MATLAB: Parfeval blocks parfor loop evaluation

Question

The existance of any parfeval job blocks the execution of a parfor loop. This occurs even when available pool workers are available, and should be free to execute the parfor loop.

On my machine, with 32 cores, running this:

function testParallelBlocking
nIter = 3;
pool = gcp;
j = parfeval(pool,@backgroundFcn,1);
for i = 1:nIter
    forLoopTime(i) = now;
end
parfor i = 1:nIter
    parforLoopTime(i) = now;
end
disp(['Parfeval completed at ' datestr(j.fetchOutputs,'HH:MM:SS:FFF')]);
for i = 1:nIter
    disp(['For loop completed at ' datestr(forLoopTime(nIter),'HH:MM:SS:FFF')]);
end
for i = 1:nIter
    disp(['Parfor loop completed at ' datestr(parforLoopTime(nIter),'HH:MM:SS:FFF')]);
end
end
function out = backgroundFcn
pause(5);
out = now;
end

produces this output:

Parfeval completed at 15:15:25:244
For loop completed at 15:15:20:232
For loop completed at 15:15:20:232
For loop completed at 15:15:20:232
Parfor loop completed at 15:15:25:269
Parfor loop completed at 15:15:25:269
Parfor loop completed at 15:15:25:269

showing that while the rest of the code continues to run, the parfor loop does not begin until the parfeval job has completed.

Is there any way to have the parfor loop execute while the parfeval job is also running in the background? I have tried limiting the number of parfeval workers by changing

parfor i = 1:nIter

to

parfor (i = 1:nIter,nWorkers)

where nWorkers < (number of cores – number of pending parfeval jobs) but it does not change the behavior.

Thanks for your help.

Answer 1

This is expected behaviour. Unfortunately, there is currently no way to overlap parfeval and parfor. Your best bet is probably to adapt your parfor loop to use parfeval instead (I appreciate that this might not be terribly convenient).