UNIVERSITY OF HOUSTON Department of Computer Science COSC 6360-Operating Systems FINAL PART I December 12, 1989 Be concise but complete. The examination is closed text but you can have four pages of notes. 1. Show that no solution of the Byzantine General Problem can accommodate one traitor and two loyal generals. (10 points) THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE READING LIST. IT WILL BE NOT BE ASKED AGAIN. 2. A time-sharing system has 20 terminals. The average think time is 8 seconds. The average disk access time is 25 ms. Each user job requires on the average 800 ms of CPU time. The CPU and the disk utilization are respectively 80% and 60%. (10 points) (a) What is the mean number of disk accesses per job? (b) What is the system's throughput? obs/s (c) What is the system's response time? (d) What is the absolute maximum for the system's throughput? (e) What is the absolute minimum for the system's response time? THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE READING LIST. IT WILL BE NOT BE ASKED AGAIN. 3. Why are authentication servers for public-key cryptosystems less likely to pose a threat to user privacy than their counterparts for conventional cryptosystems? (10 points) time? THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE READING LIST. IT WILL BE NOT BE ASKED AGAIN. 4. Sketch the solution proposed by Tanenbaum et al. to build object capabilities that can be easily downgraded without going back to the server. (10 points) THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE READING LIST. IT WILL BE NOT BE ASKED AGAIN. 5. How does Sprite maps file names into file addresses? (6 points) Contrast this approach with that used by NFS. (4 points) 6. Describe the RPC mechanism of the V System. (10 points) THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE READING LIST. IT WILL BE NOT BE ASKED AGAIN. UNIVERSITY OF HOUSTON Department of Computer Science COSC 6360-Operating Systems FINAL PART II QUALIFYING EXAMINATION PART A December 12, 1989 Be concise but complete. The examination is closed text but you can have four pages of notes. 1. Lightweight Remote Procedure Calls: Bershad et al. have recently proposed a new communication facility, called Lightweight Remote Procedure Calls, that are designed and optimized for communication between protec- tion domains on the same machine. (a) How does this mechanism differ from conventional pro- cedure calls? (10 points) (b) How makes it easier to break large kernels into smaller more manageable parts? (5 points) (c) Give one example of an earlier implementation of the same concept under a different disguise. (5 points) 2. Gray and Cheriton have recently proposed a new mechanism to solve the distributed file cache consistency problem. Under their proposal, client caches are to be given leases, i.e. contracts that give to the cache control over writes to the covered datum during the term of the lease. When a lease- holder grants approval for a write, it invalidates at the same time its local copy of the datum. When a lease expires, a new lease can be requested by the client cache from the file server or automatically renewed by the file server without any explicit request from the client cache. Assuming that all writes following a write-through policy: (a) Show that communication failures and site crashes can- not affect the correctness of the data. (8 points) (b) Discuss the trade-off(s) involved in the choice of lease terms. (6 points) (c) Propose a very simple mechanism for handling slight time variations between the server clock and the client clock (6 points).