculation being a bit more accurate.

When KeyData tried a 486DX/33, the Cyclades worked 7 of 7 times.  The
Bogomips report from that chip was 5.8 and should have been about 16.
So, that supports Cyclades theory that the problem is a bad bogomips

I'm going to try a quick & dirty fix of just multiplying the delays in
cyclades.c by 7 or 8.  That's the "hack" fix.  However, I'd like to
take a stab at the general problem.

But all I have is odd-ball ideas.  Could the caching be off during the
calculation and then magically turn itself on?  If that was true, I
could (safely?) hard-code the loops_per_sec in the kernel?

I've yet to find the source yet for the calculation (anyone know which

And does anyone have any ideas?  Odd ball or not...


How are BogoMips Calculated (or how can the calculation go wrong)

Post by Hugh McCur » Mon, 20 May 1996 04:00:00


I now know more thanks to Jeff Tranter's stand alone Bogomips program.

The computer is question reports a bogomips value of 6.55 when booting
and 49.71 when multiuser.

Of course, if anyone knows why this is, I'd love to hear about it
(mostly curious).

However, my question is - "Is it wise to change the loops_per_sec
variable in the kernel to match the 'correct' bogomips value for that


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Ive got a K6 on a GA-586TX board. Ive updated it from 2.2.8 to 2.2.12. Now it
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theres no problem.
Do anybody knows what going on?

Jens Reimann

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