Control/Indicator, Local Variable, and Value Property Node Differences

Issue Details

My VI runs differently when I use local variables as opposed to value property nodes or controls/indicators directly. What differences are there between these?

Solution

There are several performance trade-offs to consider when selecting which method to use for updating your front panel controls.  The available update methods are Controls/Indicators, Local Variables, or Value Property Nodes. Here are the benefits and negatives of each method:

Controls/Indicators
Benefits

• Built in logic to prevent front panel updates when continuously updating the same value. This prevents front panel re-draws.
• Do not need to de-reference pointers, nor make copies of the data in memory.  As such this is the fastest, and least memory intensive option.

Negatives

• You cannot write to a single item in a cluster/array, but must update the entire variable

C analogy 
(this is not a C equivalent, but helps to show the steps that must be taken to update a corresponding front panel control/indicator).

int x;   // Front Panel Indicators
x = 10;  // Write directly to the Front Panel Indicator.


Local Variables
Benefits

• Have built in logic to prevent front panel updates when continuously updating the same value.  This prevents front panel re-draws.
• Do not need to de-reference pointers, unlike property nodes.  As such they are faster than property nodes.

Negatives

• Memory intensive process.  These have the same front panel update logic as controls/indicators, however they must make a full copy of the data in memory for each local variable you create.
• You cannot write to a single item in a cluster/array, but must update the entire variable.
• Race conditions may occur if you are writing to and reading from the same local variable at different rates.

C analogy 
(this is not a C equivalent, but helps to show the steps that must be taken to update a corresponding front panel control/indicator).

int x; // Front Panel Indicator
int y; // Local variable
y=10;  // Write to local variable
x=y;   // LabVIEW does this for you to update the front panel.


Property Nodes
Benefits

• Do not create copies of data in memory unlike Local Variables.
• If you are using a non-strict control reference, then the data type of the value may be a variant rather than the actual data type of the control.
• Capable of reading and writing of a single control within a cluster
• Can be used to update front panel controls from within a SubVI

Negatives

• Required to update the front panel item every single time they are called.  
• They are a pass by reference function as opposed to a pass by value function.  This means they are essentially pointers to specific memory locations.  The pointers must be de-referenced, and then the value in memory updated.  The process of de-referencing the variables causes them to be slower than Controls/Indicators, or Local Variables.
• Property Nodes cause the front panel of a SubVI to remain in memory, which increases memory use.  If the front panel of a SubVI is not displayed, remove property nodes to decrease memory use.

C analogy 
(this is not a C equivalent, but helps to show the steps that must be taken to update a corresponding front panel control/indicator).

int x;                  // Front Panel Indicator
int *x_pointer = &x;    // Create pointer to Front Panel Indicator 
                        // (Property Node)
*x_pointer = 1;  // * is used to dereference the pointer and update
                 // variable X. This is the operation that happens
                 // every time a property node executes.