(AUTONOMOUS)
B. Tech VII Semester Supplementary Examinations, May 2018
(Regulations: VCE-R14)
DISTRIBUTED OPERATING SYSTEMS
(Common to Computer Science and Engineering Information Technology)
Date: 28 May, 2018 AN Time: 3 hours Max Marks: 75
Answer ONE question from each Unit
All Questions Carry Equal Marks
Unit I
1. What are the drawbacks of having caches in Bus-Based Multiprocessors system? 7M
What are Microkernels? Explain how microkernels can be used to organize an operating
system in a client-server model.
8M
2. Why are decentralized algorithms preferred over centralized algorithms in a distributed
system? Explain.
10M
An experimental file server is up 3/4 of the time and down 1/4 of the time, due to bugs.
How many times does this file server have to be replicated to give an availability of at
least 99?
5M
Unit II
3. If the communication primitives in a client-server system are nonblocking, then a call to
send will complete before the message has actually been sent. To reduce the overhead,
some systems do not copy the data to the kernel, but transmit it directly from user
space. For such a system, devise two ways in which the sender can be told that the
transmission has been completed and the buffer can be reused.
9M
Suppose that the time to do a null RPC (i.e., 0 data bytes) is 1.0msec, with an additional
1.5msec for every 1K of data. How long does it take to read 32K from the file server in a
single 32K RPC How about as 321K RPCs?
6M
4. Messages can get lost, which affects the semantics of the message passing model.
Suppose that blocking primitives are being used. When a client sends a message, it is
suspended until the message has been sent. However, when it is restarted, there is no
guarantee that the message has been delivered. The message might have been lost.
Explain any three different approaches to this problem.
9M
In many communication systems, calls to send set a timer to guard against hanging the
client forever if the server crashes. Suppose that a fault-tolerant system is implemented
using multiple processors for all clients and all servers, so the probability of a client or
server crashing is effectively zero. Do you think it is safe to get rid of timeouts in this
system?
6M
Unit III
5. Consider the behavior of two machines in a distributed system. Both have clocks that are
supposed to tick 1000 times per millisecond. One of them actually does, but the other
ticks only 990 times per millisecond. If UTC updates come in once a minute, what is the
maximum clock skew that will occur?
7M
Systems that use locking for concurrency control usually distinguish read locks from write
locks. What should happen if a process has already acquired a read lock and now wants
to change it into a write lock? What about changing a write lock into a read lock?
8M
6. Write a brief note on nested transactions. 9M
Give the full algorithm for whether an attempt to lock a file should succeed or fail.
Consider both read and write locks, and the possibility that the file was unlocked, read
locked, or write locked.
6M
Cont…2

Unit IV
7. List the advantages of Stateless and statefull servers. 6M
Consider a multithreaded Web server, show why it is better than a single-threaded server.
Are there any circumstances in which a single-threaded server might be better? Give an
example.
9M
8. Explain briefly two solutions for solving the update problems in distributed file systems. 8M
Consider a system in which threads are implemented entirely in user space, with the runtime
system getting a clock interrupt once a second. Suppose that a clock interrupt occurs
while some thread is executing in the run-time system. What problem might occur? Can
you suggest a way to solve it?
7M
Unit V
9. Explain briefly the architecture of the Memnet. 8M
Discuss different ways for finding the owner of the page.
7M
10. What are the three conditions required to be met for a memory to exhibit entry
consistency.
8M
Discuss synchronization in a Distributed Shared Memory system. 7M