CS6/73201 "Advanced Operating Systems" 

            HOMEWORK #4
	
        Due in class 11/9/00


The numbers in parenthesis refer to "Advanced Concepts in Operating
Systems" (AOS) and  "Distributed Operating System" (DOS)  textbooks.
Each question is worth 5 points.

1. Briefly describe Christian's and Berkeley physical clock
   synchronization algorithms. List the algorithms relative
   advantages and disadvantages. Do they algorithms require a
   server to have a UTC receiver to synchronize clocks (between
   each other) of the machines on the local network? Exaplain
   why or why not.

2. (DOS 6.10)

3. Explain why lamport's logical clocks are insufficient in certain
   cases and why vector clocks need to be used instead. Give an example
   execution where vector clocks provide additional information that 
   lamport's clocks don't. Draw a diagram to support your example.

4. (AOS 6.2)

5. (AOS 6.3)

6. (AOS 6.5)

7. Suzuki-Kasami's DMX algorithm a queue of requesting nodes is
   attached to the token. Explain why it is necessary and give
   an example as to how the algorithm would fail to satisfy
   the properties of DMX problem if this queue is not attached.

8. Consider the network structure below. The processes are executing
   Raymond's token circulation DMX algorithm. Process A initally holds
   the token and processes C,D and E are in CS contention. Is it
   possible that requesting processes enter critical section
   in the following order D,C,E? Explain your answer. Draw a sequence
   of diagrams outlining the possible computation.

	     oA
	    / \
	   oB  oC
	  / \
	 oD  oE

9. Explain the difference between robust and stabilizing algorithm.
   Explain what fault masking means. Can an algoritm be both
   robust and stabilizing? Explain your answer.

10. The impossibility result states that consensus is not possible
   in asynchronous systems in the presence of failures. Yet there
   is an algorithm that solves consensus in the presense of initially
   dead process. Does the existence of such algorithm contradict
   the impossibility result? Explain your answer. Briefly explain the 
   idea of this algorithm.