Книга: Distributed operating systems

PROBLEMS

PROBLEMS

1. The Amoeba designers assumed that memory would soon be available in large amounts for low prices. What impact did this assumption have on the design?

2. State an advantage and a disadvantage of the processor pool model compared to the personal multiprocessor model.

3. List three functions of the Amoeba microkernel.

4. Some Amoeba servers can be run in the kernel as well as in user space. Their clients cannot tell the difference (except by timing them). What is it about Amoeba that makes it impossible for clients to tell the difference?

5. A malicious user is trying to guess the bullet server's get-port by picking a random 48-bit number, running it through the well-known one-way function, and seeing if the put-port comes out. It takes 1 msec per trial. How long will it take to guess the get-port, on the average?

6. How does a server tell that a capability is an owner capability, as opposed to a restricted capability? How are owner capabilities verified?

7. If a capability is not an owner capability, how do servers check it for validity?

8. Explain what a glocal variable is.

9. Why does the trans call have parameters for both sending and receiving? Would it not have been better and simpler to have two calls, send_request and get_reply, one for sending and one for receiving?

10. Amoeba claims to guarantee at-most-once semantics on RPCs. Suppose that three file servers offer the same service. A client does an RPC with one of them, which carries out the request and then crashes. Then the RPC is repeated with another server, resulting in the work being done twice. Is this possible? If so, what does the guarantee mean? If not, how is it prevented?

11. Why does the sequencer need a history buffer?

12. Two algorithms for broadcasting in Amoeba were presented in the text. In method 1, the sender sends a point-to-point message to the sequencer, which then broadcasts it. In method 2, the sender does the broadcast, with the sequencer then broadcasting a small acknowledgement packet. Consider a 10-Mbps network on which processing a packet-arrived interrupt takes 500 microsec, independent of the packet size. If all data packets are 1K bytes, and acknowledgement packets are 100 bytes, how much bandwidth and how much CPU time are consumed per 1000 broadcasts by the two methods?

13. What property of FLIP addressing makes it possible to handle process migration and automatic network reconfiguration in a straightforward way?

14. The bullet server supports immutable files for its users. Are the bullet server's own tables also immutable?

15.  Why does the bullet server have uncommitted and committed files?

16.  In Amoeba, links to a file can be created by putting capabilities with different rights in different directories. These give different users different permissions. This feature is not present in UNIX. Why?

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