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

Subject: Re: [boost] [boost.chrono] Slow down process (if specifing c++11)
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-08-28 16:48:05

Le 26/08/13 15:54, Gaetano Mendola a écrit :
> 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
I don't know if changing the order could changes the results.

   gettimeofday(&myStart, 0);

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

and

   std::cout << "t1: " << t1 << std::endl;

   gettimeofday(&myStop, 0);

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

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

Or to be neutral execute either the chrono computation either the timeval one.

HTH,
Vicente

>
> 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;
> }
>
>
>
>