CSSE4004 - Lecture 5

Question 1 of 16

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Which of the following is false about the Repulsion forces approach in large scale coordinator election

Select one of the following:

  • each token is “repulsed” by nearby tokens (token holder sends token to another peer if too many tokens nearby)

  • token must be held by a node for a set time period before node can become superpeer

  • each node holding a token learns about other tokenholders

  • a superpeer which receives another token sends it to the lowest prime numbered neighbour

Question 2 of 16

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How can a coordinator failure be addressed in a centralised mutual exclusion system?

Select one of the following:

  • By using election algorithms to change the coordinator

  • By using triple modular redundancy to compare the results of two coordinators

  • By emitting a heartbeat that informs other nodes when the coordinator cannot provide a lock

  • By moving the coordinator periodically to the next node in the token ring

Question 3 of 16

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Which of these is not a drawback of a typical distributed mutual exclusion algorithm?

Select one of the following:

  • large number of messages required

  • one slow process slows down others

  • more points of failure

  • locks may be granted unfairly

Question 4 of 16

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Which of the following is true about Lamport's algorithm

Select one or more of the following:

  • when a message is received, its time is compared against the local clock. If the local clock is less than C(b), it is set to C(b) + 1

  • messages carry their sending time according to the receiver's clock e.g., C(b)

  • between any two events, the clock must tick at least twice (i.e. one on the transmitting side, one on the receiving side)

  • Events occurring in processes that do not interact (even indirectly through third parties) are said to be concurrent

Question 5 of 16

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In the bully algorithm with nodes 1,2,3,4,5,6,7 if 7 is the coordinator and 4 notices the coordinator has crashed, who will be the new coordinator?

Select one of the following:

  • 6

  • 4

  • 1

  • 7 after it reboots

Question 6 of 16

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Which of the following is false about wireless network clock synchronisation algorithms

Select one of the following:

  • Clock synchronisation is difficult because nodes cannot always contact one another

  • Nodes are resource-constrained

  • Receivers record difference between reference message timestamp and their own clock

  • The coordinating node must transmit at a higher power level to ensure all nodes can receive clock broadcasts

Question 7 of 16

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Which of the following is false about Lamport's algorithm?

Select one of the following:

  • if a and b are events in the same process, and a occurs before b, then a - > b is true

  • if a is the event of a message being sent by one process, and b is the receipt of that message in another process, then a ->b is also true

  • a -> b and b -> c, then a -> c

  • if a is the event of a message being sent by one process, and b is the receipt of that message in another process, then b -> a is also true

Question 8 of 16

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Which of these are clock synchronisation protocols

Select one of the following:

  • Network Time Protocol, The Berkeley Algorithm

  • Network Time Protocol, Town Clock Algorithm, The Berkeley Algorithm

  • UTC lock algorithm, NTP

  • Network Time Protocol, Town Clock Algorithm

Question 9 of 16

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Which is true about NTP?

Select one or more of the following:

  • Stands for Numerical Time Protocol

  • Daemon calculates new time based on average of reported values

  • In general, synchronising with stratum-k server makes you stratum-k+1

  • Stratum-1 servers typically have direct access to a reference clock

Question 10 of 16

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What issue is addressed by a vector clock?

Select one of the following:

  • Lamport’s algorithm does not capture causality

  • Sometimes messages are received out of order

  • Sometimes messages are created at precisely the same time

  • Sometimes events must be recorded before an event which occurred after it

Question 11 of 16

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Which is true about the Berkeley algorithm?

Select one or more of the following:

  • Uses averaging approach to correct clocks

  • Daemon tells each node how to adjust its clock

  • Servers are divided into strata reflecting the accuracy of their clocks

  • Lower stratum servers are assumed to be more accurate => A will try to synchronise with B’s clock

Question 12 of 16

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Which of the following is false about superpeers

Select one of the following:

  • Large-scale systems may require many local coordinators, creating the need for superpeers

  • Should offer low-latency access to other superpeers

  • Serve no more than a set number of regular nodes

  • Can be selected by reserving a fraction of the identifier space in a distributed hash table containing all nodes

Question 13 of 16

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In the ring election algorithm with nodes 1,2,3,4,5,6,7 if 7 is the coordinator and 5 and 2 both notice which node becomes the new coordinator?

Select one of the following:

  • 6

  • 5

  • 5 or 2

  • 2

Question 14 of 16

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In a token ring mutual exclusion algorithm, how does a node respond if it currently holds the resource?

Select one of the following:

  • The token is passed on to the next node in the ring when it is finished accessing the shared resource

  • It sends a message in the opposite direction back to the requester stating that it holds the lock

  • It sends a message in the forward direction to the requester stating that it holds the lock

  • The previous node sends a message back to the requester when it doesn't get a response from the node holding the resource

Question 15 of 16

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In a regular distributed mutual exclusion system, how does a node reply if it currently holds the resource?

Select one of the following:

  • It ignores the message

  • It sends a NACK response

  • It queues the message until it is finished with the resource then replies OK

  • It replies to the next node in the token ring

Question 16 of 16

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Which of these is not an advantage of centralised mutual exclusion?

Select one of the following:

  • easy to implement

  • low message overhead

  • fair (access requests are processed in order)

  • improves performance with high message volumes

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CSSE4004 - Lecture 5

tb22
Quiz by , created over 1 year ago

CSSE4004 - Lecture 5

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tb22
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