Generational garbage collection algorithms achieve efficiency because newer records point to older records; the only way an older record can point to a newer record is by a store operation to a previously-created record, and such operations are rare in many languages. A garbage collector that concentrates just on recently allocated records can take advantage of this fact. Such a garbage collector can be so efficient that the allocation of records costs more than their disposal. A scheme for quick record allocation attacks this bottleneck. Many garbage-collected environments don't know when to ask the operating system for more memory. A robust heuristic solves this problem. This paper presents a simple, efficient, low-overhead version of generational garbage collection with fast allocation, suitable for implementation in a Unix environment.

Memory Management Units (MMUs) are traditionally used by operating systems to implement disk-paged virtual memory. Some operating systems allow user programs to specify the protection level (inaccessible, readonly. read-write) of pages, and allow user programs t.o handle protection violations. bur. these mechanisms are not. always robust, efficient, or well-mat. ched to the needs of applications.

types : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 14 2.3 Imperative features : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 15 2.3.1 References : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 15 2.3.2 Exceptions : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 16 2.3.3 Continuations : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 17 v 2.4 An example --- functional queues : : : : : : : : : : : : : : : : : : : : : : : : 18 II Design 19 3 Concurrent Programming Languages 21 3.1 Processes and threads : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 22 3.2 Shared-memory languages : : : : : : : : : : : : : : : : : : : : : : : : : : : : 23 3.2.1 Low-level synchronization mechanisms : : : : : : : : : : : : : : : : : 23 3.2.2 Monitors : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 25 3.2.3 Shared-memory concurrency and ML : : : : : : : : : : : : : : : : : 25 3.3 Distributed-memory l...

Now that the use of garbage collection in languages like Java is becoming widely accepted due to the safety and software engineering benefits it provides, there is significant interest in applying garbage collection to hard real-time systems. Past approaches have generally suffered from one of two major flaws: either they were not provably real-time, or they imposed large space overheads to meet the real-time bounds. We present a mostly non-moving, dynamically defragmenting collector that overcomes both of these limitations: by avoiding copying in most cases, space requirements are kept low; and by fully incrementalizing the collector we are able to meet real-time bounds. We implemented our algorithm in the Jikes RVM and show that at real-time resolution we are able to obtain mutator utilization rates of 45% with only 1.6--2.5 times the actual space required by the application, a factor of 4 improvement in utilization over the best previously published results. Defragmentation causes no more than 4% of the traced data to be copied.

This paper presents the design and implementation of a "quasi real-time" garbage collector for Concurrent Caml Light, an implementation of ML with threads. This two-generation system combines a fast, asynchronous copying collector on the young generation with a nondisruptive concurrent marking collector on the old generation. This design crucially relies on the ML compiletime distinction between mutable and immutable objects. 1 Introduction This paper presents the design and implementation of a garbage collector for Concurrent Caml Light, an implementation of the ML language that provides multiple threads of control executing concurrently in a shared address space. Garbage collection --- the automatic reclamation of unused memory space --- is one of the most problematic components of run-time systems for multi-threaded languages. The naive "stop-the-world" approach, where all threads synchronously stop executing the user's program to perform garbage collection, is clearly inadequate,...

Program-synchronous exceptions, for example, breakpoints, watchpoints, illegal opcodes, and memory access violations, provide information about exceptional conditions, interrupting the program and vectoring to an operating system handler. Over the last decade, however, programs and run-time systems have increasingly employed these mechanisms as a performance optimization to detect normal and expected conditions. Unfortunately, current architecture and operating system structures are designed for exceptional or erroneous conditions, where performance is of secondary importance, rather than normal conditions. Consequently, this has limited the practicality of such hardware-based detection mechanisms. We propose both hardware and software structures that permit efficient handling of synchronous exceptions by user-level code. We demonstrate a software implementation that reduces exceptiondelivery cost by an order-of-magnitude on current RISC processors, and show the performance benefits o...

The complexity of systems for automatic control and other safety-critical applications grows rapidly. Computer software represents an increasing part of the complexity. As larger systems are developed, we need to find scalable techniques to manage the complexity in order to guarantee high product quality. Memory management is a key quality factor for these systems. Automatic memory management, or garbage collection, is a technique that significantly reduces the complex problem of correct memory management. The risk of software errors decreases and development time is reduced. Garbage collection techniques suitable for interactive and soft real-time systems exist, but few approaches are suitable for systems with hard real-time requirements, such as control systems (embedded systems). One part of the problem is solved by incremental garbage collection algorithms, which have been presented before. We focus on the scheduling problem which forms the second part of the problem, i.e. how the work of a garbage collector should be scheduled in order

The runtime data structures of the Standard ML of New Jersey compiler are simple yet general. As a result, code generators are easy to implement, programs execute quickly, garbage collectors are easy to implement and work efficiently, and a variety of runtime facilities can be provided with ease.

Many modern programming environments use tag bits at runtime to distinguish objects of different types. This is particularly common in systems with garbage collection, since the garbage collector must be able to distinguish pointers from non-pointers, and to learn the length of records pointed to. The use of tag bits leads to inefficiency. In addition to the obvious space overhead (tag bits and record descriptors occupy memory space), there is a time overhead: tag bits must be stripped off of data before arithmetic operations are performed, and re-attached to the data when it is stored into memory. This takes either extra instructions at runtime, or special tag-handling hardware, or both. This paper shows how the use of tag bits, record descriptor words, explicit type parameters, and the like can be avoided in languages (like ML) with static polymorphic typechecking. Though a form of tag will still be required for user-defined variant records, all other type information ...

We present the design and implementation of a new garbage collection framework that significantly generalizes existing copying collectors. The Beltway framework exploits and separates object age and incrementality. It groups objects in one or more increments on queues called belts, collects belts independently, and collects increments on a belt in first-in-first-out order. We show that Beltway configurations, selected by command line options, act and perform the same as semi-space, generational, and older-first collectors, and encompass all previous copying collectors of which we are aware.