ClockBench.cpp

// Copyright 2014 by Bill Torpey. All Rights Reserved.
// This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 United States License.
// http://creativecommons.org/licenses/by-nc-nd/3.0/us/deed.en

#include <time.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

#include <vector>
#include <cmath>
using namespace std;


#define ONE_BILLION  1000000000000L

double CPU_FREQ = 1;
const int BUCKETS = 256;         // how many samples to collect per iteration
const int ITERS = 100;           // how many iterations to run

inline unsigned long long cpuid_rdtsc() {
  unsigned int lo, hi;
  asm volatile (
     "cpuid \n"
     "rdtsc"
   : "=a"(lo), "=d"(hi) /* outputs */
   : "a"(0)             /* inputs */
   : "%ebx", "%ecx");     /* clobbers*/
  return ((unsigned long long)lo) | (((unsigned long long)hi) << 32);
}

inline unsigned long long rdtsc() {
  unsigned int lo, hi;
  asm volatile (
     "rdtsc"
   : "=a"(lo), "=d"(hi) /* outputs */
   : "a"(0)             /* inputs */
   : "%ebx", "%ecx");     /* clobbers*/
  return ((unsigned long long)lo) | (((unsigned long long)hi) << 32);
}

inline unsigned long long rdtscp() {
  unsigned int lo, hi;
  asm volatile (
     "rdtscp"
   : "=a"(lo), "=d"(hi) /* outputs */
   : "a"(0)             /* inputs */
   : "%ebx", "%ecx");     /* clobbers*/
  return ((unsigned long long)lo) | (((unsigned long long)hi) << 32);
}

// macro to call clock_gettime w/different values for CLOCK
#define do_clock(CLOCK) \
do { \
   for (int i = 0; i < ITERS * BUCKETS; ++i) { \
      struct timespec x;  \
      clock_gettime(CLOCK, &x); \
      int n = i % (BUCKETS); \
      timestamp[n] = (x.tv_sec * ONE_BILLION) + x.tv_nsec; \
   } \
   deltaT x(#CLOCK, timestamp); \
   x.print(); \
} while(0)



struct deltaT   {

   deltaT(const char* name, vector<long> v) : sum(0), sum2(0), min(-1), max(0), avg(0), median(0), stdev(0), name(name)
   {
      // create vector with deltas between adjacent entries
      x = v;
      count = x.size() -1;
      for (int i = 0; i < count; ++i) {
         x[i] = x[i+1] - x[i];
      }

      for (int i = 0; i < count; ++i) {
         sum    += x[i];
         sum2   += (x[i] * x[i]);
         if (x[i] > max) max = x[i];
         if ((min == -1) || (x[i] < min)) min = x[i];
      }

      avg = sum / count;
      median = min + ((max - min) / 2);
      stdev =  sqrt((count  * sum2 - (sum * sum)) / (count * count));
   }

   void print()
   {
      printf("%25s\t", name);
      printf("%ld\t%7.2f\t%7.2f\t%7.2f\t%7.2f\t%7.2f\n", count, min, max, avg, median, stdev);
   }

   void dump(FILE* file)
   {
      if (file == NULL)
         return;

      fprintf(file, "%s", name);
      for (int i = 0 ; i < count; ++i ) {
         fprintf(file, "\t%ld", x[i]);
      }
      fprintf(file, "\n");
   }

   vector<long> x;
   long   count;
   double sum;
   double sum2;
   double min;
   double max;
   double avg;
   double median;
   double stdev;
   const char* name;
};





int main(int argc, char** argv)
{
   if (argc > 1) {
      CPU_FREQ = strtod(argv[1], NULL);
   }

   FILE* file = NULL;
   if (argc > 2) {
      file = fopen(argv[2], "w");
   }


   vector<long>    timestamp;
   timestamp.resize(BUCKETS);

   printf("%25s\t", "Method");
   printf("%s\t%7s\t%7s\t%7s\t%7s\t%7s\n", "samples", "min", "max", "avg", "median", "stdev");

#if _POSIX_TIMERS > 0
   #ifdef CLOCK_REALTIME
   do_clock(CLOCK_REALTIME);
   #endif

   #ifdef CLOCK_REALTIME_COARSE
   do_clock(CLOCK_REALTIME_COARSE);
   #endif

   #ifdef CLOCK_REALTIME_HR
   do_clock(CLOCK_REALTIME_HR);
   #endif

   #ifdef CLOCK_MONOTONIC
   do_clock(CLOCK_MONOTONIC);
   #endif

   #ifdef CLOCK_MONOTONIC_RAW
   do_clock(CLOCK_MONOTONIC_RAW);
   #endif

   #ifdef CLOCK_MONOTONIC_COARSE
   do_clock(CLOCK_MONOTONIC_COARSE);
   #endif
#endif


   {
      for (int i = 0; i < ITERS * BUCKETS; ++i) {
         int n = i & 0xff;
         timestamp[n] = cpuid_rdtsc();
      }
      for (int i = 0; i < BUCKETS; ++i) {
         timestamp[i] = (long) ((double) timestamp[i] / CPU_FREQ);
      }
      deltaT x("cpuid+rdtsc", timestamp);
      x.print();
      x.dump(file);
   }

   // will throw SIGILL on machine w/o rdtscp instruction
   #ifdef RDTSCP
   {
      for (int i = 0; i < ITERS * BUCKETS; ++i) {
         int n = i & 0xff;
         timestamp[n] = rdtscp();
      }
      for (int i = 0; i < BUCKETS; ++i) {
         timestamp[i] = (long) ((double) timestamp[i] / CPU_FREQ);
      }
      deltaT x("rdtscp", timestamp);
      x.print();
      x.dump(file);
   }
   #endif

   {
      for (int i = 0; i < ITERS * BUCKETS; ++i) {
         int n = i & 0xff;
         timestamp[n] = rdtsc();
      }
      for (int i = 0; i < BUCKETS; ++i) {
         timestamp[i] = (long) ((double) timestamp[i] / CPU_FREQ);
      }
      deltaT x("rdtsc", timestamp);
      x.print();
      x.dump(file);
   }

   printf("Using CPU frequency = %f\n", CPU_FREQ);

   return 0;
}