[boost] [boost.chrono] Slow down process (if specifing c++11)

Subject: [boost] [boost.chrono] Slow down process (if specifing c++11)
From: Gaetano Mendola (mendola_at_[hidden])
Date: 2013-08-26 09:54:39

Hi have tried the following code compiling it with gcc 4.6.3,
with four combinations:

1) -O3
2) -O3 (commeting out the chrono call)
3) -O3 -std=c++0x
4) -O3 -std=c++0x (commenting out the chrono call)

and I'm obtaining the following runtime values:

1) 8921 ms
2) 8915 ms
3) 9400 ms
4) 8933 ms

as you can see the combination:  "c++11 and chrono call" slows down the entire
process. I have tried the same with gcc 4.8.1 and all times are more or less
the same.

At this point I'm not sure if it's a problem of chrono with gcc 4.6.3 or
simply an issue of 4.6.3 disappeared in 4.8.1 series.

Regards
Gaetano Mendola

Here the code I have used:

#include <iostream>
#include <boost/chrono.hpp>
#include <sys/time.h>
#include <math.h>

int main() {

  cpu_set_t  myAffinityMask;
  CPU_ZERO( &myAffinityMask );
  CPU_SET(0, &myAffinityMask );
  sched_setaffinity(0, sizeof(myAffinityMask), &myAffinityMask);

  volatile float* myMemoryA = new float[(1<<24)];
  volatile float* myMemoryB = new float[(1<<24)];

  struct timeval myStart;
  struct timeval myStop;
  struct timeval myResult;

  boost::chrono::time_point<boost::chrono::steady_clock> t1 = boost::chrono::high_resolution_clock::now();

  gettimeofday(&myStart, 0);

  for (size_t i = 0; i < (1<<24); ++i) {
    myMemoryA[i] = i;
    myMemoryB[i] = i+1;
  }
  delete []myMemoryA;
  delete []myMemoryB;

  for (size_t j = 0; j < 100; ++j) {
    volatile float* myMemoryA = new float[(1<<24)];
    volatile float* myMemoryB = new float[(1<<24)];
    for (size_t i = 0; i < (1<<24); ++i) {
      myMemoryA[i] *= sqrtf(myMemoryB[i]);
    }
    delete []myMemoryA;
    delete []myMemoryB;
  }
  gettimeofday(&myStop, 0);

  timersub(&myStop,&myStart,&myResult);

  std::cout << "Time: " <<  myResult.tv_sec*1000 + myResult.tv_usec/1000.0 << std::endl;
  std::cout << "t1: " << t1 << std::endl;
}