Variable size: does it reflect real size after computation finishes?

Hi all,

I have a program which produces a 40227 by 50 double matrix. I'd like to optimize storage so I want to check total memory usage. It is in structural variable "canti". If I run whos, MATLAB shows:

K>> whos
  Name                Size            Bytes  Class              Attributes

    INPname             1x60              120  char                         
    a                   1x1                 8  double                       
    ans                 1x1                 1  logical                      
    bondL1              1x1                 8  double                       
    bondL2              1x1                 8  double                       
    bondR1              1x1                 8  double                       
    bondR2              1x1                 8  double                       
    canti               1x1               112  canbeam

However, structural variable "canti" contains a few large variables as follows:

K>> canti.dis
ans = 
           inpt: [40227x1 double]
            val: [40227x50 double]
           full: [40227x50 double]
          trial: [40227x50 double]

Why would "canti" only uses 112 bytes of memory while it contains variables of 40227 by 50 matrix? According to my test, if I do:

>> a = rand(40227, 50);
>> whos
  Name          Size               Bytes  Class     Attributes

    a         40227x50            16090800  double

A 40227 by 50 double should cost 16090800 bytes in memory, which is 15.3454 MB. If MATLAB is not showing the real memory usage for some reason, then how can I check the memory usage during and after my program?

Many thanks!

Answer 1

[Stephen23]

A structure does not really contain any arrays, but is a list of links to other arrays in memory (some might call this an "an array of pointers"). As such, whos correctly returns the size of the structure itself, and not of the arrays that it has links to. See:

http://blogs.mathworks.com/loren/2008/02/01/structure-initialization/

http://www.mathworks.com/help/matlab/matlab_prog/memory-requirements-for-a-structure-array.html

https://www.mathworks.com/matlabcentral/answers/48807-how-preallocate-structure-for-better-memory

Unfortunately the standard whos cannot read the memory size (bytes) of arrays in a structure. You could try this FEX submission:

http://www.mathworks.com/matlabcentral/fileexchange/12278-vsize--a-pedestrian-variable-decoder

[Xh Du]

Now I understand, thanks!

Answer 2

[Walter Roberson]

Notice you have

      canti               1x1               112  canbeam

here, "canbeam" is the data class of the variable. It is not struct, not cell array, not numeric, but rather "canbeam", which is not a Mathwork-supplied data type. It must be an object oriented object from a class you have provided.

I suspect, from the size, that canbeam is a handle class. If so then because you can have multiple copies of the same handle, it would be incorrect to count the referenced memory against the handle: the 112 bytes is the size of the handle itself.

[Xh Du]

Thank you, you were right, "canbeam" is an OO class. The size only reflects the class handle itself, not the content inside.