Книга: Distributed operating systems

PROBLEMS

PROBLEMS

1. In many layered protocols, each layer has its own header. Surely it would be more efficient to have a single header at the front of each message with all the control in it than all these separate headers. Why is this not done?

2. What is meant by an open system? Why are some systems not open?

3. What is the difference between a connection-oriented and connectionless communication protocol?

4. An ATM system is transmitting cells at the OC-3 rate. Each packet is 48 bytes long, and thus fits into a cell. An interrupt takes 1 ?sec. What fraction of the CPU is devoted to interrupt handling? Now repeat this problem for 1024-byte packets.

5. What is the probability that a totally garbled ATM header will be accepted as being correct?

6. Suggest a simple modification to Fig. 2-9 that reduces network traffic.

7. If the communication primitives in a client-server system are nonblocking, a call to send will complete before the message has actually been sent. To reduce 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.

8. 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?

9. When buffered communication is used, a primitive is normally available for user processes to create mailboxes. In the text it was not specified whether this primitive must specify the size of the mailbox. Give an argument each way.

10. In all the examples in this chapter, a server can only listen to a single address. In practice, it is sometimes convenient for a server to listen to multiple addresses at the same time, for example, if the same process performs a set of closely related services that have been assigned separate addresses. Invent a scheme by which this goal can be accomplished.

11. Consider a procedure incr with two integer parameters. The procedure adds one to each parameter. Now suppose that it is called with the same variable twice, for example, as incr(i, i). If i is initially 0, what value will it have afterward if call-by-reference is used? How about if copy/restore is used?

12. Pascal has a construction called a record variant, in which a field of a record can hold any one of several alternatives. At run time, there is no sure-fire way to tell which one is in there. Does this feature of Pascal have any implications for remote procedure call? Explain your answer.

13. The usual sequence of steps in an RPC involves trapping to the kernel to have the message sent from the client to the server. Suppose that a special co-processor chip for doing network I/O exists and that this chip is directly addressable from user space. Would it be worth having? What steps would an RPC consist of in that case?

14. The SPARC chip uses a 32-bit word in big endian format. If a SPARC sends the integer 2 to a 486, which is little endian, what numerical value does the 486 see?

15. One way to handle parameter conversion in RPC systems is to have each machine send parameters in its native representation, with the other one doing the translation, if need be. In the text it was suggested that the native system could be indicated by a code in the first byte. However, since locating the first byte in the first word is precisely the problem, can this work, or is the book wrong?

16. In Fig. 2-23 the deregister call to the binder has the unique identifier as one of the parameters. Is this really necessary? After all, the name and version are also provided, which uniquely identifies the service.

17. Reading the first block of a file from a remote file server is an idempotent operation. What about writing the first block?

18. For each of the following applications, do you think at least once semantics or at most once semantics is best? Discuss.

 (a)  Reading and writing files from a file server.

 (b)  Compiling a program.

 (c)  Remote banking.

19. Suppose that the time to do a null RPC (i.e., 0 data bytes) is 1.0 msec, with an additional 1.5 msec 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 32 1K RPCs?

20. How can atomic broadcast be used to manage group membership?

21. When a computation runs for a long time, it is sometimes wise to make checkpoints periodically, that is, to save the state of the process on disk in case it crashes. In that way, the process can be restarted from the check point instead of from the beginning. Try to devise a way of checkpointing a computation that consists of multiple processes running in parallel.

22. Imagine that in a particular distributed system all the machines are redundant multiprocessors, so that the possibility of a machine crashing is so low that it can be ignored. Devise a simple method for implementing global time-ordered atomic broadcast using only unicasting. (Hint: Arrange the machines in a logical ring.)

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