Run Simulink Model via parsim() and save after each simulation

I am trying to run my simulink model in parallel while saving the results to a matfile after each simulation has completed.

Each worker should take the following action:

1. Simulate model with simIn( j )
2. Create matfile for above simulation
3. Repeate with simIn ( K ) until all simulations are complete.

The below code is my starting point. "test_model" is a simulink model of only a sin wave going into a gain, integrator, and to workspace block simply to test out parsim().

%% Example of how to run simulink models in parallel
clear; close all; clc;

mdl = "test_model";
amp= 1:10;
freq = 1:10;

nSims = length(amp);
simIn(1:nSims) = Simulink.SimulationInput(mdl);

% Setup model workspace
k = 5;

for i =1:nSims
    nm = [num2str(i) '.mat'];

    simIn(i) = setVariable(simIn(i), "amp", amp(i), 'Workspace', mdl);
    simIn(i) = setVariable(simIn(i), "freq", freq(i), 'Workspace', mdl);
    simIn(i) = setPostSimFcn(simIn(i), @(x)postSim(x, simIn(i), nm));

end

out = parsim (simIn, 'UseFastRestart','on', 'TransferBaseWorkspaceVariables','on');

function postSim(out, in, nm)
% postSim - Run @ completion of each simulation
% Saves simulation input, output, and name to file "nm"
    m = matfile(nm, 'Writable', true);
    m.out = out;
    m.in = in;
    m.nm = nm;
end

I understand that save() does not work as expected in parallel, so I've implemented the example from the question below as a method of creating my mat files. The above code works exactly as expected if "parsim" is replaced with "sim"; however, no files are created when using "parsim".

https://www.mathworks.com/matlabcentral/answers/135285-how-do-i-use-save-with-a-parfor-loop-using-parallel-computing-toolbox

Note: setting 'TransferBaseWorkspaceVariables' to 'off' and setting k via setVariable does not have any impact on this behavior.

Ask: How can I modify the above code to also work with the parsim command?

Answer 1

[Trent Gatz]

I still believe this to be an error with parsim(); however, one workaround is to use:

%out = parsim(simIn);
parfor i = 1:nSims
    out(i) = sim(simIn(i));
end

[Paul]

Are there any functional or behavioral differences between calling parsim or calling sim inside a parfor?

If the latter is the same as the former, why would parsim even be an option?

[Trent Gatz]

As far as I can tell, they behave identically. Matlab gives a warning that parsim is prefered over sim in a parfor, but I cannot tell why.

This link has information on both, again stating to use parsim, but it's not clear what the benefit would be: https://www.mathworks.com/help/simulink/ug/not-recommended-using-sim-function-within-parfor.html

Here it says that parsim avoids potential difficulties encountered with sim inside parfor, but doesn't list them out:

https://www.mathworks.com/matlabcentral/answers/1929645-using-sim-command-in-parfor

[Paul]

Why is it necessary to create the .mat files on each worker, as opposed to saving all the .mat files on the client after parsim completes, should there be a future need or desire to go back to parsim? Asking because the solution or workaround, should there be one, may depend on the answer.

Also, if you think there's a bug with parsim, you can submit a report to Tech Support and see if they agree. They might even provide a workaround themselves.

[Trent Gatz]

I need to save to a matfile after each simulation as the output is large and keeping it all in RAM wouldn't be possible. Additionally, if MATLAB were to crash or my computer were to restart / lose power, I'd like to maintain this checkpoint of progress completed rather than starting over with no simulations completed.

[Paul]

According to Data Logging for Multiple Simulations you can go to Model Settings -> Data Import/Export check the box for "Log data set to file" and specify the file name (call it 'logfile.mat' for sake of discussion)

After executing

simOut = parsim(simIn)

the files logfile_1.mat, log_file_2.mat, etc. will be in the directory from which parsim was executed (maybe the output directory can be changed?).

When I experimented, these file contain the signals that were marked for logging in the block diagram, but do not include anything else that shows up in simOut, like ToWorkspace data and the simulation metadata (and the simOut will not include the logged data) and won't include the simIn either (but simIn is known a priori and can be saved before starting parsim).

You can also turn on the logging to file in the SimIn object if you don't want to change the model itself. See this thread.

Don't know when these log files are actually written back so have no idea what the state of things would be if the parsim stops executing before it's complete.