Matlab still busy a long time after last line of script is executed

Summary: Script runs fine on first execution, Matlab stays busy very long after a subsequent execution of the exactly same script.

I am learning Matlab and am running a simple script with a couple of for- and if-loops and very small matrices (<1000 entries).

After first starting up Matlab and running the script it takes about 2 seconds to finish the script and the variables appear in the workspace shortly after. This leads me to believe that the code that I have is somewhat reasonable (no crazy infinite loops or anything like that...).

Then, after clearing the workspace, I do another, subsequent run of the same script without changing anything, the script executes in about 0.5s which is what I would expect.

However, after the last line is executed (in my case it is "toc"), Matlab remains busy for a solid 1-2 minutes. Only then, after this long time, the variables appear in the workspace again.

During all of this, I am monitoring CPU and RAM usage, and it stays well below 50% so I assume that is not the problem. What am I missing?

[Adam Danz]

How are you clearing the workspace?

If you're using clear all, that could explain the increased processing time after clearing the workspace. Clear-all decreases code performance and is usually unnecessary.

You could run the profiler to get more details about what's sucking up the time.

[Gunther Breitenberger]

Hi Adam,

thanks for your reply. I am typing "clear" into the command window. Also, the first line of code in the script is "clear".

It looks like this:

tic
clear
%my script here...
toc

I added these lines because I wanted to make sure not to have any leftover indices from the previous run or anything like that (total newbie). The problem seems to be independent on whether or not I am clearing the workspace in between runs.

[Adam Danz]

"clear all" is different than just "clear".

I assum you're working from a script. If you were working from a function, it would have its own workspace and you wouldnt' have to worry about variable contamination from the previous run.

Anyway, the clear command is not causing the problem. Run the profiler and share the results. That may provide some insight.

[Gunther Breitenberger]

I tried to run the profiler during the 190s from starting my script to the appearance of my variables in the workspace, but I am not sure if this is what you meant.

[Adam Danz]

That report shows where the time is spent but I don't know enough what how that line fits into your code for it to be helpful.

I recommend using the debugger to step through your code line-by-line using the F10 shortcut to determine which line is causing the problem.

You can't use the command window to precisely determine where the problem is because the code execution is usually ahead of the command window display updates.

[Bruno Luong]

Just wonder if you have any graphic object created (figure, Gui, AppDesigner and such) by the script ? MATLAB flushs the events, etc... now and then, and also when the script ends.

Check also if you have any (invalid) network drive and MATLAB for some reason try to access it.

[Gunther Breitenberger]

Hi Bruno, I don't create any graphic object.

It seems to me like the issue is somehow caused by the output window.

Clearing the output window (Clear All Output) before a rerun of the script will reduce the busy time from from 190s to 14s.

So the situation presents itself like this:

Fresh Matlab startup and running script: 8s busy

Rerunning script: 190s busy

Rerunning script with clearing workspace: 190s busy

Rerunning script with clearing output window: 14s busy

[Adam Danz]

ahhhh...... so you're using Live Editor. That's an important detail.

Answer 1

[Adam Danz]

This is a known issue with Live Editor.

See this potential solution.

https://www.mathworks.com/matlabcentral/answers/535869-why-do-the-live-editor-command-window-and-simulink-get-slow-after-several-runs-of-a-live-script

Note that the solution also mentions a point I made in the comments under your question. Make sure your script does not contain the command, "clear all".

[Gunther Breitenberger]

Thank you Adam!

[Adam Danz]

Glad I can help.

If live editor is not needed, use a standard m-file instead. They are faster.