So far we have seen that mark-sweep has comparatively cheap collection costs but may suffer from fragmentation. Given that garbage collection should account for only a small proportion of overall execution time in any well configured system, it is essential that overheads on the mutator are kept to a minimum and, in particular, that allocation is fast, since mutator costs dominate those of the collector. Mark-compact collectors eliminate fragmentation and support very fast, ‘bump a pointer’ allocation (see Chapter 7) but require multiple passes over live objects, and significantly increase collection times. In this chapter, we discuss a third style of tracing garbage collection, semispace copying [Fenichel ...
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