Execute 3 functions in parallel on 3 workers - matlab

Question

I've got three functions (qrcalc, zcalc, pcalc) with three unique set of inputs which I want to run in parallel. This is my attempt but it doesn't work:
function [outall]=parallelfunc(in1,in2,in3)
if parpool('size') == 0 % checking to see if pool is already open
A=feature('numCores');
parpool('local',A);
else
parpool close
A=feature('numCores');
parpool('local',A);
end
spmd
if labindex==2
out1=qrcalc(in1);
elseif labindex==3
out2=zcalc(in2);
elseif labindex==4
out3=pcalc(in3);
end
outall=[out1;out2;out3];
end
Error: Error using parallelattempt>(spmd body) (line 20) Error
detected on worker 3. An UndefinedFunction error was thrown on the
workers for 'out1'. This may be because the file containing 'out1' is
not accessible on the workers. Specify the required files for this
parallel pool using the command: addAttachedFiles(pool, ...). See the
documentation for parpool for more details.
Error in parallelattempt>(spmd) (line 11) spmd
Error in parallelattempt (line 11) spmd
Are there any suggestions for how this can be done?
Here is a version of the code that does not require the custom functions. Therefore I replaced them with zeros, magic and ones:
function [outall]=parallelattempt(in1,in2,in3)
poolobj = gcp;
addAttachedFiles(poolobj,{'zeros.m','ones.m','magic.m'})
spmd
if labindex==2
out1=zeros(in1);
elseif labindex==3
out2=magic(in2);
elseif labindex==4
out3=ones(in3);
end
outall=[out1;out2;out3];
end

Answer 1

If you use an spmd-statement, the code inside will be sent to all workers. By the use of labindex you only create the variables outX on one specific worker. The problem is, that outall=[out1;out2;out3]; should now be executed on workers where two outX-variables are not declared. The direct fix for this error is to declare the variables before the spmd-statement (out1=[]; out2=[]; out3=[];). But this is not the best solution.
You can use a single variable inside the spmd-statement instead of several ones (outX), lets call this variable out. Now the code executes on each worker and stores its result in out, which is a Composite-object. Concatenation is not necessary because it is done automatically. Additionally, you can specify with spmd (3) at the beginning of the block that only 3 workers should be used. Composite-objects can be indexed like cell arrays where the index equals to the number of the worker/lab. Therefore we can concatenate it after the block.
This is the specific code for that part:
spmd (3)
if labindex==1
out = qrcalc(in1);
elseif labindex==2
out = zcalc(in2);
elseif labindex==3
out = pcalc(in3);
end
end
outall = [out{1};out{2};out{3}];
Note that the creation of the pool will be done automatically, if none exists. You may need to attach the files of your functions before the statement.
An even better approach in my opinion is to use parfeval here. This does exactly what you want to achieve in the first place - it solves your initial problem. The outX variables get calculated in parallel (non-blocking). With the function fetchOutputs you can block execution until the result is calculated. Use it on all the outX-variables and concatenate it in the same line.
Here is the code for that:
out1 = parfeval(#qrcalc,1,in1);
out2 = parfeval(#zcalc,1,in2);
out3 = parfeval(#pcalc,1,in3);
outall = [fetchOutputs(out1);fetchOutputs(out2);fetchOutputs(out3)];