8270 performance

[Jaap-Jan Boor]

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

>
> Ricardo,
> Jaap-Jan,
>
>>> 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,
> --------------------------------------------
>  Jean-Denis Boyer, Eng.
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