Measure executing time on ARM Cortex-A8 using hardware counter

Question

I'm using a Exynos 3110 processor (1 GHz Single-core ARM Cortex-A8, e.g. used in the Nexus S) and try to measure execution times of particular functions. I have an Android 4.0.3 running on the Nexus S. I tried the method from

[1] How to measure program execution time in ARM Cortex-A8 processor?

I loaded the kernel module to allow reading the register values in user mode. I am using the following program to test the counter:

static inline unsigned int get_cyclecount (void)
{
    unsigned int value;
    // Read CCNT Register
    asm volatile ("MRC p15, 0, %0, c9, c13, 0\t\n": "=r"(value));
    return value;
}


static inline void init_perfcounters (int do_reset, int enable_divider)
{
    // in general enable all counters (including cycle counter)
    int value = 1;

    // peform reset:  
    if (do_reset)
    {
        value |= 2;     // reset all counters to zero.
        value |= 4;     // reset cycle counter to zero.
    } 

    if (enable_divider)
        value |= 8;     // enable "by 64" divider for CCNT.

    value |= 16;

    // program the performance-counter control-register:
    asm volatile ("MCR p15, 0, %0, c9, c12, 0\t\n" :: "r"(value));  

    // enable all counters:  
    asm volatile ("MCR p15, 0, %0, c9, c12, 1\t\n" :: "r"(0x8000000f));  

    // clear overflows:
    asm volatile ("MCR p15, 0, %0, c9, c12, 3\t\n" :: "r"(0x8000000f));
}


int main(int argc, char **argv)
{
    int i = 0;
    unsigned int start = 0;
    unsigned int end = 0;

    printf("Hello Counter\n");

    init_perfcounters(1,0);

    for(i=0;i<10;i++)
    {
        start = get_cyclecount();
        sleep(1); // sleep one second
        end = get_cyclecount();

        printf("%u %u %u\n", start, end, end - start);
    }

    return 0;
}

According to [1] the counter is incremented with each clock cycle. I switched the scaling_governor to userspace and set the CPU frequency to 1GHz to make sure that the clock frequency is not change by Android.

If I run the program the sleeps of 1 second are executed, but the counter values are in the range of ~200e6, instead of the expected 1e9. Is there anything processor specific I am missing here? Is the clock rate of the counters different to the clock rate of the processor ?

Answer 1

Check out this professor's page: http://users.ece.utexas.edu/~valvano/arm/ He has multiple full example programs that have to do with time/periodic-timers/measuring-execution-time, they are developed for ARM Cortex-M3 based microcontrollers. I hope this isn't very different from what you are working on. I think you would be interested in Performance.c