Smalltalk and Object Orientation: an Introduction - Free

30.3.3 PermSpace
This is used to house objects which are rarely eligible for garbage collection. Examples of such objects
are the system classes, tools and compilers. None of the garbage collections attempt to process this area
of memory except the “Global garbage collector”. This is initiated by the user from the VisualLauncher.
Again this is a performance “tweak” and means that most of the time items such as a class do not have
to be checked to see if the memory they occupy should be reclaimed nor must PermSpace be
compacted.
30.4 Garbage collection and memory compaction
There are a number of different ways in which garbage collection and memory compaction can occur.
The first is the “Generation Scavenger”. This is called whenever NewSpace exceeds the scavenge
threshold. It only processes objects in NewSpace (although it does place objects in OldSpace).
Another garbage collector is the incremental garbage collector (or IGC). This is called from
Smalltalk code (as opposed to the scavenger which is called automatically by the Virtual Machine). The
IGC reclaims objects in OldSpace, NewSpace and LargeSpace. It is called the incremental garbage
collector because it works in an incremental manner to avoid disturbing the executing VisualWorks
environment.
The two remaining garbage collectors are the “Compacting Garbage Collector” and the “Global
Garbage Collector”. These two garbage collect ors are under user control and are initiated from the
VisualLauncher. The compacting garbage collector is run when the user selects the “Garbage
Collection” option on the special menu on the Launcher. The global garbage collector is run when the
user select the “Global Garbage Collection” option on the same menu (and processes all of the memory
available).
Finally, there is a data compactor which runs on the OldSpace which does not do any garbage
collection, only memory compaction. This runs considerably faster than the garbage collectors.
30.5 Placing objects in PermSpace
PermSpace is an error of memory which is intended to hold objects which will never die or which will
only rarely die. This means that most of the garbage collection methods do not need to worry about
what is in PermSpace, thus reducing the amount of memory they must scan. Obviously some of the
objects in PermSpace may die off, the global garbage collector therefore scans PermSpace as well.
It is possible to move objects from OldSpace to PermSpac e to increase the performance of the
garbage collector. This effect is achieved because by moving objects out of OldSpace the number of
objects which must be considered is reduced. This can have a major effect on the performance of the
garbage collector.
When you start up your “clean” image, most of the objects (including classes) present in the system
will be located in PermSpace. When you create new objects these will reside in NewSpace or OldSpace.
To relocate them into PermSpace it is necessary to save the current image using the “Perm Save As ...”
option from the “ File” menu option in the VisualLauncher. This writes the objects in OldSpace into
PermSpace in the saved image. It does not affect where objects are located in the current (executing)
image, only in the saved image. It is therefore necessary to exit the image and start up the newly saved
image to have an executing image, in which the objects are stored in PermSpace.
There will however be a number of objects in PermSpace which are transient (i.e . likely to die in the
short term). These objects will have been placed into PermSpace, because all objects, whether or not
you wanted them to, will have been saved into PermSpace. Example of such objects may included
windows, objects with which you were w orking etc. To remove these from the image (thus improving
the performance of the garbage collector) perform a global garbage collection. Once you have done this
save the image in the normal manner (i.e. “ Save as...”). Having started up the newly saved ima ge you
could increase the time taken to start the image by performing one last save of the image. This is
necessary because the global garbage collector compacts the objects in PermSpace. This means that
when the image is started up, the system must reloca te these objects. If you make one more image, then
the objects will already have been relocated and the startup time will be minimized.
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