Description
Problem 1
You have been hired as a Boston MBTA platform inspector. Your job is to respond to requests for inspection
of the platform conditions at specific stations.
To do your job, you are given a mobile app where you can receive requests for inspection when they are
sent by the users. Once a request for platform inspection is received, you are expected to respond to it
at some point in the future, but not necessarily in the order received. Your boss wants you to be smart
about answering these requests so that you are able to satisfy as many as possible. As a reward for swiftly
completed inspections, your boss will foot the bill for a whole pint of rosted marshmellow frappuccino at the
end of your shift.
Since you are familiar with the B-line, you have been assigned to service all 24 stations on that line, which
are shown at https://mbta.com/schedules/Green-B/line?direction_id=0 (Government Center is stop
#0 and BU is stop #8.) The following assumptions hold: (i) once you make it to a station, the amount of
time to service the request is negligible—all you have to do is take a picture with your mobile phone, and
(ii) from any station, you can travel in either direction.
a) What class of scheduling strategies are best suited to determine the order in which you should handle
inspection requests? Motivate your answer.
b) To minimize the danger from switching platforms, you decide to minimize the number of times you
switch directions. What scheduling strategy is consistent with that request?
c) Now, consider the following set of platform inspection requests currently on your mobile app after
inspecting platform #14 (Harvard Ave) while going eastbound (towards BU): #17, #6, #13, #19,
#2, #15, #16, #0. In what order will you serve these requests?
d) You have received a complaint that you let many requests linger for too long because you were responding to these requests “out of order”. Your boss now wants you to modify your strategy by not
allowing more than 3 requests to go ahead of a pending request received before them. Describe your
modified scheduling strategy and apply it to the scenario in Part c).
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CS-350 – Fundamentals of Computing Systems::Homework Assignment #6 Problem 2
Problem 2
A disk has 12 cylinders and that are concurrently used by 4 processes. Requests coming from process P1
always target cylinder 10 and arrive every 12 time units. Requests form P2 target cylinder 7 and arrive
every 6 time units. Requests from P3 target cylinder 5 and arrive every 5 time units. Requests from P4
target cylinder 2 and arrive every 18 time units.
Assume the following: (1) the first request of each task arrives at time 0; (2) the disk head is able to move
by TWO cylinders every time unit; (3) when the disk head is in the right position, it takes ONE extra time
unit to complete serving a request. During this time, the disk head cannot move; (4) only after a request
has been fully served, a new scheduling decision can be made; (5) the disk head is initially positioned at
cylinder 2.
a) Use the grid below to visualize the schedule produced by the Shortest Scan Next scheduling algorithm
until time 36. Carefully read the 5 rules described above.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
1
2
3
4
5
6
7
8
9
10
11
12
Cylinder #
Time
b) Use the same style of grid as in the previous Part a) to visualize the schedule produced by the SCAN
scheduling algorithm until time 36. Assume that the disk head travel direction at time 0 is UP. Carefully
read the 5 rules described above.
c) Use the same style of grid as in the previous Part a) to visualize the schedule produced by the C-SCAN
scheduling algorithm until time 36. Carefully read the 5 rules described above.
d) Use the same style of grid as in the previous Part a) to visualize the schedule produced by a static
priority scheduler that uses the Rate Monotonic rule to assign a fixed priority to each process. Draw
only until time 36. Carefully read the 5 rules described above.
e) Which algorithm appears to exhibit the best performance if the objective is to minimize the overall
distance traveled by the disk head between time 0 and 36 while serving the largest number of requests?
Motivate your answer.
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CS-350 – Fundamentals of Computing Systems::Homework Assignment #6 Problem 3
Problem 3
The engine controller of a Boeing 747 airliner is comprised of four control tasks, namely the Fuel Injector
(FI) task, the Spin Rate Controller (SR), the Air Pressure Regulator (AP), and the Anomaly Detector (AD).
The FI, SR, AP, and AD tasks are released every 7, 9, 12, and 17 milliseconds, respectively. The distribution
of runtimes of the 4 tasks has been estimated. The FI task has a runtime of 1.5±0.5 milliseconds, i.e. a mean
runtime of 1.5 milliseconds, a min runtime of 1.5-0.5 milliseconds, and a max runtime of 1.5+0.5 milliseconds.
The SR task has a runtime of 1.8±0.2. The AP task has a runtime of 2.3±0.7 milliseconds. Finally, the AD
task has a runtime of 3.9±0.1 milliseconds.
a) Consider only the system comprised of tasks FI, SR, and AP. Is the system schedulable on a single-CPU
processor using RM? Motivate your answer.
b) From now on, consider the system with all the 4 tasks. Is the system schedulable on a single-CPU
processor using RM? Motivate your answer.
c) Is the system schedulable on a single-CPU processor using EDF? Motivate your answer.
d) Draw the schedule produced by EDF until time 34.
e) Starting from time 0, at what time in the future the same exact pattern of task activations and
executions will repeat? Hint: there is no need to draw anything. Just think about it.
f) Consider a 2-CPU processor which is slower than the one considered in the original system. In particular, in the new processor, each task takes 60% more time to execute — but the periods do not
change! Is it possible to schedule the system by first splitting the 4 tasks across the 2 CPUs and then
using RM on each CPU? Explain your reasoning.
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CS-350 – Fundamentals of Computing Systems::Homework Assignment #6 Problem 4
Problem 4
Code: In this problem you will write two functions to navigate complex structures that are based on the
ability to crack MD5 hashes.
a) Extend the class Dispatcher.java that you implemented as part of HW5 to parallelize the cracking of
a set of hashes. Use Java support for multi-threading to define as many woorker threads as the number
of CPUs N in the machine—passed as a parameter to your code. The Dispatcher should construct a
global work queue where each hash to crack is a unit of work and provided to the first available worker
for cracking. Define a dispatch(…) method to handle the distribution of work units to worker
threads.
Apart from implementing the dispatch(…) method, the class should also include a public static
void main(String [] args) function. The main(…) function should accept 2 parameters from the
calling environment. First, the input data (list of hashes to crack) is given in a file. The path to the
file is passed as the first parameter by the calling environment. The input file is structured as a list of
MD5 hashes to crack, one per line, with each line terminated with a newline (\n) character. Second
the number of CPUs available on the machine N is passed as the second parameter.
It is responsibility of the main(…) function to internally initialize and start the dispatcher. It is the
job of the dispatcher to (1) read the input file; (2) distribute the work to the worker threads and let the
threads print the result. The result of each of the unhash operations, i.e. the cracked hashes, should be
printed in output, with a single line per decoded hash. Apart from the list of decoded hashes, nothing
else should be printed in output.
b) Modify the dispatcher to be able to handle impossible hashes. For this part, you will be given an input
file that contains a few hashes that are impossible to crack. Extend the dispatcher to handle timeouts
in the processing of hashes so that the workers do not get stuck trying to reverse impossible hashes.
For this part, the input file will appear identical to the previous part, at least on the surface. The
second parameter passed to your code will still be the number of available CPUs. A third parameter
is added for this part, that encodes the (recommended) length of the timeout for impossible hashes
expressed in milliseconds.
Just like the previous part, it is responsibility of the main(…) initialize and start the dispatcher. It
is the job of the dispatcher to (1) read the input file; (2) distribute the work to the worker threads and
let the threads print the result. Importantly, (3) the dispatcher should handle the timeout for all the
threads. If a worker does not complete on time, the dispatcher should order the thread to move on. In
this case, the uncrackable hash as it was read from the input file should be print in output.
HINT: in Java, thread signaling works in a very counter-intuitive way. Use variables to control the
execution flow on the worker threads instead.
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CS-350 – Fundamentals of Computing Systems::Homework Assignment #6 Problem 5
Submission Instructions: in order to submit this homework, please follow the instructions below for
exercises and code.
The solutions for Problem 1-3 should be provided in PDF format, placed inside a single PDF file named
hw6.pdf and submitted via Gradescope. Follow the instructions on the class syllabus if you have not received
an invitation to join the Gradescope page for this class. You can perform a partial submission before the
deadline, and a second late submission before the late submission deadline.
The solution for Problem 4 should be provided in the form of Java source code. To submit your code,
place all the .java files inside a compressed folder named hw6.zip. Make sure they compile and run correctly according to the provided instructions. The first round of grading will be done by running your code.
Use CodeBuddy to submit the entire hw6.zip archive at https://cs-people.bu.edu/rmancuso/courses/
cs350-fa22/codebuddy.php?hw=hw6. You can submit your homework multiple times until the deadline.
Only your most recently updated version will be graded. You will be given instructions on Piazza on how
to interpret the feedback on the correctness of your code before the deadline.
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