Книга: Distributed operating systems



1. In this problem you are to compare reading a file using a single-threaded file server and a multithreaded server. It takes 15 msec to get a request for work, dispatch it, and do the rest of the necessary processing, assuming that the data needed are in the block cache. If a disk operation is needed, as is the case one-third of the time, an additional 75 msec is required, during which time the thread sleeps. How many requests/sec can the server handle if it is single threaded? If it is multithreaded?

2. In Fig. 4-3 the register set is listed as a per-thread rather than a per-process item. Why? After all, the machine has only one set of registers.

3. In the text, we described a multithreaded file server, showing why it is better than a single-threaded server and a finite-state machine server. Are there any circumstances in which a single-threaded server might be better? Give an example.

4. In the discussion on global variables in threads, we used a procedure create_global to allocate storage for a pointer to the variable, rather than the variable itself. Is this essential, or could the procedures work with the values themselves just as well?

5. 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 runtime system. What problem might occur? Can you suggest a way to solve it?

6. Suppose that an operating system does not have anything like the SELECT system call to see in advance if it is safe to read from a file, pipe, or device, but it does allow alarm clocks to be set that interrupt blocked system calls. Is it possible to implement a threads package in user space under these conditions? Discuss.

7. In a certain workstation-based system, the workstations have local disks that hold the system binaries. When a new binary is released, it is sent to each workstation. However, some workstations may be down (or switched off) when this happens. Devise an algorithm that allows the updating to be done automatically, even though workstations are occasionally down.

8. Can you think of any other kinds of files that can safely be stored on user workstations of the type described in the preceding problem?

9. Would the scheme of Bershad et al. to make local RPCs go faster also work in a system with only one thread per process? How about with Peregrine?

10. When two users examine the registry in Fig. 4-12 simultaneously, they may accidentally pick the same idle workstation. How can the algorithm be subtly changed to prevent this race?

11. Imagine that a process is running remotely on a previously idle workstation, which, like all the workstations, is diskless. For each of the following UNIX system calls, tell whether it has to be forwarded back to the home machine:

 (a) READ (get data from a file).

 (b) IOCTL (change the mode of the controlling terminal).

 (c) GETPID (return the process id).

12. Compute the response ratios for Fig. 4-15 using processor 1 as the benchmark processor. Which assignment minimizes the average response ratio?

13. In the discussion of processor allocation algorithms, we pointed out that one choice is between centralized and distributed and another is between optimal and suboptimal. Devise two optimal location algorithms, one centralized and one decentralized.

14. In Fig. 4-17 we see two different allocation schemes, with different amounts of network traffic. Are there any other allocations that are better still? Assume that no machine may run more than four processes.

15. The up-down algorithm described in the text is a centralized algorithm design to allocate processors fairly. Invent a centralized algorithm whose goal is not fairness but distributing the load uniformly.

16. When a certain distributed system is overloaded, it makes m attempts to find an idle workstation to offload work to. The probability of a workstation having k jobs is given by the Poisson formula

where ? is the mean number of jobs per workstation. What is the probability that an overloaded workstation finds an idle one (i.e., one with k=0) in m attempts? Evaluate your answer numerically for m=3 and values of X from 1 to 4.

17. Using the data of Fig. 4-20, what is the longest UNIX pipeline that can be co-scheduled?

18. Can the model of triple modular redundancy described in the text handle Byzantine failures?

19. How many failed elements (devices plus voters) can Fig. 4-21 handle? Given an example of the worst case that can be masked.

20. Does TMR generalize to five elements per group instead of three? If so, what properties does it have?

21. Eloise lives at the Plaza Hotel. Her favorite activity is standing in the main lobby pushing the elevator. She can do this over and hour, for hours on end. The Plaza is installing a new elevator system. They can choose between a time-triggered system and an event-triggered system. Which one should they choose? Discuss.

22. A real-time system has periodic processes with the following computational requirements and periods:

 P1: 20 msec every 40 msec

 P2: 60 msec every 500 msec

 P3: 5 msec every 20 msec

 P4: 15 msec every 100 msec

Is this system schedulable on one CPU?

23. Is it possible to determine the priorities that the rate monotonic algorithm would assign to the processes in the preceding problem? If so, what are they? If not, what information is lacking?

24. A network consists of two parallel wires: the forward link, on which packets travel from left to right, and the reverse link, on which they travel from right to left. A generator at the head of each wire generates a continuous stream of packet frames, each holding an empty/full bit (initially empty). All the computers are located between the two wires, attached to both. To send a packet, a computer determines which wire to use (depending on whether the destination is to the left or right of it), waits for an empty frame, puts a packet in it, and marks the frame as full. Does this network satisfy the requirements for a real-time system? Explain.

25. The assignment of processes to slots in Fig. 4-32 is arbitrary. Other assignments are also possible. Find an alternative assignment that improves the performance of the second example.

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