Smarter garbage collection with simplifiers

We introduce a method for providing lightweight daemons, called simplifiers, that attach themselves to program data. If a data item has a simplifier, the simplifier may be run automatically from time to time, seeking an opportunity to "simplify" the object in some way that improves the program's time or space performance.It is not uncommon for programs to improve their data structures as they traverse them, but these improvements must wait until such a traversal occurs. Simplifiers provide an alternative mechanism for making improvements that is not tied to the vagaries of normal control flow.Tracing garbage collectors can both support the simplifier abstraction and benefit from it. Because tracing collectors traverse program data structures, they can trigger simplifiers as part of the tracing process. (In fact, it is possible to view simplifiers as analogous to finalizers; whereas an object can have a finalizer that is run automatically when the object found to be dead, a simplifier can be run when the object is found to be live.)Simplifiers can aid efficient collection by simplifying objects before they are traced, thereby eliminating some data that would otherwise have been traced and saved by the collector. We present performance data to show that appropriately chosen simplifiers can lead to tangible space and speed benefits in practice.Different variations of simplifiers are possible, depending on the triggering mechanism and the synchronization policy. Some kinds of simplifier are already in use in mainstream systems in the form of ad-hoc garbage-collector extensions. For one kind of simplifier we include a complete and portable Java implementation that is less than thirty lines long.