Matlab: Does calling the same mex function repeatedly from a loop incur too much overhead?

Question

I have some Matlab code which needs to be speeded up. Through profiling, I\'ve identified a particular function as the culprit in slowing down the execution. This function i

Answer 1

You should absolutely without any hesitation move the loop inside the mex file. The example below demonstrates a 1000 times speedup for a virtually empty work unit in a for loop. Obviously as the amount of work in the for loop changes this speedup will decrease.

Here is an example of the difference:

Mex function without internal loop:

#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[ ], int nrhs, const mxArray *prhs[ ]) 
{      
    int i=1;    
    plhs[0] = mxCreateDoubleScalar(i);
}

Called in Matlab:

tic;for i=1:1000000;donothing();end;toc
Elapsed time is 3.683634 seconds.

Mex function with internal loop:

#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[ ], int nrhs, const mxArray *prhs[ ]) 
{      
    int M = mxGetScalar(prhs[0]);
    plhs[0] = mxCreateNumericMatrix(M, 1, mxDOUBLE_CLASS, mxREAL);
    double* mymat = mxGetPr(plhs[0]);
    for (int i=0; i< M; i++)
        mymat[i] = M-i;
}

Called in Matlab:

tic; a = donothing(1000000); toc
Elapsed time is 0.003350 seconds.