Matlab vs c++ speed comparison in this code

Question

I have written simple C++ code and tested it in C++, then I have adapted the same code for MATLAB by mex file_name.cpp and run the same code in MATLAB which is using the same compiler as C++. Here is the code:

int k;
for(int j = 0; j < 100;j++){
    for(int i = 0; i < 10000000; i++){
        k++;
    }
    k/=10000000
}

Here is MATLAB code:

double a;int j;int i;
double* k;

for(j = 0; j < 100;j++){
    for(i = 0; i < 10000000; i++){
        a = a+1;
    }
    a = a / 10000000;
}

plhs[0] = mxCreateDoubleMatrix(1,1,mxREAL);
k = mxGetPr(plhs[0]);
*k = (double)a;

I have edited this code for MATLAB, i.e changing to suitable types, adding MEX-function etc and the results are approx 900ms in MATLAB as opposed to 3100 ms in C++.

What I don't understand is both are running the same code and with the same compiler (in MATLAB I write mex -setup in command line and selected Visual Studio compiler as MEX compiler), however, MATLAB is around 3.5 times faster.

What is MATLAB doing to be that faster and what is C++ not doing? Could somebody please explain me why there is so huge difference? I have tried some other codes, all are 3-6 times faster in MATLAB.

My PC is 64-bit Windows 7, Visual Studio 2010 is used for C++, MATLAB is R2012b.

Is it possible this is because of my Visual Studio version? If I change it to VS2012, would it be faster?

mex -v output is here.

Thanks,

Answer 1

Performance is highly dependent on platform, OS, compiler, etc. Whatever Matlab is doing in this case, it somehow has managed to find an optimization that the VS2010 compiler did not. I would venture to guess that upgrading to VS2012 would not make a substantial difference, but I could be wrong. It is, after all, a different compiler.

I will admit that this is somewhat surprising, but check your compilation flags and try profiling with different configurations. If your Matlab install is 32-bit, that could make a difference, as well.

There could also be slight differences in your code, possibly slight enough that you might not have noticed. Your code might be linking against other libraries that could also have wide variation in performance.

The lesson here is that it can be very difficult to pin down exactly why one thing performs better than another.

EDIT: You have mentioned that the code is compiled for debugging. This only further increases the variation of what compilers will output, since activating debug options may also turn off other optimizations, and each compiler has a different idea of what sort of debug information is important and worth sticking in your code.

I would recommend turning off all debug options to get a more consistent output. I would also recommend making sure you are compiling with similar levels of optimization, probably either the greatest possible or not at all.