Re: 8270 performance

On 8-dec-03, at 22:11, Jean-Denis Boyer wrote:

Ricardo,
Jaap-Jan,

quoted

quoted

I would expect the bogomips number
to be somewhere near the core clock
(like 8xx).

Well, I think the numbers are pretty fair, assuming that the
bogomips number
reflect a certain amount of external memory accesses at 66
MHz external bus
clock.

It has more to do with pipelining!
Hopefully, there ain't any memory access during calibration...
(except, of course, when the instruction cache is disabled!)

I didn't look at the documentation, but the execution of a branch
probably occurs later in the pipeline of a 8260 than for the 860. It
probably looses one more cycle. The calibration relies on the
execution speed of function __delay (arch/ppc/kernel/misc.S), which
loops on a branch instruction 'bdnz'.

Fortunately, the number of bogomips will increase linearly with the
core clock.
(These are approximations)

For 8260:
 200MHz => 66M bdnz per second => 133BogoMips
 266    => 88M                 => 176
 400    => 133M                => 266

For 860:
 50MHz => 25M bdnz per second => 50BogoMips

Ok, that's a nice explanation, thanks!

Jaap-Jan

Regards,
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