Using parfor and labSend/labRecieve - parallel-processing

Question

I want to run two matlab scripts in parallel for a project and communicate between them. The purpose of this is to have one script do image analysis and sending the results to the other which will use it for more calculations (time consuming, but not related to the task of finding stuff in the images). Since both tasks are time consuming, and should preferably be done in real time, I believe that parallelization is necessary.
To get a feel for how this should be done I created a test script to find out how to communicate between the two scripts.
The first script takes a user input using the built in function input, and then using labSend sends it to the other, which recieves it, and prints it.
function [blarg] = inputStuff(blarg)
mpiInit(); %added because of error message, but do not work...
for i=1:2
labBarrier; % added because of error message
inp = input('Enter a number to write');
labSend(inp);
if (inp == 0)
break;
else
i = 1;
end
end
end
function [ blarg ] = testWrite( blarg )
mpiInit(); % added because of error message, but does not help
par = 0;
if ( blarg == 0)
par = 1;
end
for i = 1:10
if (par == 1)
labBarrier
delta = labReceive();
i = 1;
else
delta = input('Enter number to write');
end
if (delta == 0)
break;
end
s = strcat('This lab no', num2str(labindex), '. Delta is = ')
delta
end
end
%%This is the file test_parfor.m
funlist = {#inputStuff, #testWrite};
matlabpool(2);
mpiInit(); % added because of error message, but does not help
parfor i=1:2
funlist{i}(0);
end
matlabpool close;
Then, when the code is run, the following error message appears:
Starting matlabpool using the 'local' profile ... connected to 2 labs.
Error using parallel_function (line 589)
The MPI implementation has not yet been loaded. Please
call mpiInit.
Error stack:
testWrite.m at 11
Error in test_parfor (line 8)
parfor i=1:2
Calling the method mpiInit does not help... (Called as shown in the code above.)
And nowhere in the examples that mathworks have in the documentation, or on their website, show this error or what to do with it.
Any help is appreciated!

Answer 1

You would typically use constructs such as labSend, labRecieve and labBarrier within an spmd block, rather than a parfor block.
parfor is intended for implementing embarrassingly parallel algorithms, in other words algorithms that consist of multiple independent tasks that can be run in parallel, and do not require communication between tasks.
I'm stretching my knowledge here (perhaps someone more expert can correct me), but as I understand things, it does not set up an MPI ring for communication between workers, which is probably the explanation for the (rather uninformative) error message you're getting.
An spmd block enables communication between workers using labSend, labRecieve and labBarrier. There are quite a few examples of using them all in the documentation.

Answer 2

Sam is right that the MPI functionality is not enabled during parfor, only during spmd. You need to do something more like this:
spmd
funlist{labindex}(0);
end
(Sam is also quite right that the error message you saw is pretty unhelpful)