Abstract

This paper discusses garbage collection techniques used in a high-performance Lisp implementation with a large virtual memory, the Symbolics 3600. Particular attention is paid to practical issues and experience. In a large system problems of scale appear and the most straightforward garbage-collection techniques do not work well. Many of these problems involve the interaction of the garbage collector with demand-paged virtual memory. Some of the solutions adopted in the 3600 are presented, including incremental copying garbage collection, approximately depth-first copying, ephemeral objects, tagged architecture, and hardware assists. We discuss techniques for improving the efficiency of garbage collection by recognizing that objects in the Lisp world have a variety of lifetimes. The importance of designing the architecture and the hardware to facilitate garbage collection is stressed. Automatic Storage Management Storage management is the part of a Lisp implementation that controls the use of memory to contain representations of objects. When a new object is created, memory must be allocated to contain its representation. When an object is no longer in use, the memory occupied by its representation can be reused for other purposes. Storage management can have a major impact on the efficiency and usability of a Lisp system. Automatic storage management allows the user to think entirely in terms of objects while the system takes care of the memory behind the scenes. Its most important aspect is automatic storage reclamation: the system finds all the objects that can be proved to be no longer in use and Permission to copy without fee all oz " part of this material is granted provided that the copies arc not made or distributed for direct commercial advantagc, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by

Citations

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