CS533 Concepts of Operating Systems On the Duality of Operating System Structures

1. CS533 Concepts of Operating SystemsOn the Duality of Operating System Structures Hugh Lauer Xerox Corporation Roger Needham Cambridge University Presented by Thanh Dang Acknowledgements: Yahia Mahmoud, Jonathan Walpole

2. What the paper is about? CS533 - Concepts of Operating Systems

3. Outlines • Concepts • Duality: What “=“ means? • Operating System Structures • Messasge-oriented vs. procedure-oriented • Observations on duality • Duality mapping • Logically identical • Identical performance • Implications • Comments CS533 - Concepts of Operating Systems

4. Concepts • Duality (Optimization): The solution of one is also the solution of another. • Two systems are dual if they • From outside • Are logically identical to each other • Have identical performance. • From inside • Have primitives defined by one can be mapped directly into the other • Have Primitive operations of one are as efficient as the other CS533 - Concepts of Operating Systems

5. Are operating systems dual? CS533 - Concepts of Operating Systems

6. Models of operating systems • How do processes communicate and synchronize? • Message passing : Message-oriented model • Small number of big processes • Explicit message channels • Identification of the context of execution with processes • Shared memory : Procedure-oriented system • Large number of small processes • Direct sharing and interlocking of data • Identification of the context of execution with functions CS533 - Concepts of Operating Systems

7. Message-oriented system • Synchronization: Message queue • Data passed by reference • Peripheral devices are processes • Static context, no global naming • Process creation and deletion are difficult CS533 - Concepts of Operating Systems

8. Message-oriented model CS533 - Concepts of Operating Systems

9. Message-oriented model CS533 - Concepts of Operating Systems

10. Message-oriented model • Messages, message identifiers • Message channels, message ports • SendMessage[ messageChannel, messageBody ] • returns[ messageId ] • AwaitReply[ messageId ] returns [ messageBody ] • WaitForMessage[ set of messagePort ] returns [ messageBody, messageld, messagePort ] • SendReply[ messageId, messageBody ] • Process declaration • CreateProcess CS533 - Concepts of Operating Systems

11. Procedure-oriented system • Synchronization: process waiting for lock • Data shared among processes • “interrupts” from peripheral devices – locks or shared data • Global naming • Process creation and deletion are cheap CS533 - Concepts of Operating Systems

12. Procedure-oriented model CS533 - Concepts of Operating Systems

13. Procedure-oriented model CS533 - Concepts of Operating Systems

14. Procedure-oriented model • Procedures, process identifiers • Procedure call facilities, synchronous and asynchronous • processld <- FORK procedureName[ parameterList ] • [ resultList ] <- JOIN processld • Modules and monitors • Module instantiation with NEW and START • Condition variables • WAIT conditionVariable • SIGNAL condition Variable CS533 - Concepts of Operating Systems

15. Concepts • Duality: The solution of one is also the solution of another. • Two systems are duality if • From outside • Logically identical to each other • Identical performance. • From inside • Primitives defined by one can be mapped directly into the other one • Primitive operations of one is as efficient as the dual model. CS533 - Concepts of Operating Systems

16. Observation 1: Duality Mapping • Message-oriented system • Procedure-oriented system Processes, CreateProcess message channels message message ports SendMessage; AwaitReply (immediate) SendMessage;... AwaitReply (delayed) SendReply main message loop arms of the case statement selective waiting for messages monitors, NEW/START External procedure identifiers ENTRY procedure identifiers simple procedure call FORK; . . .JOIN RETURN (from procedure) monitor lock, ENTRY attribute ENTRY procedure declarations condition variables, WAIT, SIGNAL CS533 - Concepts of Operating Systems

17. Observation 2: Logically identical • Control structures are directly translated • No algorithms are changed • No data structures are replaced • Semantic content is invariant • Logic of the program is preserved CS533 - Concepts of Operating Systems

18. Observation 3: Identical performance • Performance P = E + S + Q • E: Execution time of the program • Same data structure, same operations • S: System call overhead • Method calls = message sends • Process switching can be equally fast • Virtual memory and paging can be equally fast • Q: Queuing and waiting time • Message queuing = lock queuing CS533 - Concepts of Operating Systems

19. Outlines • Concepts • Duality: What “=“ means? • Operating System Structures • Messasge-oriented vs. procedure-oriented • Duality • Program Similarity • Performance • Implications • Comments CS533 - Concepts of Operating Systems

20. Implications • No model is inherently preferable • Choosing one or another depends on • Machine structure • Not application CS533 - Concepts of Operating Systems

21. Other comments • Empirical analysis but refer to only a few practical operating systems • Informal arguments, not sure if formal and rigorous proof is possible • A mechanism which has no dual: Good or Bad? • Unclear if it is still true when we have a zoo of operating systems nowadays – empirical data is different. Has time already proved if they are correct? CS533 - Concepts of Operating Systems