David B. Lomet

Process structuring, synchronization, and recovery using atomic actions

This paper explores the notion of an atomic action as a method of process structuring. This notion, first introduced explicitly by Eswaren et al [6] in the context of data base systems, reduces the problem of coping with many processes to that of coping with a single process within the atomic action . A form of process synchronization, the await statement, is adapted to work naturally with atomic actions. System recovery is also considered and we show how atomic actions can be used to isolate recovery action to a single process. Explicit control of recovery is provided by a reset procedure that permits information from rejected control paths to be passed to subsequent alternative paths.

A New Method for Fast Data Searches with Keys

Bounded disorder uses hashing and tree indexing to speed key-associative access to data. It supports range searches and is robust, coping well with arbitrary key distributions.

Bounded index exponential hashing

Bounded index exponential hashing, a new form of extendible hashing, is described. It has the important advantages over most of the other extendible hashing variants of both (i) providing random access to any record of a file in close to one disk access and (ii) having performance which does not vary with file size. It is straightforward to implement and demands only a fixed and specifiable amount of main storage to achieve this performance. Its underlying physical disk storage is readily managed and record overflow is handled so as to insure that unsuccessful searches never take more than two accesses. The methods ability to access data in close to a single disk access makes it possible to organize a database, in which files have a primary key and multiple secondary keys, such that the result is a significant performance advantage over existing organizations.

A Formalization of Transition Diagram Systems

The transition diagram systems first introduced by Conway are formalized in terms of a restricted deterministic pushdown acceptor (DPDA) called a nested DPDA. It is then established that the class of nested DPDAs is capable of accepting all deterministic context-free languages. The proof of this involves demonstrating that left recursion can be eliminated from deterministic (or LR(k)) grammars without destroying the deterministic property. Using various structural properties of nested DPDAs, one can then establish equivalence results for certain classes of deterministic languages.

The bounded disorder access method

A new key associative access method, called the bounded disorder method, is described. The method uses a combination of hashing and tree indexing. The method has very good random access performance, being comparable to the best hashing methods if its small index is stored entirely in main memory. The methods advantage compared with hashing is that range searches are possible while searching only a portion of the file proportional to the size of the range. It is possible to control index size by controlling node size. Node size can be increased without increasing the amount of data transferred during a random probe. Further, increasing node size has only a minor effect on key sequential access performance. Even quite large nodes, so long as they can be read into memory in their entirety, have good key sequential performance. The bounded disorder method is the only one of the methods employing large nodes that can cope well with arbitrary key distributions. These properties make the bounded disorder method a good choice as the only access method of a data base system.

Data flow analysis in the presence of procedure calls

The aliasing that results in a variable being known by more than one name has greatly complicated efforts to derive data flow information. The approach we take involves the use of a series of claims that, after we compute the data flow for some of the aliasing possibilities, allows us to produce good approximations for the remaining cases. The method can thus limit the potential combinatorial explosion of aliasing computations while providing results that are frequently exact and almost always very good. The method is illustrated in the context of data flow analysis involving multiple procedures and their calling interactions. It is applicable also in the treatment of recursive procedures.

Subsystems of Processes with Deadlock Avoidance

A graph-oriented approach to deadlock avoidance, which treates both shared and exclusive locking, has been described [6]. The method is particularly suited for database systems. With enhancements introduced here, the problem of indefimite delay, i.e., the possibility that a process will not run to completion (will be delayed indefinitely) can be eliminated. The approach taken is to partition the resource system into subsystems, each of which can be scheduled independently. Indefinite delay is avoided by the construction of subsystems that guarantee the completion of a process or the granting of a resource request. Further, we show how the subsystem approach can be applied systematically so as to approximate FIFO scheduling of resource requests, while avoiding deadlock and indefmite delay. Other scheduling disciplines can also be realized. A lock manager program utilizing the FIFO method has been implemented.

A practical deadlock avoidance algorithm for data base systems

A new algorithm is presented for avoiding system deadlocks. Because its performance has little dependence on the number of lockable resources, it is particularly well suited to use in data base systems which may have millions of individually lockable records. A further advantage is that both exclusive and shared locking are accommodated. The algorithm is presented in some detail and its performance and functional characteristics discussed.

A high performance, universal, key associative access method

A new file organization is proposed that combines the advantages of digital B-trees and extendible hashing methods into one organization that can be used universally. The method, like these predecessors, relies on digital searching. The key notions are: (i) that multipage nodes are addressed by the root and can have both data and index entries, the mix of entries changing over time; and (ii) that these nodes can be doubled with file growth and, when this occurs, data nodes at the next level of the tree are absorbed into the pages of these nodes, frequently keeping data closer to the root and simultaneously improving utilization. The result is an unbalanced tree that we call a digital lopsided tree or DL-tree. The paper describes DL-trees and their operations, and examines their properties. The most important engineering issues involve the doubling process and the methods used to optimize the tree properties. Ways of dealing with these issues are suggested.

Multi-table search for B-tree files

A new method of organizing index entries in nodes of a B-tree is presented. The method is designed specifically to work with variable length keys. Thus it is particularly suited to take advantage of the variable length entries that result when key compression techniques are used. The paper analyses the characteristics of compressed keys. Based on this analysis, it is shown that the method results in significant space savings, leading to reduced disk accesses during random searches of a B-tree organized file. The search algorithms are shown to be competitive with the best of previous methods for treating variable length entries.