why such strange results for boost::timer? - c++

Question

Why boost::timer gives me such strange results?
My working solution is to use wrapper about gettimeofday function from <time.h>, but I don't understand why boost::timer is not working for me here. What do I do wrong?
class Timer {
private:
timeval startTime;
public:
void start(){
gettimeofday(&startTime, NULL);
}
double stop(){
timeval endTime;
long seconds, useconds;
double duration;
gettimeofday(&endTime, NULL);
seconds = endTime.tv_sec - startTime.tv_sec;
useconds = endTime.tv_usec - startTime.tv_usec;
duration = seconds + useconds/1000000.0;
return duration;
}
long stop_useconds(){
timeval endTime;
long useconds;
gettimeofday(&endTime, NULL);
useconds = endTime.tv_usec - startTime.tv_usec;
return useconds;
}
static void printTime(double duration){
printf("%5.6f seconds\n", duration);
}
};
test:
//test
for (int i = 0; i < 10; i++) {
void *vp = malloc(1024*sizeof(int));
memset((int *)vp, 0, 1024);
void* itab = malloc(sizeof(int)*1024*256); //1MiB table
if (itab) {
memset ( (int*)itab, 0, 1024*256*sizeof (int) );
float elapsed;
boost::timer t;
Timer timer = Timer();
timer.start();
Munge64(itab, 1024*256);
double duration = timer.stop();
long lt = timer.stop_useconds();
timer.printTime(duration);
cout << t.elapsed() << endl;
elapsed = t.elapsed();
cout << ios::fixed << setprecision(10) << elapsed << endl;
cout << ios::fixed << setprecision(10) << t.elapsed() << endl;
printf("Munge8 elapsed:%ld useconds\n", lt);
elapsed = 0;
free(vp);
free(itab);
//printf("Munge8 elapsed:%d\n", elapsed);
}
}
results:
0.000100 seconds
0 << ??????????
40 << ????????????????
40 << ???????????????????????????????????
Munge8 elapsed:100 useconds
0.000100 seconds
0
40
40
Munge8 elapsed:100 useconds
0.000099 seconds
0
40
40
Munge8 elapsed:99 useconds

Answer 1

You should not use boost::timer - http://www.boost.org/doc/libs/1_54_0/libs/timer/doc/original_timer.html#Class timer
On POSIX like it measures CPU time - not wall clock time.
Consider using boost::chrono or std::chrono - you would want to look at steady_clock - as other clocks when implementing a timer if you want to isolate yourself from drift or shift in system wall clock. I expect on POSIX this will use clock_gettime on CLOCK_MONOTONIC.