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

Subject: Re: [boost] [boost.chrono] Slow down process (if specifing c++11)
From: Julien Nitard (julien.nitard_at_[hidden])
Date: 2013-08-26 10:29:16

Hi

I've seen something similar before. I'd check whether the call
boost::chrono triggers a fork() in C++11 mode. If it does, Linux will mark
your pages as copy-on-write and slow down the entire thing.

To make check whether that's the case move the memory allocation after the
call to chrono : the pages allocated after fork() won't be affected.

Regards,

Julien

On Mon, Aug 26, 2013 at 9:54 PM, Gaetano Mendola <mendola_at_[hidden]> wrote:

> 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;
> }
>
>
>
>
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