Garbage collection and memory
management
Contents
The PowerBuilder garbage collection mechanism checks memory
automatically for unreferenced and orphaned objects and removes any it
finds, thus taking care of most memory leaks. You can use garbage
collection to destroy objects instead of explicitly destroying them using
the DESTROY statement. This lets you avoid runtime errors that occur when
you destroy an object that was being used by another process or had been
passed by reference to a posted event or function.
A reference to an object is any variable whose value is the object.
When the variable goes out of scope, or when it is assigned a different
value, PowerBuilder removes a reference to the object and counts the
remaining references, and the garbage collection process destroys the
object if no references remain.
Garbage collection occurs:
-
When the garbage collection interval has been exceeded and the
PowerBuilder application becomes idle and -
When you explicitly call the GarbageCollect function.
When PowerBuilder completes the execution of a system-triggered
event, it makes a garbage collection pass if the set interval between
garbage collection passes has been exceeded. The default interval is 0.5
seconds. Note that this system-triggered garbage collection pass only
occurs when the PowerBuilder application is idle, therefore if a long
computation or process is in progress when the interval is exceeded,
garbage collection does not occur immediately.
You can force immediate garbage collection by invoking the
GarbageCollect function. When you use dot notation and OLEObjects,
temporary variables are created. These temporary variables are released
only during the garbage collection process. You might want to invoke
GarbageCollect inside a loop that appears to be causing memory
leaks.
The garbage collection pass removes any objects and classes that
cannot be referenced, including those containing circular references
(otherwise unreferenced objects that reference each other).
Posting events and functions
When you post an event or function and pass an object reference,
PowerBuilder adds an internal reference to the object to prevent its
memory from being reclaimed by the garbage collector between the time of
the post and the actual execution of the event or function. This
reference is removed when the event or function is executed.
Exceptions to garbage
collection
There are a few objects that are prevented from being
collected:
-
Visual objects
Any object that is visible on your screen is not collected
because when the object is created and displayed on your screen, an
internal reference is added to the object. When any visual object is
closed, it is explicitly destroyed. -
Timing objects
Any Timing object that is currently running is not collected
because the Start function for a Timing object adds an internal
reference. The Stop function removes the reference. -
Shared objects
Registered shared objects are not collected because the
SharedObjectRegister function adds an internal reference.
SharedObjectUnregister removes the internal reference.
Controlling when garbage collection
occurs
Garbage collection occurs automatically in PowerBuilder, but you can
use functions to force immediate garbage collection or to change the
interval between reference count checks. Three functions let you control
when garbage collection occurs: GarbageCollect,
GarbageCollectGetTimeLimit, and GarbageCollectSetTimeLimit.
For information about these functions, see PowerScript Reference. For an example illustrating their
use, see the Code Examples sample application, described in Using Sample
Applications.
Performance concerns
You can use tracing and profiling to examine the effect of changing
the garbage collection interval on performance.
For information about tracing and profiling, see the section called “Tracing and Profiling Applications” in Users Guide.