Hello,
I would like to measure the performance of the GCC c++11 Poisson random
number generator, but the in-program measurement of time intervals is
confusing me. I have two pieces of code (below): in the first I use the
ctime clock() function and get sensible results, in the second I use
std::chrono:steady_clock and get results that I think are wrong. Could
somebody please tell me what I'm doing wrong. I use g++ (GCC) 4.8.0 on a
Linux X.X.X 2.6.32-358.0.1.el6.x86_64 #1 SMP Tue Feb 26 14:48:51 CST
2013 x86_64 x86_64 x86_64 GNU/Linux system.
BR,
Henrik Mannerström
***** Typical output of first program *****
>g++ -Wall -Wextra -pedantic -std=c++11 -O2 test7.cc -o test7
>time ./test7
C++11 library RNG - C clock
4999561509
Total 7.86 s
786 ns/rnv
real 0m7.870s
user 0m7.864s
sys 0m0.002s
***** Typical output of second program *****
>g++ -Wall -Wextra -pedantic -std=c++11 -O2 test5.cc -o test5
>time ./test5
C++11 library RNG - steady_clock
5000562918
Total 3.59925 s
359.925 ns/rnv
real 0m7.900s
user 0m7.894s
sys 0m0.002s
***** First file *****
#include
<iostream>
#include
<random>
#include
<ctime>
unsigned int const sz =
10000000;
int
main()
{
std::random_device
rd;
std::mt19937
gen(rd());
std::uniform_real_distribution<> unif(0,
1e3);
int unsigned long sum =
0;
clock_t start =
clock();
for (unsigned int k=0;k!=sz;k+=1)
{
std::poisson_distribution<unsigned int>
poiss(unif(gen));
sum +=
poiss(gen);
}
double elapsed_seconds = static_cast<double>(clock() - start) /
static_cast<double>(CLOCKS_PER_SEC);
std::cout << "C++11 library RNG - C clock" <<
std::endl;
std::cout << sum <<
std::endl;
std::cout << "Total " << elapsed_seconds << " s" <<
std::endl;
std::cout << 1e9 * elapsed_seconds / static_cast<double>(sz) << "
ns/rnv" <<
std::endl;
}
***** Second file *****
#include
<iostream>
#include
<random>
#include
<chrono>
unsigned int const sz =
10000000;
typedef std::chrono::steady_clock
mclock;
int
main()
{
std::chrono::time_point<mclock> start,
end;
std::random_device
rd;
std::mt19937
gen(rd());
std::uniform_real_distribution<> unif(0,
1e3);
int unsigned long sum =
0;
start =
mclock::now();
for (unsigned int k=0;k!=sz;k+=1)
{
std::poisson_distribution<unsigned int>
poiss(unif(gen));
sum +=
poiss(gen);
}
end =
mclock::now();
int unsigned elapsed_nseconds =
std::chrono::duration_cast<std::chrono::nanoseconds>
(end-start).count();
std::cout << "C++11 library RNG - steady_clock" <<
std::endl;
std::cout << sum <<
std::endl;
std::cout << "Total " << static_cast<double>(elapsed_nseconds)/1e9 <<
" s" <<
std::endl;
std::cout << static_cast<double>(elapsed_nseconds) /
static_cast<double>(sz) << " ns/rnv" <<
std::endl;
}
