Reference counting

Abstract: In previous heap storage systems, the cost of creating objects and garbage collection is independent of the lifetime of the object. Since objects with short lifetimes account for a large portion of storage use, it is worth optimizing a garbage collector to reclaim storage for these objects more quickly. The garbage collector should spend proportionately less effort reclaiming objects with longer lifetimes. We present a garbage collection algorithm that (1) makes storage for short-lived objects cheaper than storage for long-lived objects, (2) that operates in real time—object creation and access times are bounded, (3) increases locality of reference, for better virtual memory performance, (4) works well with multiple processors and a large address space.

Abstract: This paper explores and quantifies garbage collection behavior for three whole heap collectors and generational counterparts: copying semi-space, mark-sweep, and reference counting, the canonical algorithms from which essentially all other collection algorithms are derived. Efficient implementations in MMTk, a Java memory management toolkit, in IBM's Jikes RVM share all common mechanisms to provide a clean experimental platform. Instrumentation separates collector and program behavior, and performance counters measure timing and memory behavior on three architectures.Our experimental design reveals key algorithmic features and how they match program characteristics to explain the direct and indirect costs of garbage collection as a function of heap size on the SPEC JVM benchmarks. For example, we find that the contiguous allocation of copying collectors attains significant locality benefits over free-list allocators. The reduced collection costs of the generational algorithms together with the locality benefit of contiguous allocation motivates a copying nursery for newly allocated objects. These benefits dominate the overheads of generational collectors compared with non-generational and no collection, disputing the myth that "no garbage collection is good garbage collection." Performance is less sensitive to the mature space collection algorithm in our benchmarks. However the locality and pointer mutation characteristics for a given program occasionally prefer copying or mark-sweep. This study is unique in its breadth of garbage collection algorithms and its depth of analysis.

Abstract: A distributed client-server application is modified while running. The application is not stopped so that updating of objects is transparent. A meta server catalogs all object classes for both the server and the clients. Modifications are specified by a run-time update tool and converted to change commands. The meta server receives the change commands and updates the structure of an application database. Object class definitions are read from the meta server and modified by the meta server to access the new structure of the application database. The modified object-class definitions are written back to persistent storage for the meta server, and compiled and linked to form new object classes. An object adaptor receives a list of modified object classes from the meta server and notifies all server and client caches of the object classes on the list. The obselete objects are invalidated by the caches and new objects are created using the most up-to-date class definitions. New references to the objects are sent to the new objects, although the old objects continue to process existing references until their reference count reaches zero and they are deleted. Old and new objects co-exist for a period of time during the run-time update.

Abstract: The analysis of the lifetime of objects in a garbage-collected system may be accomplished quickly and effectively using reference counts and cyclic garbage analysis. A reference count is maintained for each of the objects to indicate the number of incoming pointers. Each time the graph structure is altered, the reference counts are updated. Timestamps are recorded each time the reference count for objects change. If a reference count goes to zero, the corresponding object may be indicated as dead. A garbage collection need only be run once (perhaps at the end), and after it is run the system may indicate which objects are cyclic garbage. The timestamps for objects which are cyclic garbage are then reviewed in reverse chronological order. For each timestamp found, the corresponding object and any object reachable from the corresponding object are indicated as dead. These objects are then removed from the set of cyclic garbage.