计算机体系结构代写 - EE524/CPTS561 Mid term exam
时间:2020-10-12

EE524/CPTS561 Mid term exam
Friday, 9 October 2020. Submit by 8:00 am, 10 October 2020
Name: ID:
FOR FULL CREDIT:
• Please show your work, justify the claims you make and point to a source if you are using any.
You may lose points if you use a resource without appropriate references.
• Please, DO NOT collaborate on your answers. You should work on the exam questions by
yourself.
Please type or hand-write your answers and upload them to Blackboard. Please convert your files or
images to PDF format and upload the PDF. It makes annotations and returning feedback to you
much easier.
Submit your exams by 8:00 am on Saturday, 10 October 2020. Late submissions will be
penalized at 10 points/hour for each hour of delay.
Question 1 [10/20 points]
You are given the task to choose the number of cores in the next generation of a multicore chip.
Your teammates collected the data summarized below:
Figure 1: Execution time and power consumption with respect to the number of cores
Looking at this data, assume the following:
• The power consumption can be expressed as: P (n) = nP (1), where P (1) is the total power
consumption of a single core. Further assume a fixed Cdyn and Ileakage throughout this problem,
and operating voltage V is proportional to the frequency (i.e., you can assume V = af , where
a > 0).
• The execution time can be expressed as: texe = tc + tsn , where tc is a constant term which
does not change with the number of cores, and tsn denotes the time that scales with number of
cores. Both tc and ts are inversely proportional with frequency.

Name: EE524/CPTS561, Fall 2020
(a) What is the energy-optimal number of cores? Write one-sentence intuition for this answer.
Recall that energy is the product of power and execution time.
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Name: EE524/CPTS561, Fall 2020
(b) Suppose that 10 ms execution time provides sufficient performance for your target market.
Propose a design technique to reduce the energy consumption compared to the minimum energy you
found in part (a). You need to quantify the new energy consumption and show that it is smaller
than the result in part (a). You can assume that Pdynamic > Pleakage.
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Name: EE524/CPTS561, Fall 2020
Question 2 [5/7/7/6 points]
As part of optimizing the cache design, you recently switched to using a two-way set associative
cache, as opposed to a direct-mapped cache. However, adding associativity to a cache increases the
access time. One of the ways to reduce the access time is to use a way predicting cache. On every
cache access, we make a prediction about the way in which the data is present and select the way. If
the way is correctly predicted, it generates a fast access, similar to a direct-mapped cache. If the
prediction is incorrect, the other way is accessed to check if the data is present in the second way. If
the data is not present in the cache, then a normal cache miss occurs. This process is described in
the figure below:
Figure 2: Way prediction scheme
The way prediction consumes one bit, since there are two ways present in the cache. Moreover,
the bit value is used as the prediction. For this question, you can assume that a prediction is always
available on a cache access, and it is not part of the critical path of the cache access.
(a) First, we will determine the cache parameters for our 2-way set associative cache. Assuming a
64-bit address, 64-byte blocks, and 12-bit cache index, complete the following table. You can use the
next page to show your work.
Table 1: Cache parameters
2-way set associative cache
Tag bits
Block offset bits
No. of sets
Cache size (KB)
We will use these cache parameters for the rest of this question.
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Name: EE524/CPTS561, Fall 2020
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Name: EE524/CPTS561, Fall 2020
(b) Next, we will determine the cache hit time by looking at the critical path in the cache hit. The
critical path is the sequence of steps that have to be done for a fast access. For this purpose, we will
use the figure below that shows the architecture of the way predicting cache.
OffsetIndexTag
Prediction
MUX MUX
Data bus
Data Drivers
Buffer Drivers
Figure 3: Architecture of way-predicting cache
Using the figure and the table below, calculate the delay of the critical path. You can assume
that the prediction bit generation is not on the critical path as it is readily available for every cache
access. (Hint: To determine the critical path, you need to check the sequence of operations that have
to be done for a fast cache access. Modules not in the direct path do not contribute to the critical
path delay.)
Table 2: Critical path calculation
Component Equation for delay (ps) Total (ps)
Decoder 20·(# of index bits) + 60
Memory array 10· log2(#rows) + 10· log2(# bits in each row) + 200
N to 1 Mux 50· log2N + 200
Buffer driver 160
Data output driver 90·(associativity) + 100
Critical Path Delay
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Name: EE524/CPTS561, Fall 2020
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Name: EE524/CPTS561, Fall 2020
(c)Way-predicting caches can potentially offer advantages in instruction caches. Assume a prediction
scheme where each set has a prediction bit that keeps track of the last way accessed for the set.
Next, assume that the cache uses a least recently used replacement policy and the parameters in
Table 1 whenever there are conflicts. In what situations would the predictions would be accurate?
Under what conditions will the way prediction scheme result in mispredictions?
(d) For this part, assume that the page size is 8 KB for the machine on which the way-prective cache is
implemented. The cache parameters remain the same as in Table 1. If the cache is physically-tagged
and virtually-indexed, does the way-predicting cache suffer from aliasing? Why or why not? Explain
your answer.
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Name: EE524/CPTS561, Fall 2020
Question 3 [10 points]
You are trying to optimize the classical five-stage pipeline used in your company. The current
pipeline uses forwarding to avoid stalls as much as possible. After doing some profiling, you discover
that the ALU stage of the pipeline is taking the longest to execute, which slows down the clock. As a
result, the execution time of the applications is higher. In order to address this issue, you propose to
split the ALU into two pipeline stages. With the ALU split into two pipeline stages, you now have a
six-stage pipeline. Note that the result of the ALU is only available after the second ALU stage
and the data after the first ALU state is not useful. Describe how this change of adding a pipeline
stage affects Read after write (RAW) hazards. How would you resolve the new complications for
RAW hazards that result from the six-stage pipeline? Please list changes and/or hardware needed
for resolving the RAW hazards. You may use two consecutive ALU instructions with a RAW hazard
for the purposes of this question.
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Name: EE524/CPTS561, Fall 2020
Question 4 (Short answer questions) [20 points]
(a) Amdahl’s law provides a very useful metric to measure speedup when we make applications
parallel using more resources. However, Amdahl’s law’s basic form makes an assumption about
the application that may not hold in real-world scenarios. State the assumption made by
Amdahl’s law and describe conditions that may break the assumption.
(b) What type of misses are reduced by each of these cache optimization techniques. List all
types of misses reduced for full credit. In addition, list the possible disadvantages of using the
optimization technique.
• Data and instruction prefetching:
• Pipelined cache accesses:
• Higher associativity:
• Larger cache capacity:
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Name: EE524/CPTS561, Fall 2020
(c) How does each of these changes in an ISA or pipelining change the number of instructions in a
program and the cycles per instruction. Provide reasoning for your answers.
• Changing the ISA from load-store to register-memory.
• Merging pipeline stages.
• Using branch instructions for jumps instead of having dedicated jump instructions.
• Increasing the number of registers from 32 to 64.
(d) Many processors optimize their interrupt handling performance by taking only the bare
minimum action on an interrupt and queuing the bulk of processing for a later time. Can the
same optimization apply to exceptions? Explain your reasoning.
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Name: EE524/CPTS561, Fall 2020
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Name: EE524/CPTS561, Fall 2020
Question 5 [15 points]
Consider a four-way set associative cache for the following questions. We explore different replacement
and insertion policies with this cache.
(a) To implement a least recently used (LRU) replacement policy, we form an array of 4 indices
for each set to represent the line reference orders of the set. Assume the initial reference order
of a particular set is 0, 1, 2, and 3 (lines 0, 1, 2, and 3 are most recently used (MRU), 2nd
MRU, 2nd LRU, and LRU, respectively), as shown in the diagram. Moreover, whenever a
cache miss happens, the new line is inserted in the most recently used position. After a miss,
a hit to line 1, and then a hit to line 0, what is the new reference order? (Please fill in the
up-to-date array of indices)
0 1 2 3
MRU
2nd
MRU
2nd
LRU LRU
(b) Next, we make a modification to the insertion policy of the cache. Instead of inserting the
new line to the MRU position, we insert it to the LRU position. It is promoted to the MRU
position only when it is reused. Given this insertion policy, what is the reference order after 2
misses, hit to line 3, and hit to line 1.
0 1 2 3
MRU
2nd
MRU
2nd
LRU LRU
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Name: EE524/CPTS561, Fall 2020
(c) A Partial LRU (or Pseudo LRU) replacement algorithm can be described with a binary tree.
That is, we form a binary tree of 4 leaf nodes for each set to represent the 4 lines in the set. In
the non-leaf node, a binary bit is used to show the LRU or MRU order of the two branches
(if the left branch is LRU (MRU), the bit is 0 (1)). Assume the initial reference order of a
particular set is (1, 1, 1), i.e., the 3 bits at nonleaf nodes A, B, and C of the figure are 1, 1,
and 1 respectively, which indicates the pair of lines 0 and 1 is accessed more recently than the
pair of lines 2 and 3, line 0 is accessed more recently than line 1, and line 2 is accessed more
recently than line 3.
0 1 2 3
1
1 1
MRU
MRU MRU
LRU
LRU
LRU
After a miss, a hit to line 1, and then a hit to line 0, what is the new reference order in the
tree:
(A, B, C) = ( , , )
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