On the Duality of Operating System Structures 1

Hugh C. Lauer Xerox Corporation Palo Alto, Californi a Roger M. Needham Cambridge University Cambridge, England Proc. Second International Symposium on Operating Systems, IRIA, Oct

 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 3

“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 4

 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 5

Two Models  Message-Oriented Systems  Procedure-Oriented Systems 6

Characterized by a relatively small, static number of processes with an explicit message system for communicating among them Message-Oriented Systems 7

 Specific communication paths  Relatively static number of processes and connections  Deletion of processes tends to be very difficult Hallmarks 8

 Messages  A message is a data structure meant for sending information from one process to another  Message Identifiers  A message identifier is a handle by which a particular message can be identified  Message Channels  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 9

 SendMessage [messageChannel, messageBody] returns [messageId]  AwaitReply [messageId] returns [messageBody]  WaitForMessage [set of messagePort] returns [messageBody, messageId, messagePort]  SendReply [messageId, messageBody] Message Transmission Operations 10

 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 11

Characterized by a large, rapidly changing number of small processes and a process synchronization mechanism based on shared data Procedure-Oriented Systems 12

 Global data can be both protected and efficiently accessed  Process creation is very easy Hallmarks 13

 Procedures  A procedure is a piece of Mesa text containing algorithms, local data, parameters and results.  Procedure call facilities – synchronous and asynchronous  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 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  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 14

 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 15

 The Duality Mapping  Similarity of Programs  Preservation of Performance Characteristics of the Models 16

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 17

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 18

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 19

 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 20

Comparison of the Two Models 21

 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 22

 Machine Architecture  Programming Environment Underlying Differences between Styles 23

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 24

Presented By: Amna Rafiq Graduate Student VT Thank you! 25