xtensor and xsimd: improve performance on reduction

Question

I'm trying to get the same performance with xtensor on the reduction operations (e.g. sum of elements) as in NumPy.

I enable xsimd for parallel computing, but it has no effect.

The following is the benchmark code:

#include <iostream>

#include "xtensor/xreducer.hpp"
#include "xtensor/xrandom.hpp"
#include <ctime>

using namespace std;


pair<double, double> timeit(int size, int n=30){
    double total_clocks = 0;
    double total_sum = 0;
    for (int i=0;i<n;i++){
        xt::xtensor<double, 1> a = xt::random::rand({size}, 0., 1.);
        int start = clock();

        double s = xt::sum(a, xt::evaluation_strategy::immediate)();

        int end = clock();
        total_sum += s; total_clocks += end-start;
    }
    return pair<double, double>(total_clocks/CLOCKS_PER_SEC/n, total_sum); 
}

int main(int argc, char *argv[])
{
    for (int i=5;i<8;i++){
        int size = pow(10, i);
        pair<double, double> ret = timeit(size);
        cout<<"size: "<<size<< " \t " <<ret.first<<" sec\t"<<ret.second<<endl;

    }
    return 0;
}

And compile this with and without enabling xsimd and all optimisations enabled(-O3):

$ g++ -DXTENSOR_USE_XSIMD -O3  -march=native -I/home/--user--/install_path/include "./18. test speed 2.cpp" -o a && ./a
size: 100000     0.0001456 sec     1.49984e+06
size: 1000000    0.0013149 sec     1.50002e+07
size: 10000000   0.0125417 sec     1.49995e+08

$ g++ -O3  -march=native -I/home/--user--/install_path/include "./18. test speed 2.cpp" -o a && ./a 
size: 100000     0.0001433 sec     1.49984e+06
size: 1000000    0.0012621 sec     1.50002e+07
size: 10000000   0.0124868 sec     1.49995e+08

By the way, the same operation using numpy:

$ python bench.py
size: 100000     0.000030 sec
size: 1000000    0.000430 sec
size: 10000000   0.005144 sec

About 4 times faster!

Setup

Ubuntu 18.04
Core i7 CPU
Latest versions of packages

How can I improve xtensor performance? Thanks in advance))

score 3 · Accepted Answer · answered Aug 11 '19 at 17:07

According to this github issue that I have opened
-mavx2 and -ffast-math flags should be enabled!

$ g++ -mavx2 -ffast-math -DXTENSOR_USE_XSIMD -O3 -I/home/--user--/install_path/include ./bench.cpp -o a && ./a
size: 100000        3.489e-05 sec   4.99932e+06
size: 1000000       0.00050792 sec  4.99989e+07
size: 10000000      0.00544542 sec  4.99997e+08

Thanks to dengbangjie!

xtensor and xsimd: improve performance on reduction

1 Answers1