UNIVERSITY OF HOUSTON
                 Department of Computer Science
                                
                  COSC 6360--Operating Systems
                     QUALIFYING EXAMINATION
                          Graduate Part
                       FINAL  EXAMINATION
                             Part I
                           May 8, 1996
                                
This exam is closed book.  You can have two sheets (i.e., four
pages) of notes.

1.   Extensible Operating Systems:
     There are two ways to build extensible operating systems.
     The first approach is to have a small microkernel and put
     many services traditionally included in the kernel into user
     space.  Another approach is to allow incremental changes to
     the kernel.  What is the major advantage and the major
     danger of this second approach?  What would you propose to
     do to make the approach safer (or at least less dangerous)?
     (20 points)

2.   Object Server:
     Consider a distributed storage system providing long-lived
     data objects to a large number of clients over a network.
     This server could export:

     a)  small-grain objects that are semantically meaningful to
         its clients, or

     b)   fixed-size data blocks that reflect its own disk
          organization.

     Discuss in some detail the trade-off between the two
     solutions focusing on their impact  on the management of the
     server's memory hierarchy.  (20 points)
     You should know that (a) main memory is a critical resource
     and (b) all modified objects must be immediately recorded
     somewhere in stable storage
     

                      UNIVERSITY OF HOUSTON
                 Department of Computer Science
                                
                  COSC 6360--Operating Systems
                       FINAL  EXAMINATION
                             Part II
                           May 8, 1996
                                
This exam is closed book.  You can have two sheets (i.e., four
pages) of notes.

                                
1.   Given m clocks Ci (t) that are within  time units of each
     other and are neither very fast nor very slow, that is,  with 1,
     describe a protocol that would keep these clocks sufficiently
     synchronized to prevent most anomalous behaviors? (2×5 points)

2.   What is the function of authenticators in Kerberos?  (5 points).
     What could be done to defeat their purpose?  (5 points)

3.   What was the problem raised by large distribution lists in
     Grapevine and what was done to solve it?  (2×5 points)

4.   What is sequential sharing in Sprite? (5 points)  Why is there
     no sequential sharing in AFS and Coda? (5 points)


5.   Hartman and Ousterhout argue in their Zebra paper that log-
     structured file systems are good candidates for file striping.
     What is the essence of their argument?  (5 points)

6.  According to Carter et al., what is the best consistency protocol
    for distributed applications where all conflicting shared-memory
    accesses are synchronized through semaphores or some other
    mechanism and why? (5 points)

7.  It has been proposed to store witnesses in volatile memory
    to improve their access times.  What needs then to be done to
    prevent the replicated data from remaining permanently
    inaccessible? (5 points)
     
8.  What did Tanenbaum et al. do to make their Bullet file
    system as fast as possible?  (5 points)