Parallel Program using SPMD command in Parallel Computing Toolbox in MATLAB!!!

Hello,

I just want to know that if we are reading an image normally by using imread command than it takes lesser time than executing same imread command in individual lab(which is written below)..plz help me..I am very much confused about parallel computing toolbox in MATLAB.

Sequential--

tic

I = imread('cameraman.tif');

I = I + 5;

J = imread('cameraman.tif');

J = J * 5;

K = imread('cameraman.tif');

K = K + 15;

L = imread('cameraman.tif');

L = L + 115;

toc

Parallel Program--

matlabpool open local 4

spmd(4)

    if labindex == 1

        tic

        I = imread('cameraman.tif');

        toc

        I = I + 5;

    elseif labindex == 2

       tic

        I = imread('cameraman.tif');

        I = I  + 5;

        toc

    elseif labindex == 3

        tic

        I = imread('cameraman.tif');

        I = I + 15;

        toc

    else

        tic

        I = imread('cameraman.tif');

        I = I + 115;

        toc

    end
end

For the First Sequential program overall Elapsed time is 0.070329 seconds.

And for the parallel one

Lab 1:

Elapsed time is 0.228841 seconds.
Lab 2: 

Elapsed time is 0.228596 seconds.

Lab 3:

Elapsed time is 0.227497 seconds.

Lab 4:

Elapsed time is 0.306381 seconds.

Answer 1

[Walter Roberson]

Keep in mind that you are asking the file system to work harder, having to read the file four separate times. I do not know how such things are handled in MS Windows; in Linux and OS-X the first read would probably end up caching the data in memory (for reasonable sized files), but still copies of the data would need to be transferred to each process.