MATLAB: Breakpoint does not work

breakpointMATLAB

Greetings,

I meet a very weird problem about breakpoint and none of my friends cannot solve it.

I put two breakpoints in 43 and 44 lines of Model class as Figure 1 shows.

When I runned the program, it stopped as Figure 2. From Figure 2, we can see the program never stopped at 43 and 44 lines, but directly threw the error and even skipped the 43 line disp(t).

However, if I put breakpoints at 42 lines and rerunned it, it can stop normally as Figure 3. But it still skipped 43 line and threw the error as Figure 2; and cannot stop at 43 and 44 lines, when exitflag was not 1.

In addition, I used "clear" in the main script, not "clear all". My Matlab version is R2018a.

Best Answer

The fact that the stack for the error includes the function parfinitedifferences suggests to me that the condition to enter that if statement is only satisfied on one of the workers used to evaluate the finite difference in parallel (you're passing the UseParallel option to fmincon, right?) In that case the breakpoint that was set on the client is not present on the workers.

You probably wouldn't want to enter debug mode on the workers anyway as they fall into the same general category as input and keyboard as called out on this documentation page. The worker would wait at the breakpoint for input that you couldn't give it.

Related Solutions

MATLAB: How to send data on the fly to a worker when using parfeval

I think it's possible to do this sort of thing by getting the workers to create a parallel.pool.PollableDataQueue, and getting the client to send messages to the worker. It's a bit awkward to set up, and in the example below, I'm going to use a parallel.pool.Constant to get the worker to hold onto the queue instance.

The general plan is this:

Get the worker to construct a pollable dataqueue
Send the dataqueue instance back to the client
Set the worker off into an loop waiting for messages from the client
Send messages from the client to get the worker to do stuff
When done, get the client to send a "poison pill" which gets the worker to return the result.

Note the code below only works for a pool of size 1. To run on a larger pool, you would need to use parfevalOnAll rather than parfeval, and you'd also need to manage the multiple queues from the workers.

% First, create a parallel pool if necessary
if isempty(gcp())
    parpool('local', 1);
end
% Get the worker to construct a data queue on which it can receive
% messages from the client
workerQueueConstant = parallel.pool.Constant(@parallel.pool.PollableDataQueue);
% Get the worker to send the queue object back to the client
workerQueueClient = fetchOutputs(parfeval(@(x) x.Value, 1, workerQueueConstant));
% Get the worker to start waiting for messages
future = parfeval(@doStuff, 1, workerQueueConstant);
% Send a few messages to the worker
for idx = 1:10
    send(workerQueueClient, idx);
end
% Send [] as a "poison pill" to the worker to get it to stop
send(workerQueueClient, []);
% Get the result
fetchOutputs(future)
% This function gets the worker to keep processing messages from the client
function out = doStuff(qConstant)
q = qConstant.Value;
out = 0;
while true
    % Wait for a message
    data = poll(q, Inf);
    if isempty(data)
        return
    else
        out = out + data;
    end
end
end

MATLAB: How to get parpool to pause at breakpoints

Unfortunately, it is not possible to use the MATLAB debugger to debug code running on workers. The normal workaround is to liberally sprinkle fprintf or disp calls through your code that will be run on the workers. Often this is sufficient to narrow down the problematic portion of code to something you can debug on the client.

Best Answer

Related Solutions

MATLAB: How to send data on the fly to a worker when using parfeval

MATLAB: How to get parpool to pause at breakpoints

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