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On the Duality of Operating System Structures Hugh C. Lauer Xerox Corporation Palo Alto, California Roger M. Needham Cambridge University Cambridge, England Proc. Second International Symposium on Operating Systems, IRIA, Oct. 1978 Overview Nature of the Paper Two Models Message-Oriented Systems Procedure-Oriented Systems Characteristics of the Models Empirical Support Underlying Similarities Underlying Differences Conclusions Outline “Many operating system designs can be placed into one of two very rough categories, depending upon how they implement and use the notions of process and synchronization.” Overview The paper is empirical in nature The universe in the investigation is the class of operating systems The properties of interest are processes, synchronization and inter-process communication Nature of the Paper Message-Oriented Systems Two Models Procedure-Oriented Systems Characterized by a relatively small, static number of processes with an explicit message system for communicating among them Message-Oriented Systems Specific communication paths Relatively static number of processes and connections Deletion of processes tends to be very difficult Hallmarks Messages Message Identifiers A message identifier is a handle by which a particular message can be identified Message Channels A message is a data structure meant for sending information from one process to another A message channel is an abstract structure which identifies the destination of the message Message Ports A message port is a queue capable of holding messages of a certain type or class Facilities SendMessage [messageChannel, messageBody] returns [messageId] AwaitReply [messageId] returns [messageBody] WaitForMessage [set of messagePort] returns [messageBody, messageId, messagePort] SendReply [messageId, messageBody] Message Transmission Operations IBM’s OS/360 A discontinued batch processing operating system developed by the IBM Corporation for their then-new System/360 mainframe computer, announced in 1964 GEC 4080 16-bit minicomputer with its unique Nucleus feature (Marconi-Eliot Division) Examples Characterized by a large, rapidly changing number of small processes and a process synchronization mechanism based on shared data Procedure-Oriented Systems Global data can be both protected and efficiently accessed Process creation is very easy Hallmarks Procedures Procedure call facilities – synchronous and asynchronous A module is the primitive Mesa unit of compilation A monitor is a special kind of Mesa module which has associated with it a lock to prevent more than one process from executing inside of it at any one time Module Instantiation The synchronous procedure call mechanism is just the ordinary Mesa procedure call statement The asynchronous procedure call mechanism is represented by the FORK and JOIN statements Modules and Monitors A procedure is a piece of Mesa text containing algorithms, local data, parameters and results. Modules may be instantiated in Mesa by means of the NEW and START statements Condition Variables Condition Variables provide more flexible synchronization among events Facilities HYDRA An early capability-based, object-oriented, microkernel implemented in the programming language BLISS as part of the C.mmp project at Carnegie-Mellon University Plessey System 250 Capability-based multiprocessing operating system kernel Examples The Duality Mapping Similarity of Programs Preservation of Performance Characteristics of the Models A program or subsystem constructed strictly according to the primitives defined by one model can be mapped directly into a dual program or subsystem which fits the other model. The Duality Mapping The dual programs or subsystems are logically identical to each other. They can also be made textually very similar, differing only in non-essential details. Similarity of Programs The performance of a program or subsystem from one model, as reflected by its queue lengths, waiting times, service rates, etc. is identical to that of its dual system given identical scheduling strategies. The primitive operations provided by the operating system of one model can be made as efficient as their duals of the other model. Preservation of Performance It is not very easy to change the structure of most operating systems in a way which would reflect the duality The underlying address structures, use of global data, and styles of communication are usually bound to the design and implementation The Cambridge CAP Computer Empirical Support Comparison of the Two Models The two styles lead to client systems with similar program structure and performance The computational complexity of the implementation of the system facilities to provide the two styles is similar Underlying Similarities between Styles Machine Architecture Programming Environment Underlying Differences between Styles There lies merit in both styles with respect to structure, performance, logical soundness, elegance and “correctness”. Neither model is inherently preferable. The main consideration is the machine architecture upon which the system is being built, not the application which the system will ultimately support. Conclusions Presented By: Amna Rafiq Graduate Student CS @ VT Thank you!
