Answers to Homework #3

Due Date: February 27, 2001
Points: 70

  1. (20 points) Show that in Lamport's algorithm the critical section is accessed according to the increasing order of timestamps. (text, problem 6.7, p. 149)

    Answer: Recall that two basic assumptions of Lamport's algorithm (or any other distributed mutual exclusion algorithm, for that matter) is that messages sent from process p to process q arrive in the order they are sent, and if a message is sent then it will arrive (i.e., no messages are lost).

    Proof by contradiction. Suppose process p1 issues a request to enter the critical section at time t1, p2 issues a similar request at time t2 with t1 < t2, and p2 enters first. This means that p2's request is at the head of its queue. As the queues are ordered by timestamp, this means p1's request has not arrived. If p2 enters, though, it also received a message from p1 with a timestamp higher than t2. This implies that p1's request has a timestamp higher than t2 (which is false as t1 < t2) or p2 never received p1's request. The latter is possible only if either p1's request was lost, or messages from p1 to p2 arrive out of order. Both these contradict the above basic assumptions. Hence p2 cannot enter the critical section first, proving the claim.

  2. (20 points) Show that in the Ricart-Agrawala algorithm, the critical section is accessed according to the increasing order of timestamps. (text, problem 6.5, part 1, p. 149)

    Answer: Proof by contradiction. Suppose process p1 issues a request to enter the critical section at time t1t1, p2 issues a similar request at time t2 with t1 < t2, and p2 enters first. This means that p2 has received reply messages from all other processes including p1. But p1 will send such a message only if it is neither requesting nor executing the critical section (which is false) or if p2's request's timestamp is smaller than that of p1's request (which is also false). Hence p1 will not send a reply to p2's request, and so p2 cannot enter the critical section first. This contradicts hypothesis, proving the claim.

  3. (30 points) On p. 145, the text discusses the greedy strategy for Raymond's tree-based algorithm, and notes that it can cause starvation. Please give an example of the application of this algorithm to a situation in which the greedy strategy causes starvation, but the regular algorithm does not.

    Answer: There are two answers to this question, depending on how one views "site."

    If there are multiple processes at each site, the processes can genetate a stream of requests to enter the critical section. As the greedy nature of the algorithm requires the site to honor requests generated at that site first, the token stays at the site and any other site with a request to enter the critical section starves.

    If there is a single process at each site, starvation will not occur. Observe that, after the process finishes execuing in the critical section, the token will be forwarded as indicated by the holdier variable. Given this observation, the proof showing no starvation in both the greedy and non-greedy cases are the same.

Extra Credit

  1. (30 points) Does Maekawa's algorithm access the critical section according to the increasing order of timestamps? Either show that it does or provide a counterexample. (text, problem 6.5, part 2, p. 149)

    Answer: The claim is false. Consider the following situation, with three sites:

    R1 = { S1, S2 }
    R2 = { S2, S3 }
    R3 = { S1, S3 }

    These satisfy the conditions for Maekawa's algorithm.

    Let the clocks at sites 1, 2, and 3 be C1 = 10, C2 = 20, and C3 = 30, respectively. Then:
    S2 sends REQUEST(2, 20) to S2 and S3
    S2 receives REQUEST(2,20) from S2
    S2 sends REPLY(2, 21) to S2
    S2 receives REPLY(2, 21) from S2
    S3 sends REQUEST(3, 30) to S1 and S3
    S3 receives REQUEST(3,30) from S3
    S3 sends REPLY(3, 31) to S3
    S3 receives REPLY(3, 31) from S3
    S1 receives REQUEST(3, 30) from S3
    S1 sends REPLY(1, 31) to S3
    S3 receives REPLY(1, 31) from S1

    At this point, S3 enters the critical section even though its request has a timestamp greater than that of S2.

    This works because Maekawa's algorithm sends a REPLY to the first message that a process receives. If a later request comes with a lower timestamp, either a FAILED message is sent or the REPLY is held.

    Matt Bishop
    Office: 3059 Engineering Unit II Phone: +1 (530) 752-8060
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    Email: [email protected]
    		Copyright Matt Bishop, 2001. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print.
    Page last modified on 3/19/2001