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FURTHER READING

Reference counting [Collins 1960] and mark-sweep collection [McCarthy 1960] are almost as old as languages with pointers. The pointer-reversal idea is attributed by Knuth [1967] to Peter Deutsch and to Herbert Schorr and W. M. Waite. Fenichel and Yochelson [1969] designed the first two-space copying collector, using depth-first search; Cheney [1970] designed the algorithm that uses the unscanned nodes in to-space as the queue of a breadth-first search, and also the semi-depth-first copying that improves the locality of a linked list. Steele [1975] designed the first concurrent mark-and-sweep algorithm. Dijkstra et al. [1978] formalized the notion of tricolor marking, and designed a concurrent algorithm that they could prove correct, trying to keep the synchronization requirements as weak as possible. Baker [1978] invented the incremental copying algorithm in which the mutator sees only to-space pointers. Generational garbage collection, taking advantage of the fact that newer objects die quickly and that there are few old-to-new pointers, was invented by Lieberman and Hewitt [1983]; Ungar [1986] developed a simpler and more efficient remembered set mechanism. The Symbolics Lisp Machine [Moon 1984] had special hardware to assist with incremental and generational garbage collection. The microcoded memory-fetch instructions enforced the invariant of Baker's algorithm; the microcoded memory-store instructions maintained the remembered set for generational collection. This collector was the first to explicitly improve locality of reference by keeping related objects on the same virtual-memory page. As modern computers rarely use microcode, and a modern general-purpose processor embedded in a general-purpose memory hierarchy tends to be an order of magnitude faster and cheaper than a computer with special-purpose instructions and memory tags, attention turned in the late 1980s to algorithms that could be implemented with standard RISC instructions and standard virtual-memory hardware. Appel et al. [1988] use virtual memory to implement a read barrier in a truly concurrent variant of Baker's algorithm. Shaw [1988] uses virtual-memory dirty bits to implement a write barrier for generational collection, and Boehm et al. [1991] make the same simple write barrier serve for concurrent generational mark-and-sweep. Write barriers are cheaper to implement than read barriers, because stores to old pages are rarer than fetches from to-space, and a write barrier merely needs to set a dirty bit and continue with minimal interruption of the mutator. Sobalvarro [1988] invented the card marking technique, which uses ordinary RISC instructions without requiring interaction with the virtual-memory system. Appel and Shao [1996] describe techniques for fast allocation of heap records and discuss several other efficiency issues related to garbage-collected systems. Branquart and Lewi [1971] describe pointer maps communicated from a compiler to its garbage collector; Diwan et al. [1992] tie pointer maps to return addresses, show how to handle derived pointers, and compress the maps to save space. Appel [1992, ] shows that compilers for functional languages must be careful about closure representations; using simple static links (for example) can keep enormous amounts of data reachable, preventing the collector from reclaiming it. Boehm and Weiser [1988] describe conservative collection, where the compiler does not inform the collector which variables and record fields contain pointers, so the collector must "guess." Any bit pattern pointing into the allocated heap is assumed to be a possible pointer and keeps the pointed-to record live. However, since the bit pattern might really be meant as an integer, the object cannot be moved (which would change the possible integer), and some garbage objects may not be reclaimed. Wentworth [1990] points out that such an integer may (coincidentally) point to the root of a huge garbage data structure, which therefore will not be reclaimed; so conservative collection will occasionally suffer from a disastrous space leak. Boehm [1993] describes several techniques for making these disasters unlikely: For example, if the collector ever finds an integer pointing to address X that is not a currently allocated object, it should blacklist that address so that the allocator will never allocate an object there. Boehm [1996] points out that even a conservative collector needs some amount of compiler assistance: If a derived pointer can point outside the bounds of an object, then its base pointer must be kept live as long as the derived pointer exists. Page 481 discusses some of the literature on improving the cache performance of garbage-collected systems.

Cohen [1981] comprehensively surveys the first two decades of garbagecollection research; Wilson [1997] describes and discusses more recent work. Jones and Lins [1996] offer a comprehensive textbook on garbage collection.


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