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?

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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?

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    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)

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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)

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    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).