Subject: Re: too few bogoMips on MPC8313E-RDB ?

From: Norbert van Bolhuis <[email-masked]>
Date: Wed, 26 Nov 2008 15:20:40 +0100

Thanks for the answer, but that's not it.

I checked the jiffies variable, it increases about 250 times
per second.
So the (mpc83xx_defconfig) kernel perception (#define CONFIG_HZ 250) is OK.

It must be something else, I still think 83.20 BogoMIPS
can't be correct for a MPC8313 running at 333 MHz.

Alessandro Rubini wrote:
>> This is what a linux-2.6.x reports (for the MPC8313E running at 333 MHz):
>> Calibrating delay loop... 83.20 BogoMIPS (lpj=166400)
>>
>> Which can't be correct.
>>
>> A MPC870 (running at 133 mhz) has ~ 131.07 BogoMIPS
> 
> Actaully, one-instruction-per-clock leads to BogoMIPS = MHz.
> 
> Your "loop per jiffies" value shows you make 332800 instructions per
> jiffy (a loop is two instructions: increment and jump).
> 
> So most liker your timer tick runs at 1000 Hz but Linux is thinking
> it's at 250Hz (332800 * 250 = 83.20 millions).
> 
>> Of course it's only a benchmark figure.
> 
> No, it's not a benchmark figure. It's the metric by which udelay() is
> calculated. So your udelays (and mdelays) will be 4 times shorter than
> required, and some hardware may be misbehaving as a result.
> 
> Hope this help
> /alessandro, who however is not runing a powerPC these times
>