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并行计算考试代考 Parallel Computing代写 cs考试代写

CSCI-UA.0480-051: Parallel Computing

Midterm Exam

并行计算考试代考 Important Notes- READ BEFORE SOLVING THE EXAM • If you perceive any ambiguity in any of the questions, state your assumptions clearly and


If you perceive any ambiguity in any of the questions, state your assumptions clearly and solve the problem based on your assumptions. We will grade both your solutions and your assumptions.

This exam is take-home.

You are allowed only one submission, unlike assignments and labs.

Your answers must be very focused. You may be penalized for wrong answers and for putting irrelevant information in your answers.

Your answer sheet must have a cover page (as indicated below) and one problem answer per page. This exam has 6 problems.

The very first page of your answer must contain:

You Last Name

Your First Name

Your NetID

Copy and paste the honor code showed in the rectangle at the bottom of this page.

Honor code (copy and paste to the first page of your exam) 并行计算考试代考

• You may use the textbook, slides, and any notes you have. But you may not use the internet.

• You may NOT use communication tools to collaborate with other humans. This includes but is not limited to G-Chat, Messenger, E-mail, etc.

• Do not try to search for answers on the internet it will show in your answer and you will earn an immediate grade of 0.

• Anyone found sharing answers or communicating with another student during the exam period will earn an immediate grade of 0.

“I understand the ground rules and agree to abide by them. I will not share answers or assist another student during this exam, nor will I seek assistance from another student or attempt to view their answers.”

Problem 1

Assume we have the following task flow graph where every node is a task and an arrow from a task to another means dependencies. For example, task B cannot start before task A is done.


The following table shows the time taken by each task in nano seconds.

TaskTime Taken

a. [4 points] What is the minimum number of cores needed to get the highest performance? What is the speedup given the number of cores you calculated?

b. [4 points] What is the span? What is the work?

c. [4 points] Given what you calculated in (b) above, what is the parallelism? And what does the parallelism number you calculated mean?

d. [4 points] Is the number you calculated in (c) different than the number of you calculated in (a)? Justify.

e. [4 points] Do you think we can have a loop in the task graph above (i.e. an arrow pointing to an earlier task)? If no, how can iterations be represented? If yes, how will you calculate the span in that case?

f. [4 points] Suppose we have two scenarios: In one of them we have x cores and in the other we have y cores. Choose x and y such that x cores gives you lower speedup but higher efficiency for the graph above. than y cores? Is it possible to do that? If yes, please given an example using the above graph (and calculate both the speedup and efficiency). If not, explain in 1-2 sentences why not.

Problem 2 并行计算考试代考

Answer the following questions about technology and hardware design.

a. [4 points] What is the importance of Moore’s law that is related to multicore processors?

b. [4 points] Why it is not easy to just write a sequential program and let the compiler generate the parallel version of it? You answer must not exceed two sentences.

c. [4 points] We say that in hardware pipelining, with five stages (fetch, decode, issue, execute, commit), the hardware can execute several instructions at the same time but each instruction is in different phase of its lifetime. That is, one instruction in its fetch phase, another in its decode phase, a third in the issue phase, etc. So, if we increase the number of stages from five, as we saw in class, to ten, does this guarantee we will have ten instructions being executed in parallel most of the time? Why? or Why not?

d. [4 points] Since MIMD can implement all the other types (SISD, SIMD, and MISD). Then why did we build chips based on SIMD (like GPUs) and did not build all chips as MIMD?

e. [4 points] We saw four different categories of architecture in Flynn’s taxonomy. What is the classification of single core with speculative execution and superscalar capability? Justify your choice.

f. [4 points] As the number of cores increases, cache coherence protocols are implemented as directory-based and not snoopy, why is that (in 1-2 sentences)?

Problem 3 并行计算考试代考

Answer the following questions regarding parallel programming techniques.

a. [5 points] State five reasons, each one not exceeding two sentences, about why it is almost impossible to eliminate load imbalance in parallel programs.

b. [4 points] Why is it good to start by writing a sequential code of the problem to be solved then parallelize that code? State one reason in 1-2 sentences.

c. [10 points] For each one of the following problems, choose whether you will parallelize it using task-level, or divide and conquer. Then add 1-2 sentences justifying your choice.

ProblemTask level or divide and conquerJustification
matrix multiplication  
factorial n  
matrix transpose  
matrix addition  
find all even numbers in a list  

Problem 4

[4 points] Suppose we have the following two pieces of code.

…. x = TRUE; ….  … if(!x) x = TRUE; ….  

A program wants to decide whether to use the code on the left or the code on the right in a multithreaded program. It is obvious that both are doing the same thing. Which one do you think has the potential of being faster? and why? (Not giving a justification will not earn you any credit).

Problem 5 并行计算考试代考

[9 points]

Suppose we have a quad-core processor. Each core has its own L1 cache. There is a shared L2 cache. The block size for all caches is 32 bytes in size. We have an array of 256 integers. We want to increment each element of that list by one. We want to do that by writing a multithreaded program. The elements of this array will be divided among the four cores. State which elements of that array will be assigned to which core to reduce the possibility of false sharing and ensure load balancing, in each one of the following scenarios. Also, for each one, give one sentence justification:

a. Each L1 cache is 64KB of size and is 2-way set associative.

b. Each L1 cache is 32KB of size and is 4-way set associative.

c. Each L1 cache is 16KB and is 8-way set associate.

Problem 6 并行计算考试代考

Answer the following questions regarding MPI.

a. [4 points] If we run MPI on a shared memory machine, do we risk suffering from coherence? Justify in 1-2 sentences.

b. [4 points] In 1-2 sentences explain why MPI gives each process a rank?

c. [4 points] We saw the API MPI_Comm_size() that gives the total number of processes. But, we already specify the number of processes when we write execution command. Then, why do we need such an API? State two reasons.

d. [4 points] What is the reason for having something called communicator in MPI?

e. [4 points] Most collective calls in MPI have either a field for source process or a field for destination process, but never both. Why is that? State your reason in 1-2 sentences.


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