What is the overhead of the ClockCycles() call?
eg. delta between 2 back-to-back ClockCycles()?

The code uses an emulation that traps into the kernel and calls
the BSP timer_value() callout to calculate a delta from the last
clock. The clock tick handler maintains a 64-bit counter that is
incremented by qtime->timer_load so that ClockCycles returns:
counter + qtime->timer_value()

On 12-05-25 2:31 PM, Bruno Silva wrote:
> I'm trying to measure the tick overhead in a Beagleboard-XM-REV C. That uses an arm processor.
> I get a problem when using the ClockCycles() system call, because the counter seems to start counting backward when I 
reduce the tick period.
>
> I've made the following code to confirm this situation:
>
> int main(int argc, char *argv[]) {
>
> 	_uint64 before_clkper, after_clkper;
> 	float cpu_freq;
> 	struct _clockperiod clkper;
>
> 	{
> 		struct sched_param param;
> 		int ret;
> 		param.sched_priority = sched_get_priority_max( SCHED_RR );
> 		ret = sched_setscheduler( 0, SCHED_RR,¶m);
> 		assert ( ret != -1 );
> 	}
> 	cpu_freq = SYSPAGE_ENTRY( qtime )->cycles_per_sec;
> 	printf("Frequency of ClockCycles: %f\n", cpu_freq);
> 	
> 	clkper.nsec = 10000000;
> 	clkper.fract = 0;
> 	ClockPeriod ( CLOCK_REALTIME,&clkper, NULL, 0 );
>
>
> 	clkper.nsec = 10000;
> 	clkper.fract = 0;
> 	before_clkper= ClockCycles();
> 	ClockPeriod ( CLOCK_REALTIME,&clkper, NULL, 0 );
> 	after_clkper= ClockCycles();
> 	printf("Before clock period %lld. After clock period %lld\n", before_clkper, after_clkper);
> 	printf("Difference between clock period %lld\n", after_clkper-before_clkper);
> 	return 0;
> }
>
> The results were:
>
> Frequency of ClockCycles: 13000000.000000
> Before clock period 15088465604. After clock period 15088465368
> Difference between clock period -236
>
>
>
>
>
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