MATLAB: Time of execution of a code in matlab

Hello, To compare the time of execution of two functions, i wrote a code where i measure the time elapsed for each function and for each index ( with the command clock /etime). However, on my computer the first function is faster than the second, and on an other computer, it's the opposite. ( I tried several time with different indexes and with clearing for each time the data ,but it is still the same result).

Can you explain me why there is such difference?

On both of them, Matlab version is r2017a , the only difference is that on one computer, the compiler is MinGW 4.9.2 ,and on the other one it's Microsoft Windows SDK 7.1 .

Thanks in advance

Best Answer

Can you find the code lines inside freqxorr which are accelereated on the faster computer. It could either be multi-threading and a higher number of cores, or the data might match into a larger CPU cache, or the internally used library can use the SSE or AVX features fo the younger processor. Nevertheless, it is not surprising, that different functions can profit differently on different CPUs.

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MATLAB: Normxcorr2 function – spatial / frequency domain selection

Hi Sam, normxcorr2 computes an estimate of the computation time for spatial domain correlation vs. frequency domain. Whichever is the smallest wins.

You could save normxcorr2.m as a different name and then modify the code so that it always did one or the other. For example, to always use spatial domain, you can modify the xcorr2_fast function inside of normxcorr2 as follows

function cross_corr = xcorr2_fast(T,A)
T_size = size(T);
A_size = size(A);
outsize = A_size + T_size - 1;
% figure out when to use spatial domain vs. freq domain

%conv_time = time_conv2(T_size,A_size); % 1 conv2

%fft_time = 3*time_fft2(outsize); % 2 fft2 + 1 ifft2

%if (conv_time < fft_time)

    cross_corr = conv2(rot90(T,2),A);
%else

%    cross_corr = freqxcorr(T,A,outsize);
%end

To only use the Fourier transform:

function cross_corr = xcorr2_fast(T,A)
T_size = size(T);
A_size = size(A);
outsize = A_size + T_size - 1;
% figure out when to use spatial domain vs. freq domain
%conv_time = time_conv2(T_size,A_size); % 1 conv2
%fft_time = 3*time_fft2(outsize); % 2 fft2 + 1 ifft2
%if (conv_time < fft_time)
%    cross_corr = conv2(rot90(T,2),A);
%else
    cross_corr = freqxcorr(T,A,outsize);
%end

MATLAB: I am trying to convert this for loop from matlab to C. Could someone please help

Note: It's been ages since I've written any C code, and even then it was mostly C++ not C. Expect some bugs.

First you'll have to include the math library whatever that is on Arduino. Usually, it's math.h, so:

#define _USE_MATH_DEFINES  // to force M_PI to exist
#include <math.h>          //for pow, sin and M_PI

The main difficulty with the matlab code is that the operation on S is vectorised which is not an option in C, you have to convert that into a loop, hence you'll have a double for loop.

I would recommend that you name the size of the arrays either using a #define or a variable:

#define TSIZE 40   //or 4000
#define ASIZE 10   //for A and forigin

int TSIZE = 40;
int ASIZE = 10;

and use that for all your array declaration

double t[TSIZE];
double A[ASIZE] = {16 18 28 33 38 41 59 43 40 58};  //makes life easier if it's declared as double
double forigin[ASIZE] = {25.0 31.5 40.0 50.0 63.0 80.0 100.0 125.0 160.0 200.0};
//... code to fill t

On to the S calculation:

double S[TSIZE];
int i, k;
//initialise S to 0
for (i = 0; i < TSIZE; ++i) S[i] = 0;
//implement the for k=1:length(A)
for (k = 0; i < 10; ++i){
   //devectorise the S calculation
   for (i = 0; i < TSIZE; ++i){
       S[i] = S[i] + pow(10, A[k]/20) * sin(2*M_PI*forigin[k]*t[i]);
   }
}

edit: You could move the calculation of pow(10, A[k]/20) out of the i loop since it only depends on k.

Best Answer

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MATLAB: Normxcorr2 function – spatial / frequency domain selection

MATLAB: I am trying to convert this for loop from matlab to C. Could someone please help

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