1. CS533 Concepts of Operating Systems Class 20 Summary

2. CS533 - Concepts of Operating Systems 2 What is an Operating System?  A concurrent program o Often runs on parallel hardware  A resource manager o Shares resources among applications  A virtual (abstract) machine o Hides complex details behind abstractions o Hides applications from each other

3. CS533 - Concepts of Operating Systems 3 Problem 1: Programming Model  Threads or events? o Why threads? o Why events? o Are threads a good abstraction for CPUs? o Are they a good abstraction for I/O?  Concurrency requires synchronization o What do you want from a synchronization solution? o Why is it difficult to provide efficient synchronization?

4. CS533 - Concepts of Operating Systems 4 Problem 1: Programming Model  Procedures vs message passing o Which is best? o Why might you prefer one over the other in a modular OS?  What is the most important characteristic of an RPC mechanism?

5. CS533 - Concepts of Operating Systems 5 Problem 2: Architecture & Interfaces  What is good and bad about layers?  What is good and bad about upcalls?  Are micro-kernels a layered architecture?  What is the defining characteristic of a micro- kernel?  What abstractions do virtual machine monitors export? o How do they relate to micro-kernels?

6. CS533 - Concepts of Operating Systems 6 Problem 3 - Safety  What is good/bad about using hardware-enforced boundaries for protection in a modular OS?  What alternative approaches are there?  What is the best approach?

7. CS533 - Concepts of Operating Systems 7 Problem 4 - Performance  Is IPC (RPC) for crossing hardware protection boundaries inherently slow? o What is the minimal implementation of RPC?  Why is there a trade-off between performance and abstraction?  Does real-fast mean the same as real-time?  List some cases in which special hardware support has been used to optimize performance  How does specialization improve performance? o Does it conflict with abstraction or correctness?

8. CS533 - Concepts of Operating Systems 8 Problem 5 – Evolving Hardware  How have hardware advances affected o Synchronization? o Timing support? o File system architecture?