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Research and Advances

Memory occupancy patterns in garbage collection systems

Some programming languages and computer systems use dynamic memory allocation with garbage collection. It would be useful to understand how the utilization of memory depends on the stochastic parameters describing the size and life distributions of the cells. We consider a class of dynamic storage allocation systems which use a first-fit strategy to allocate cells and perform noncompacting garbage collections to recover free memory space when memory becomes fully occupied. A formula is derived for the expected number of holes (available cells) in memory immediately following a garbage collection which specializes to an analogue of Knuth's 'Fifty Percent' rule for nongarbage-collection systems. Simulations confirm the rule for exponentially distributed cell lifetimes. Other lifetime distributions are discussed. The memory-size requirements for noncompacting garbage collection are also analyzed.
Research and Advances

Determinants of program repair maintenance requirements

Considerable resources are devoted to the maintenance of programs including that required to correct errors not discovered until after the programs are delivered to the user. A number of factors are believed to affect the occurrence of these errors, e.g., the complexity of the programs, the intensity with which programs are used, and the programming style. Several hundred programs making up a manufacturing support system are analyzed to study the relationships between the number of delivered errors and measures of the programs' size and complexity (particularly as measured by software science metrics), frequency of use, and age. Not surprisingly, program size is found to be the best predictor of repair maintenance requirements. Repair maintenance is more highly correlated with the number of lines of source code in the program than it is to software science metrics, which is surprising in light of previously reported results. Actual error rate is found to be much higher than that which would be predicted from program characteristics.
Research and Advances

The weighted median filter

The median filter is well-known [1, 2]. However, if a user wishes to predefine a set of feature types to remove or retain, the median filter does not necessarily satisfy the requirements. A more general filter, called the Weighted Median Filter, of which the median filter is a special case, is described. It enables filters to be designed with a wide variety of properties. Particular cases of filter requirements are discussed and the corresponding filters are derived. The notion of a minimal weighted median filter, of a subclass that act identically, is introduced and discussed. The question of finding the number of distinct ways a class of filters can act is considered and solved for some classes.
Opinion

From Washington

We are entering a new era in very high performance computing that will be dominated by parallel architectured systems. It is critical for the United States to maintain its leadership as this new era, with its broadened applications, evolves over the next decade. Toward this end the National Science Foundation sponsored a workshop in November 1983 to focus the collective strength of universities, industry, and government on projects for development of knowledge-intensive industries.
Research and Advances

Training wheels in a user interface

New users of high-function application systems can become frustrated and confused by the errors they make in the early stages of learning. A training interface for a commercial word processor was designed to make typical and troublesome error states “unreachable,” thus eliminating the sources of some new-user learning problems. Creating a training environment from the basic function of the system itself afforded substantially faster learning coupled with better learning achievement and better performance on a comprehension post-test. A control group spent almost a quarter of their time recovering from the error states that the training interface blocked off. We speculate on how this training strategy might be refined, and more generally, on how function should be organized in a user interface.
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Research and Advances

A null-object detection algorithm for constructive solid geometry

Constructive solid geometry (CSG) is the primary scheme used for representing solid objects in many contemporary solid modeling systems. A CSG representation is a binary tree whose nonterminal nodes represent Boolean operations and whose terminal nodes represent primitive solids. This paper deals with algorithms that operate directly on CSG representations to solve two computationally difficult geometric problems—null-object detection (NOD) and same-object detection (SOD). The paper also shows that CSG trees representing null objects may be reduced to null trees through the use of a new concept called primitive redundancy, and that, on average, tree reduction can be done efficiently by a new technique called spatial localization. Primitive redundancy and spatial localization enable a single complex instance of NOD to be converted into a number of simpler subproblems and lead to more efficient algorithms than those previously known.
Research and Advances

Audit trail compaction for database recovery

Total elapsed recovery time from disk-based database corruption can be shortened by reprocessing the audit trail off-line and thereby avoiding excessive resource utilization penalties. Using a bit map, the audit trail is compacted by eliminating irrelevant or superseded records. The compacted trail is then partitioned, and the partitions are processed in parallel.
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Research and Advances

The TWA reservation system

Where can you find a solid, forthright overview of the computer systems and management behind airline reservations? NASA's space shuttle? Or any of the multitude of other large computer systems that support important projects or national activities? It's hard, sometimes impossible: partly because the people who worked on such systems often do not have the time to write about their experiences: and partly because many professional journalists who interview these people do not have the technical background to ferret out answers to the fundamental design questions addressed in these systems.
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Research and Advances

Faster methods for random sampling

Several new methods are presented for selecting n records at random without replacement from a file containing N records. Each algorithm selects the records for the sample in a sequential manner—in the same order the records appear in the file. The algorithms are online in that the records for the sample are selected iteratively with no preprocessing. The algorithms require a constant amount of space and are short and easy to implement. The main result of this paper is the design and analysis of Algorithm D, which does the sampling in O(n) time, on the average; roughly n uniform random variates are generated, and approximately n exponentiation operations (of the form ab, for real numbers a and b) are performed during the sampling. This solves an open problem in the literature. CPU timings on a large mainframe computer indicate that Algorithm D is significantly faster than the sampling algorithms in use today.
Research and Advances

Efficient algorithms to globally balance a binary search tree

A binary search tree can be globally balanced by readjustment of pointers or with a sorting process in O(n) time, n being the total number of nodes. This paper presents three global balancing algorithms, one of which uses folding with the other two adopting parallel procedures. These algorithms show improvement in time efficiency over some sequential algorithms [1, 2, 7] when applied to large binary search trees. A comparison of various algorithms is presented.
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Research and Advances

A large font virtual terminal interface: a software prosthesis for the visually impaired

A large font software interface running under UNIX1 is of great assistance to the severely visually impaired in the interactive use of computers. The interface, which functions as a virtual terminal as far as other processes are concerned, displays all characters (both user and system generated) at user-selectable degrees of magnification, yet is compatible with any standard CRT/keyboard terminal.

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