This paper discusses modularization as a mechanism for improving the flexibility and comprehensibility of a system while allowing the shortening of its development time. The effectiveness of a “modularization” is dependent upon the criteria used in dividing the system into modules. A system design problem is presented and both a conventional and unconventional decomposition are described. It is shown that the unconventional decompositions have distinct advantages for the goals outlined. The criteria used in arriving at the decompositions are discussed. The unconventional decomposition, if implemented with the conventional assumption that a module consists of one or more subroutines, will be less efficient in most cases. An alternative approach to implementation which does not have this effect is sketched.

/pdf/on-the-criteria-to-be-used-in-decomposing-systems-into-1wtax7v87k.pdf

On the criteria to be used in decomposing systems into modules

Two formal models for parallel computation are presented: an abstract conceptual model and a parallel-program model. The former model does not distinguish between control and data states. The latter model includes the capability for the representation of an infinite set of control states by allowing there to be arbitrarily many instruction pointers (or processes) executing the program. An induction principle is presented which treats the control and data state sets on the same ground. Through the use of “place variables,” it is observed that certain correctness conditions can be expressed without enumeration of the set of all possible control states. Examples are presented in which the induction principle is used to demonstrate proofs of mutual exclusion. It is shown that assertions-oriented proof methods are special cases of the induction principle. A special case of the assertions method, which is called parallel place assertions, is shown to be incomplete. A formalization of “deadlock” is then presented. The concept of a “norm” is introduced, which yields an extension, to the deadlock problem, of Floyd's technique for proving termination. Also discussed is an extension of the program model which allows each process to have its own local variables and permits shared global variables. Correctness of certain forms of implementation is also discussed. An Appendix is included which relates this work to previous work on the satisfiability of certain logical formulas.

Formal verification of parallel programs

Odometry is the most widely used method for determining the momentary position of a mobile robot. This paper introduces practical methods for measuring and reducing odometry errors that are caused by the two dominant error sources in differential-drive mobile robots: 1) uncertainty about the effective wheelbase; and 2) unequal wheel diameters. These errors stay almost constant over prolonged periods of time. Performing an occasional calibration as proposed here will increase the odometric accuracy of the robot and reduce operation cost because an accurate mobile robot requires fewer absolute positioning updates. Many manufacturers or end-users calibrate their robots, usually in a time-consuming and nonsystematic trial and error approach. By contrast, the method described in this paper is systematic, provides near-optimal results, and it can be performed easily and without complicated equipment. Experimental results are presented that show a consistent improvement of at least one order of magnitude in odometric accuracy (with respect to systematic errors) for a mobile robot calibrated with our method.

https://www.johnloomis.org/ece445/topics/odometry/borenstein/paper58.pdf

Measurement and correction of systematic odometry errors in mobile robots

A problem of increasing importance in the design of large multiprogramming systems is the, so-called, deadlock or deadly-embrace problem. In this article we survey the work that has been done on the treatment of deadlocks from both the theoretical and practical points of view.

System Deadlocks

The realization of end-to-end quality of service (QoS) guarantees in emerging network-based applications requires mechanisms that support first dynamic discovery and then advance or immediate reservation of resources that will often be heterogeneous in type and implementation and independently controlled and administered. We propose the Globus Architecture for Reservation and Allocation (GARA) to address these four issues. GARA treats both reservations and computational elements such as processes, network flows, and memory blocks as first-class entities, allowing them to be created, monitored, and managed independently and uniformly. It simplifies management of heterogeneous resource types by defining uniform mechanisms for computers, networks, disk, memory, and other resources. Layering on these standard mechanisms, GARA enables the construction of application-level co-reservation and co-allocation libraries that applications can use to dynamically assemble collections of resources, guided by both application QoS requirements and the local administration policy of individual resources. We describe a prototype GARA implementation that supports three different resource type-parallel computers, individual CPU under control of the dynamic soft real-time scheduler, and integrated services networks, and provide performance results that quantify the costs of our techniques.

/pdf/a-distributed-resource-management-architecture-that-supports-pqeb5lsby8.pdf

A distributed resource management architecture that supports advance reservations and co-allocation

A well-known problem in the design of operating systems is the selection of a resource allocation policy that will prevent deadlock. Deadlock is the situation in which resources have been allocated to various tasks in such a way that none of the tasks can continue. The various published solutions have been somewhat restrictive: either they do not handle the problem in sufficient generality or they suggest policies which will on occasion refuse a request which could have been safely granted. Algorithms are presented which examine a request in the light of the current allocation of resources and determi.~e whether or not the granting of the request will introduce the possibility of a deadlock. Proofs given in the appendixes show that the conditions imposed by the algorithms are both necessary and sufficient to prevent deadlock. The algorithms have been successfully used in the THE system.

Prevention of system deadlocks

A language, BLISS, is described. This language is designed so as to be especially suitable for use in writing production software systems for a specific machine (the PDP-10): compilers, operating systems, etc. Prime design goals of the design are the ability to produce highly efficient object code, to allow access to all relevant hardware features of the host machine, and to provide a rational means by which to cope with the evolutionary nature of systems programs. A major feature which contributes to the realization of these goals is a mechanism permitting the definition of the representation of all data structures in terms of the access algorithm for elements of the structure.

/pdf/bliss-a-language-for-systems-programming-51da3zb6wi.pdf

BLISS: a language for systems programming

find it a bit misleading to apply the appellation "perma-nent blocking" to a condition which under fortuitous circumstances might resolve itself. We prefer the term "indefinite postponement." The algorithm given by Holt in the final section of his paper is unnecessarily cautious and is likely to delay the resolution of the "indefinite postponement condition ." There is no need to restrict granting of requests to the first process in the safe sequence found. Safe requests by any process preceding Pi in the sequence should always be granted. Certain requests may be granted to those after Pi in the sequence without postponing the grant for P~. We would like to comment on the paper by R.C. Holt [1]. In Section 2 Holt quotes Habermann [2] and then continues as follows: "Habermann states, 'The algorithms [for deciding if a state is safe] decide only whether or not granting a request can produce a deadlock, so assignment rules (according to priority rules for instance) can be implemented freely.' The obvious conclusion would seem to be that a scheduler which grants only safe requests will avoid all deadlocks and grant all requests." Holt's "obvious conclusion" is quite clearly false. The statement is equivalent to "a scheduler which does not grant any unsafe requests will eventually grant all requests." We do not believe that anyone who read the earlier article thoughtfully would reach that conclusion. The earlier articles [2, 3] make it clear that the notion of safe state is equivalent to the existence of a sequence of resource allocations and process completions which results in the return of all resources to the common pool. From this it follows that, were one to choose a scheduling policy which restricted the set of possible sequences to a proper subset of the set of "safe sequences," we must not consider any state to be safe if its safety was predicted on sequences which are not in that subset. With this fact in mind the difficulties discussed in Holt's Section 3 can be systematically avoided. Although we hate to quibble over choice of words, we General permission to republish, but not for profit, all or part of this material is granted, provided that reference is made to this publication, to its date of issue, and to the fact that reprinting privileges were granted by permission of the Association for Computing Machinery. My note commenting on Habermann's Communications article …

A. N. Habermann

Papers

Prevention of system deadlocks

BLISS: a language for systems programming

Comment on deadlock preventive method