Languages such as Scheme and Smalltalk that provide continuations as first-class data objects present a challenge to efficient implementation. Allocating activation records in a heap has proven unsatisfactory because of increased frame linkage costs, increased garbage collection overhead, and decreased locality of reference. However, simply allocating activation records on a stack and copying them when a continuation is created results in unbounded copying overhead. This paper describes a new approach based on stack allocation that does not require the stack to be copied when a continuation is created and that allows us to place a small upper bound on the amount copied when a continuation is reinstated. This new approach is faster than the naive stack allocation approach, and it does not suffer from the problems associated with unbounded copying. For continuation-intensive programs, our approach is at worst a constant factor slower than the heap allocation approach, and for typical programs, it is significantly faster. An important additional benefit is that recovery from stack overflow is handled gracefully and efficiently.

Continuations have proven to be useful for implementing a variety of control structures, including exception handling facilities and breadth-first searching algorithms. However, traditional continuations are not useful in the presence of concurrency, because the notion of the rest of the computation represented by a continuation does not in general make sense. This paper presents a new type of continuation, called a process continuation, that may be used to control tree-structured concurrency. Just as a traditional continuation represents the rest of a computation from a given point in the computation, a process continuation represents the rest of a subcomputation, or process, from a given point in the subcomputation. Process continuations allow nonlocal exits to arbitrary points in the process tree and allow the capture of a subtree of a computation as a composable continuation for later use. Even in the absence of multiple processes, the precise control achievable with process continuations makes them more useful than traditional continuations.