Programming and Data Types

Making Efficient Use of Memory

This section discusses the following ways to conserve memory and improve memory use:

• Memory Management Functions
• Removing a Function From Memory
• Nested Function Calls
• Variables and Memory
• PC-Specific Topics
• UNIX-Specific Topics
• What Does "Out of Memory" Mean?

Memory Management Functions

MATLAB has five functions to improve how memory is handled:

• clear removes variables from memory.
• pack saves existing variables to disk, then reloads them contiguously. Because of time considerations, you should not use pack within loops or M-file functions.
• quit exits MATLAB and returns all allocated memory to the system.
• save selectively stores variables to disk.
• load reloads a data file saved with the save command.

Note    save and load are faster than MATLAB low-level file I/O routines. save and load have been optimized to run faster and reduce memory fragmentation.

On some systems, the whos function displays the amount of free memory remaining. However, be aware that:

• If you delete a variable from the workspace, the amount of free memory indicated by whos usually does not get larger unless the deleted variable occupied the highest memory addresses. The number actually indicates the amount of contiguous, unused memory. Clearing the highest variable makes the number larger, but clearing a variable beneath the highest variable has no effect. This means that you might have more free memory than is indicated by whos.
• Computers with virtual memory do not display the amount of free memory remaining because neither MATLAB nor the hardware imposes limitations.

Removing a Function From Memory

MATLAB creates a list of M- and MEX-filenames at startup for all files that reside below the matlab/toolbox directories. This list is stored in memory and is freed only when a new list is created during a call to the path function. Function M-file code and MEX-file relocatable code are loaded into memory when the corresponding function is called. The M-file code or relocatable code is removed from memory when:

• The function is called again and a new version now exists.
• The function is explicitly cleared with the clear command.
• All functions are explicitly cleared with the clear functions command.
• MATLAB runs out of memory.

Nested Function Calls

The amount of memory used by nested functions is the same as the amount used by calling them on consecutive lines. These two examples require the same amount of memory.

result = function2(function1(input99));

result = function1(input99);
result = function2(result);

Variables and Memory

Memory is allocated for variables whenever the left-hand side variable in an assignment does not exist. The statement

x = 10

allocates memory, but the statement

x(10) = 1

does not allocate memory if the 10th element of x exists.

To conserve memory:

• Avoid creating large temporary variables, and clear temporary variables when they are no longer needed.
• When working with arrays of fixed size, pre-allocate them rather than having MATLAB resize the array each time you enlarge it.
• Set variables equal to the empty matrix [] to free memory, or clear them using

clear variable_name

• Reuse variables as much as possible.

Global Variables.   Declaring variables as global merely puts a flag in a symbol table. It does not use any more memory than defining nonglobal variables. Consider the following example.

global a
a = 5;

Now there is one copy of a stored in the MATLAB workspace. Typing

clear a

removes a from the MATLAB workspace, but it still exists in the global workspace.

clear global a

removes a from the global workspace.

PC-Specific Topics

• There are no functions implemented to manipulate the way MATLAB handles Microsoft Windows system resources. Windows uses system resources to track fonts, windows, and screen objects. Resources can be depleted by using multiple figure windows, multiple fonts, or several Uicontrols. The best way to free up system resources is to close all inactive windows. Iconified windows still use resources.
• The performance of a permanent swap file is typically better than a temporary swap file.
• Typically a swap file twice the size of the installed RAM is sufficient.

UNIX-Specific Topics

• Memory that MATLAB requests from the operating system is not returned to the operating system until the MATLAB process in finished.
• MATLAB requests memory from the operating system when there is not enough memory available in the MATLAB heap to store the current variables. It reuses memory in the heap as long as the size of the memory segment required is available in the MATLAB heap.

For example, on one machine these statements use approximately 15.4 MB of RAM.

  a = rand(1e6,1);
  b = rand(1e6,1);

These statements use approximately 16.4 MB of RAM.

  c = rand(2.1e6,1);

These statements use approximately 32.4 MB of RAM.

  a = rand(1e6,1);
  b = rand(1e6,1);
  clear
  c = rand(2.1e6,1);

This is because MATLAB is not able to fit a 2.1 MB array in the space previously occupied by two 1 MB arrays. The simplest way to prevent overallocation of memory, is to preallocate the largest vector. This series of statements uses approximately 32.4 MB of RAM

  a = rand(1e6,1);
  b = rand(1e6,1);
  clear
  c = rand(2.1e6,1);

while these statements use only about 16.4 MB of RAM

  c = rand(2.1e6,1);
  clear
  a = rand(1e6,1);
  b = rand(1e6,1);

Allocating the largest vectors first allows for optimal use of the available memory.

What Does "Out of Memory" Mean?

Typically the Out of Memory message appears because MATLAB asked the operating system for a segment of memory larger than what is currently available. Use any of the techniques discussed in this section to help optimize the available memory. If the Out of Memory message still appears:

• Increase the size of the swap file.
• Make sure that there are no external constraints on the memory accessible to MATLAB (on UNIX systems use the limit command to check).
• Add more memory to the system.
• Reduce the size of your data.

Preallocating Arrays

Character Arrays (Strings)