# Mianzhi Wang

Ph.D. in Electrical Engineering

# Tracking Parallel Simulation Progress in MATLAB

Running numerical simulations with various parameter settings in MATLAB can take a very long time. Printing the current progress to the command window can help ease our anxious hearts and estimate the remaining time required to finish all simulations (so we can take a break and have a cup of tea). Usually our script will look like this:

for ii = 1:n_param
run_simulation(param(ii));
fprintf('Progress: %.1f\n', ii/n_param);
end


However, the above code does not work with parfor, the paralleled for loop, because parfor does not execute the loops in the iteration order. We will get seemingly randomly ordered outputs (just like multithreaded programming):

Progress: 60.0%
Progress: 20.0%
Progress: 70.0%
Progress: 10.0%


We may change our code into the following:

n_completed = 0;
parfor ii = 1:n_param
run_simulation(param(ii));
n_completed = n_completed + 1;
fprintf('Progress: %.1f\n', n_completed/n_param);
end


However, this will lead to race conditions as each MATLAB worker will try to update n_completed. Actually the MATLAB editor is smart enough to report an error if we write something like this. We can hack around this error by storing n_completed in an external file using file IO functions. However, race conditions still exist as two MATLAB workers may try to access the file at the same time.

Instead of using parfor, we can manually schedule the evaluation of our functions on MATLAB workers with parfeval. The first three arguments of parfeval specify the parallel pool we will use, the handle of our function, and the number of outputs. The remaining will be the passed as the input arguments of our function. Detailed documentation can be accessed via doc parfeval. The function parfeval will immediately return a FevalFuture object, which can be used to fetch the results via fetchOutputs or fetchNext. The function fetchNext will block the execution until new results are available. Therefore we can report the progress upon each return of parfeval. With these in mind, we can rewrite our code as:

p = gcp(); % Get the current parallel pool
for ii = 1:n_param
% Assuming our function has only one output
% This line of code will not block the execution.
f(ii) = parfeval(p, @run_simulation, 1, param(ii));
end
results = cell(ii, 1);
for ii = 1:n_param
% Fetch the next available result
% This will block the execution until new results are available or all
% results are fetched.
[idx, value] = fetchNext(f);
% Store the new result. Note that fetchNext also returns the index of the
% newly fetched result. We can utilize this to keep our results in the
% original iteration order.
results{idx} = value;
% Finally, we can report the progress here.
fprintf('Progress: %.1f\n', ii/n_param);
end


In this case, as soon as a worker finishes its job, fetchNext will return and the second loop will advance and report the current progress.

Note: unlike parfor, Ctrl + C will not cancel all pending jobs immediately. You will need to run cancel(f) to cancel all unfinished jobs scheduled by parfeval.

The function parfeval was introduced in MATLAB 2013b. Therefore the above method should work on all recent MATLAB versions. Interestingly, MATLAB 2017a recently introduced a new class named parallel.pool.DataQueue, making it possible to send data from workers to client using a data queue. This opens up lots of new possibilities. Actually, the official documentation actually gives a nice example on how to use this class to create a progress bar. Therefore, with MATLAB 2017a or later, the solution will become:

% Initialized the queue
q = parallel.pool.DataQueue;
% After receiving new data, update_progress() will be called
afterEach(q, @update_progress);
n_completed = 0;
parfor ii = 1:n_param
run_simulation(param(ii));
% Send data to the queue
send(q, ii);
end
% Put this function at a proper location
function update_progress()
n_completed = n_completed + 1;
fprintf('Progress: %.1f\n', n_completed/n_param);
end