multicore CPU frequency

Question

Below is code my testing my computer's CPU frequency and timing a function using rdtsc.

/* Code only works on x86 machine compiling with GCC */

/* Keep track of most recent reading of cycle counter */
static unsigned cyc_hi = 0;
static unsigned cyc_lo = 0;

void access_counter(unsigned *hi, unsigned *lo)
{
  /* Get cycle counter */
  asm("rdtsc; movl %%edx,%0; movl %%eax,%1" 
      : "=r" (*hi), "=r" (*lo)
      : /* No input */ 
      : "%edx", "%eax");
}

double get_counter()
{
  unsigned ncyc_hi, ncyc_lo;
  unsigned hi, lo, borrow;
  double result;
  /* Get cycle counter */
  access_counter(&ncyc_hi, &ncyc_lo);
  /* Do double precision subtraction */
  lo = ncyc_lo - cyc_lo;
  borrow = lo > ncyc_lo;
  hi = ncyc_hi - cyc_hi - borrow;
  return (double) hi * (1 << 30) * 4 + lo;
}

void start_counter()
{
  access_counter(&cyc_hi, &cyc_lo);
}

void p()
{
  sleep(1);
}

int main(int argc, char const *argv[])
{
    /* Determine Clock Rate of Processor */
    double MHZ;
    int sleep_time = 10;
    start_counter();
    sleep(sleep_time);
    MHZ = get_counter() / (sleep_time * 1e6);
    printf("Processor Clock Rate ~= %.1f MHz\n", MHZ);
    /* cat /proc/cpuinfo */

    /* Time Function P */
    double tsecs;
    start_counter();
    p();
    tsecs = get_counter() / (MHZ * 1e6);
  printf("%.1f seconds\n", tsecs);
    return 0;
}

After running this program, it prints the processor clock rate is around 3591.8MHz, then I run cat /proc/cpuinfo, it shows there are 8 processors (0 ~7), and some processors' cpu MHz are different, but no one is 3591.8Mhz. I am wondering how do I interpret this value, 3591.8MHz? Many thanks.

Answer 1

CPU can reclock thousands of time per second. They are designed to do that. Likely your program works correctly and shows a snapshot of a current frequency which is not observed when you manually do the cat after? Also your program creates some workload so it will likely spike one CPU to momentary come out of some deep hybernation state to quickly do the work and then go back to sleep. This is behaviour of new intel core i processors. What is your CPU? If you do cat /proc/cpuinfo multiple times do you get the same results?