21/06/2021 Quiz: COMP3221 Final Exam 2021
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COMP3221 Final Exam 2021
Started: Jun 21 at 13:00
Quiz Instructions
Answer all the questions:
There are 30 questions including multiple choices questions and essay type questions
Time Limit: 2 hours (plus 10 min reading time)
Note: 1 attempt only
6 ptsQuestion 1
The closed-form solution to L2-regularized linear regression is as follows:
where
a) What is the meaning of n and k ?
b) What is the meaning of ?
c) What are the big-O computation and storage complexities in terms of n and k to
compute as above.
= ( 𝑋 + 𝜆𝐼 𝑦𝑤∗ 𝑋𝑇 )−1𝑋𝑇
𝑋 ∈ , 𝑦 ∈𝑅𝑛×𝑘 𝑅𝑛
𝜆
𝑤∗
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4 ptsQuestion 2
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When n and k (i.e., the dimension in linear regression) are large, what are two
methods/algorithms that can reduce the complexities (e.g., linear in time and
space)?
4 ptsQuestion 3
How should we leverage distributed computing while mitigating network
communication?
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4 ptsQuestion 4
In FedAvg algorithm, where and how the “global” model is updated? Name two FL
algorithms.
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4 ptsQuestion 5
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What are key differences/challenges of Federated Learning compared with
distributed machine learning in data centers ?
3 ptsQuestion 6
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C :: 7.1 -> 7
C :: 7.3 -> 7
B :: 6.1 -> 6
C :: 7.2 -> 7
A :: 6.3 -> 7
B :: 6.1 -> 7
A :: 7.3 -> 8
A :: 7.3 -> 7
B :: 7.1 -> 7
In the distributed execution of the totally ordered multicast represented in this
figure, where a circle
represents message reception and a square represents reception of the last ack,
write the corresponding
timestamp for the three red events on the diagram.
2 ptsQuestion 7
A 51% attack is possible even if the Blockchain uses a cryptopuzzle
A 51% attack can perform a double-spend, meaning attackers issue two (conflicting)
transactions
Select which one of the following is NOT correct
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If a coalition of processes reunite more than half of the mining power, they can attack the
Blockchain
A 51% attack can remove transactions that took place before the attack
4 ptsQuestion 8
(0,0,1) ; (0,0,2) ; (3,0,0)
(1,0,0) ; (0,0,2) ; (0,0,1)
(1,1,0) ; (1,2,2) ; (3,0,3)
(2,0,0) ; (0,0,1) ; (3,0,3)
Consider the system in the previous question uses vector clocks. What are the
vector clock values (p, q, z) after the execution of each event in the figure below?
Vector clocks
Assume that the initial vector clock value for each process is zero.
2 ptsQuestion 9
For each of the following distributed execution of operations in which all variables
have an initial value zero;
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T3, T2, T1
T1, T2, T3
T3, T1, T2
Not Serializable
Indicate whether each execution is serializable at the transaction level and write
the execution order
2 ptsQuestion 10
Non of them
Creating blocks
Creating transactions
Validating transactions
What are not the main tasks of a miner?
2 ptsQuestion 11
Outline characteristics of TCP
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TCP does not care about reliability
TCP connection requires response from each packet it sends
TCP is connectionless
TCP does not need to establish a connection
2 ptsQuestion 12
UDP is connectionless
UDP is not concerned about reliability, just throughput
UDP requires synchronisation between both processes
Each UDP packet requires a response to the origin, signalling that it has received the
packet
Select characteristics of a UDP Connection
2 ptsQuestion 13
(3, 5)
(3, 3)
(3, 6)
(3, 4)
Considering RSA algorithm, suppose we choose p = 3 and q = 5, which means n =
15. What could be the pair of values (e,d) as follows?
2 ptsQuestion 14
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In Authentication, what are categories, and their examples, of Corroborative
Evidence?
3 ptsQuestion 15
Encrypt messages
Share publicly with everyone else
Decrypt messages
User credentials
Select the purposes of a public key
2 ptsQuestion 16
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What is the difference between symmetric and asymmetric encryption?
Provide an example of both schemes as part of your answer.
2 ptsQuestion 17
True
False
A hash function is a one-way function, you cannot decrypt a hash.
3 ptsQuestion 18
Explain how hashes are used to provide assist with integrity of a data between two
endpoints.
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2 ptsQuestion 19
Decrypt messages
To authenticate other users
Encrypt messages
To verify message integrity
Select the purposes of a private key
3 ptsQuestion 20
In the header of a block in Blockchain of Bitcoin, which header field contains the
information of “Proof-of-Work” ?
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4 ptsQuestion 21
Which figure shows the most desired convergence behavior of gradient
descent? Why (explain by comparing it with the other two) ?
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4 ptsQuestion 22
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We denote GD for Gradient Descent. What are:
a) Cons of mini-batch GD over Stochastic GD
b) Pros of mini-batch GD over GD.
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8 ptsQuestion 23
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In Logistic Regression given the following cost function
where , feature vector of the i-th sample, and there are n
data samples.
1. What is the name of the cost function above? Can we find directly the solution
to ? Why?
2. Assume that the Gradient Descent update for above cost function is:
and the dataset includes only 2
examples: . If the initial weight
vector , what is (assume that )?
3. What is the difference of gradient update when using Stochastic Gradient
Descent instead of Gradient Descent?
𝑓(𝐰) = − { log[𝜎( )] + (1 − ) log[1 − 𝜎( )]}∑
𝑛
𝑖=1
𝑦𝑖 𝐰
𝑇
𝐱𝑖 𝑦𝑖 𝐰
𝑇
𝐱𝑖
𝜎(𝑧) =
1
1+𝑒−𝑧
∈𝐱𝑖 𝑅
𝑑
∇𝑓(𝐰) = 0
:= − 𝜂 {𝜎( ) − }𝐰(𝑡+1) 𝐰(𝑡) ∑
𝑛
𝑖=1
𝐰
𝑇
𝐱𝑖 𝑦𝑖 𝐱𝑖
( = [1, 2], = 1), ( = [−2, 3], = 0)𝑥1 𝑦1 𝑥2 𝑦2
= [0, 0]𝑤(0) 𝑤(2) 𝜂 = 0.1
4 ptsQuestion 24
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For properties of distributed machine learning and computing, fill the missing
words below:
a) Parallelism makes computation …..
b) Network makes communication …..
c) Therefore, we should design algorithms that compute …., communicate ….
4 ptsQuestion 25
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In parallel gradient descent for least square example above. What is the local
computation and storage at each worker? What is the computation of the server
which do “reduce” operation (i.e., sum over n gradients)?
4 ptsQuestion 26
Execute the RIP protocol on the graph below. Provide the routing table for each
node. Assume that each edge has the weight of 1
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4 ptsQuestion 27
Given the following execution and the starting value of x is 5 and y is 5.
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Determine if execution is linearizable, outline your reason why/why no.
4 ptsQuestion 28
Let process p2 try to synchronize with process p1 by running Cristian’s
algorithm. Assume that both clocks increase at the same rate and that initially (at
time T1 to T3)
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when the query message is sent) the clock of p1 is Cp1(T1) = 1h2mn25s850ms
while the clock of p2 is Cp2(T1) = 1h2mn26s500ms.
What is the round trip time?
What is the offset of p1 relative to p2?
4 ptsQuestion 29
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Find the shortest path between a and z, provide the contents of your priority queue
and minimum cost on each step.
2 ptsQuestion 30
The hashes
What guarantees the integrity of stored transactions in a Blockchain?
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Saved at 13:00
The miner
Transaction signatures
The blocks
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