8270 performance

Ricardo Scop scop at digitel.com.br
Tue Dec 9 11:02:37 EST 2003

Hello Jean-Denis,

Monday, December 08, 2003, 6:11:30 PM, you wrote:

JDB> Ricardo,
JDB> 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.

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

JDB> 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
JDB> the execution speed of function __delay (arch/ppc/kernel/misc.S), which loops on a branch instruction 'bdnz'.

Ooops, I should learn to read the source code before making any
assumptions. Sorry for the mistake, and many thanks for the lesson!


JDB> Regards,
JDB> --------------------------------------------
JDB>  Jean-Denis Boyer, Eng.
JDB>  M5T Centre d'Excellence en Télécoms Inc.
JDB>  4283 Garlock Street
JDB>  Sherbrooke (Québec)
JDB>  (819)829-3972 x241
JDB> --------------------------------------------

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