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Matlab doesn't release memory when variables are cleared

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I am working with very large data sets (~50 GB) in matlab R2015a. When I clear variables, matlab frequently does not release the memory. I have tried clear all, clear classes, clear java and running the java garbage collector. The only way I can get matlab to release the memory is close and restart matlab. Is there a better way?
Science Machine
Science Machine on 8 Apr 2022
you can write a script to close matlab and continue with the next function in a new file, after saving variables to disk. Not ideal for sure, but it seems that eg. even shutting down parallel workers does not release a substantial chunk of ram, and that fully closing is indeed the only option here..

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Accepted Answer

dpb on 13 Dec 2016
It has more to do with Windows and how its memory-management routines work (or not) regarding what memory that is marked as unused by the application is actually physically released and when. Also, even though there may be sufficient total free memory, it is free contiguous memory that is limiting when creating arrays; if there isn't sufficient for the job, you're stuck.
There are guidelines to help and also some newer techniques more recently introduced you can try--

More Answers (4)

Sean de Wolski
Sean de Wolski on 13 Dec 2016
You can force clear a variable from memory by setting it to empty rather than calling clear on it.
x = [];
clear x
Bonnie Tyler
Bonnie Tyler on 15 Dec 2016
I have tried both setting the variables to [] and clearing and have the same problem.
dpb on 15 Dec 2016
Edited: dpb on 15 Dec 2016
They're the same; all an application can do is mark the memory as "unused"; it's up to the OS to reclaim it; see the above.
In the olden days in FORTRAN with nothing but static memory allocation, the "trick" was to allocate a very large chunk of memory and then use it by subscripting within it judiciously.
Perhaps reusing existing memory from one of your processing step to the next would be a similar possibility here of assigning the output of the step to the same variable as previously used. Matlab may still need to make copies if it can't tell that memory can be overwritten safely, so it may not help; there again may not be sufficient contiguous memory for the temporary, but it's a tactic you could possibly try.
Alternatively, perhaps it's time for mex files or, if you were able to illustrate specifically what your processing steps are, perhaps with some additional background others may have more efficient processing ideas.

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Adam Danz
Adam Danz on 18 Oct 2017
Bonnie mentioned that clear all, clear classes, etc didn't work but what worked for me was using:
clearvars -global
This immediately reduced memory devoted to Matlab from 3.2 gig to 0.7 gig. In my case, one or two GUIs that were closed were still occupying a lot of memory.

Arwel on 4 Jul 2019
Previously, Ive just done something this which seems to work.....
% After deleting your large variable, go....

Christian Schwermer
Christian Schwermer on 16 Aug 2020
Edited: Christian Schwermer on 19 Aug 2020
i had a similar problem in my GUI, where i used a cell array as FIFO buffer to acquire images. Memory usage increases for every session. Only closing matlab and restart releases the memory usage:
bufferSize = 450;
frame_buffer = cell(1, bufferSize);
frame_buffer(:) = {[]}=;
When i preallocate the buffer for each cell . Memory usage stays on a constant acceptable level. Nevertheless it wasn't possible to release memory without restarting:
ROI = VideoInputObj.ROIPosition;
bufferSize = 450;
frame_buffer = cell(1, bufferSize);
frame_buffer(:) = {zeros(ROI(4), ROI(3) ,'uint8')} ;

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