Run parfor as for loop

In previous versions of Matlab, if I ran a parfor loop without first calling matlabpool, it would execute as a serial for loop. This made it easy to run portions of my code either way with a simple switch. However, in recent versions, the default preference is to start a new pool automatically when a parfor is encountered, so the loop is run in parallel regardless of whether I set up a parallel pool ahead of time.

Is there a way to programatically disable this behavior while running a function? I can change the preferences in my own version, but so far I haven't found a way to make sure this setting isn't active when someone else runs the code on their computer.

Answer 1

[Edric Ellis]

Unfortunately there is no programmatic way to change the preference. What you can do is use the optional argument to parfor that specifies the number of workers to use. You could do something like this:

% Create a helper function to choose the NumWorkers argument to PARFOR
function arg = getParforArg()
p = gcp('nocreate'); % get the pool object if it exists, but never open a pool
if isempty(p)
  arg = 0;
else
  arg = p.NumWorkers;
end
end

% Then, use that in your PARFOR loops.
parfor (idx = 1:10, getParforArg())
  ... do stuff ...
end

The optional argument to parfor is described in the reference page.

[Kelly Kearney]

Thanks, that does the trick!

[Matt J]

Unfortunately, this approach doesn't allow you to set breakpoints in the body of the parfor loop, even when getParforArg() returns zero

Answer 2

[Moritz Poll]

In case anyone comes across this question in 2021 or later, it seems that setting the number of workers to zero does the trick. For example:

if feature('numcores') > 0
    M = feature('numcores');
else
    M = 0;
end
parfor (i = 1:10, M)
    % do something in parallel or serially depending on how many cores the
    % machine has you are running on. Useful when running on a server that
    % let's you decide how many CPUs to allocate. If you allocate only one,
    % this will run serially.
end