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Re: Newer laptops & CPU speed

Jochen Voss <voss@seehuhn.de> wrote:
> On Sat, Mar 19, 2005 at 10:29:28PM +0000, Joerg Sommer wrote:
>> Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
>> > On Mon, 2005-03-14 at 16:01 +0000, Jochen Voss wrote:
>> >
>> >> From other people's posts I conclude that bogomips should approximately
>> >> equal clock?
>> >
>> > It depends on the CPU, but on a 7447A, yes.
>> Do you know why it is for i386 nearly the double of cpu clock? Or reverse,
>> why is it only equal the clock on ppc?
> I am no expert on this, but the delay loop in question is
>     _GLOBAL(__delay)
> 	    cmpwi   0,r3,0
> 	    mtctr   r3
> 	    beqlr
>     1:      bdnz    1b
> 	    blr
> in file "arch/ppc/kernel/misc.S".  I do not yet speak powerpc
> assembler, but I read bdnz as decrease (register r3?) and branch
> backwards to label 1 if the result is not zero.  Probably a 7447A CPU

Yes, this is it.

> can do this within 1 CPU clock cycle?  I didn't check the loop for
> i386 but maybe it consists of two instructions or of one which takes
> two CPU clock cycles?

I found a statement to this in /usr/share/doc/HOWTO/en-txt/BogoMips.gz.

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.

But I found there a PPC should have the double too:
   PowerPC G4/500          992.87  K. Gustilo <gustilo@_mail.med.upenn.edu>              
   PowerPC G4/500          996.64  Heinz Nabielek <h.nabielek@_fz-juelich.de>            
   PowerPC G4/500          999.42  Stephen D. Scotti <sdscotti@_earthlink.net>           
   PowerPC G4/999         1992.29  Paula <canthavespam@_yahoo.com>                       

Are these wrong values or did something changed in the calculation?


Mathematiker beim Kuchenessen (aus dem wahren Leben):
J: Du überlegst wohl, wie du das Stück am optimalsten teilst?
K: Ja, ich wende gerade den Simplex-Algorithmus darauf an.
C: Schau mal, da hast du schon vier Ecken.

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