On The Duality of Operating System Structures

1. On The Duality of Operating System Structures Hugh Lauer, Xerox Roger Needham, Cambridge University 1979 Presented by Catherine Vilhauer CS533 - Concepts of Operating Systems

2. Introduction • Classic paper by Lauer and Needham(1979) • Operating Systems fall into two categories • Message-oriented systems • Procedure-oriented systems • These two systems are duals • can be mapped to one another • are logically identical • performance is also identical • As a result, which you choose based on architecture constraints CS533 - Concepts of Operating Systems

3. What I’m Going to Cover • Message-Oriented System • Procedure-Oriented System • Comparison of the Two • Argument for Duality • Evidence? CS533 - Concepts of Operating Systems

4. Message-Oriented SystemsWhat are they? • Pass messages (or events) easily and efficiently among processes • Queue messages at destination process • Primitive operations for processes • Pre-emption occurs when higher priority message received CS533 - Concepts of Operating Systems

5. Message-Oriented SystemWhat’s a good design? • Specific communication paths (message channels, sockets etc) • Relatively static number of processes and connections • Deletion and creation of processes can be difficult • Relatively static context for each process • virtual memories or address spaces in one-to-one correspondence with process. • Processes rarely share data in memory • Processes tend to be associated with system resources CS533 - Concepts of Operating Systems

6. Message-Oriented SystemsCharacteristics • Synchronization implemented in message queues • Data structures passed by reference in messages • Peripheral devices treated as processes • Priorities statically assigned at time of design • Processes operate on small number of messages at a time and complete those operations before looking at message queue again • Procedural interfaces or global naming schemes not useful CS533 - Concepts of Operating Systems

7. Message-Oriented SystemsCanonical Model • Idealized Operating System kernel contains: • Messages and message identifiers • Message channels and Message Ports • Four message transmission operations • SendMessage(MessageChannel, MessageBody) • AwaitReply(MessageID) • WaitForMessage(set of MessagePorts) • SendReply(MsgId, MsgBody) • Process Declarations • Create Process CS533 - Concepts of Operating Systems

8. An Example: SEDA CS533 - Concepts of Operating Systems

9. Procedure-Oriented SystemsWhat are they? • Oriented around procedure • Efficient context switching • Synchronization via locks, mutexes and semaphores • Process may have to wait for a lock • Preemption occurs when release performed on a lock CS533 - Concepts of Operating Systems

10. Procedure-Oriented SystemsWhat’s a good design? • Global data protected and accessed by procedural interfaces which do synchronization in controlled ways • Process creation and deletion easy • Process has just one task, but wanders over system • Encoded in common / global data structures CS533 - Concepts of Operating Systems

11. Procedure-Oriented SystemsCharacteristics • Synchronization - queues of processes waiting for locks • Data shared directly among processes • Control of and interrupts from devices - manipulating locks and / or shared data in memory • Processes inherit priorities dynamically • Global naming schemes CS533 - Concepts of Operating Systems

12. Procedure-Oriented SystemCanonical Model • Idealized Operating System contains: • Procedures • local data • algorithms • parameters • results • Procedure call facilities • synchronous • asynchronous • Modules and monitors • Condition Variables • Fork and Join CS533 - Concepts of Operating Systems

13. Comparison of the Two Systems CS533 - Concepts of Operating Systems

14. An Example: KNOT • Knot platform built by Behren, Condit and Brewer in following paper • Interesting that Brewer was co-author of SEDA paper arguing for event-based system 2 years before CS533 - Concepts of Operating Systems

15. The Duality Mapping CS533 - Concepts of Operating Systems

16. The Duality ArgumentSimilarity of Programs • Can transform one system to the other • If written in exact style shown on previous slide • Logic unchanged Process declaration Monitor Procedure Calls SendMessage AwaitReply CS533 - Concepts of Operating Systems

17. The Duality ArgumentPerformance is Preserved • Performance of both systems the same • Execution time of program themselves • But since we have already shown that the programs are identical, then the execution time of each system will be exactly the same too. • computational overhead of primitive operations they call • The authors assert that the two canonical models can be made execute as efficiently as the other • GEC 4080 and MESA - different systems, but same speed • queuing and waiting times (sharing resources) • can make basi operations of the two models behave identically with scheduling operations. • Will happen just as quickly and in the same order in both CS533 - Concepts of Operating Systems

18. Evidence? • Not much empirical evidence • Cambridge CAP Computer • Hugely influential and still considered a classic CS533 - Concepts of Operating Systems

19. Conclusions • No inherent different between the two styles • Produce systems of similar structure and performance • Computational complexity is similar • Why choose one over the other? • Machine architecture • Programming environment CS533 - Concepts of Operating Systems