COMP9334 Capacity Planning of Computer Systems and
Networks
Assignment (Version 1.01), Term 1, 2023
Due 5:00pm, Fri 17 March 2023 (Friday Week 5)
Change log and version info
Updates, changes and clarifications will appear in this box.
Version 1.01 (7 March 2023) revises the wording in Question 1.
Version 1.00 issued on 27 February 2023
Instructions
(1) There are 3 questions in this assignment. Answer all questions.
(2) The total mark for this assignment is 20 marks.
(3) The submission deadline is 5:00pm Friday 17 March 2023. Submissions made after the
deadline will incur a penalty of 5% per day. Late submissions will only be accepted
until 5:00pm Wednesday 22 March 2023, after which no submissions will be accepted.
(4) In answering the questions, it is important for you to show your intermediate steps and
state what arguments you have made to obtain the results. You need to note that both
the intermediate steps and the arguments carry marks. Please note that we are not just
interested in whether you can get the final numerical answer right, we are more inter-
ested to find out whether you understand the subject matter. We do that by looking at
your intermediate steps and the arguments that you have made to obtain the answer.
Thus, if you can show us the perfect intermediate steps and the in-between arguments
but get the numerical values wrong for some reason, we will still award you marks for
having understood the subject matter.
You can take a look at the solution to revision problems to get some ideas the level of
explanation that is required.
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(5) If you use a computer program to perform any part of your work, you must submit the
program or you lose marks for the steps.
(6) This is an individual assignment.
(7) Your submission should consist of:
(a) A report describing the solution to the problems. This report can be typewritten
or a scan of handwritten pages. This report must be in pdf format and must be
named report.pdf. The submission system will only accept the name report.pdf.
(b) One or more computer programs if you use them to solve the problems numerically.
You should use zip to archive all the computer programs into one file with the name
supp.zip. The submission system will only accept this name. The report must refer
to the programs so that we know which program is used for which part.
(8) Submission can be made via the course website.
(9) You can submit as many times as you wish before the deadline. A later submission will
over-write the earlier one. We will only mark the last submission that you make.
(10) If you want to ask questions on the assignment, you can attend a consultation (see the
Timetable section of the course website for dates and times) or post your question on
the forum. Please note that if your forum post shows part of your solution or code, you
must mark that forum post private.
(11) Additional assignment conditions:
Joint work is not permitted on this assignment.
– This is an individual assignment. The work you submit must be entirely your
own work: submission of work even partly written by any other person is not
permitted.
– Do not request help from anyone other than the teaching staff of COMP9344.
– Do not post your assignment work or code to the course forum.
– Assignment submissions are routinely examined both automatically and man-
ually for work written by others.
Rationale: this assignment is designed to develop the individual skills needed to
solve problems. Using work/code written by, or taken from, other people will stop
you learning these skills. Other CSE courses focus on skills needed for working in
a team.
The use of AI generative tools, such as ChatGPT, is not permitted on this assign-
ment.
Rationale: this assignment is designed to develop your understanding of basic con-
cepts. Using AI tools will stop you learning these fundamental concepts, which will
significantly impact your ability to complete future courses. Moreover, ChatGPT
has been found to give incorrect answers for advanced problems covered in this
course.
? Sharing, publishing, or distributing your assignment work is not permitted.
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– Do not provide or show your assignment work to any other person, other than
the teaching staff of COMP9334. For example, do not message your work to
friends.
– Do not publish your assignment code via the Internet. For example, do not
place your assignment in a public GitHub repository.
Rationale: by publishing or sharing your work, you are facilitating other students
using your work. If other students find your assignment work and submit part
or all of it as their own work, you may become involved in an academic integrity
investigation.
Sharing, publishing, or distributing your assignment work after the completion of
COMP9334 is not permitted.
– For example, do not place your assignment in a public GitHub repository after
this offering of COMP9334 is over.
Rationale: COMP9334 may reuse assignment themes covering similar concepts and
content. If students in future terms find your assignment work and submit part
or all of it as their own work, you may become involved in an academic integrity
investigation.
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Question 1 (5 marks)
Assuming that you are the administrator of an interactive computer system. The computer
system consists of a multi-core CPU and a disk. During an observation time of 1800 seconds,
you obtained the following measurements from the system:
Average busy time per core 1575 s
Disk busy time 1124 s
Number of requests completed by the computer system 57
This computer system is used by 16 interactive users and the thinking time per interactive
user is 45 seconds.
You consider the current throughput of the system is too low. You are considering a
proposal to upgrade the current CPU, which has 4 cores, to a new CPU with 6 cores and the
same processing speed per core as that of the current CPU.
For this question, you can assume that the total workload remains the same before and
after the upgrade. You can also assume that the workload is requests (note the plural) are
almost evenly distributed among the cores at the moment and the workload requests can still
be evenly distributed among the cores after the upgrade. As the system administrator, you
know that when the workload each request (note the singular) uses the CPU, it uses only one
core at a time, i.e., the workload a request does not use multiple cores concurrently.
Answer the following questions.
(a) Determine the current average service demand of a core.
(b) What will the average service demand per core be if the proposed CPU upgrade is
carried out?
Hint: The service demand of a core depends on two factors: the number of visits to the
core and the service time needed per visit to the core. For the set up of this question,
one of these two factors remains the same after the upgrade, while the other factor will
change.
(c) What will the throughput bound of the computer system be if the proposed CPU
upgrade is carried out?
Reminder: If you use a computer program to derive your numerical answers, you must
include your computer program in your submission. Do not forget to show us your steps to
obtain your answer.
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Question 2 (7 marks)
Assuming that you are the owner of a CPU and you are happy for outsiders to use your CPU
as long as these outsiders’ work does not interrupt yours and your work takes precedence over
theirs. This question is inspired by people who donate their spare CPU time for scientific
research.
In this question, we will use the term primary user to refer to the CPU owner (which is
you) and the term external users to refer to the outsiders. We make the following assumptions:
The CPU is configured as a single processing unit without any queueing spaces.
If a request (which can be from the p
rimary user or an external user) arrives when the
CPU is idle, the request will be admitted to the CPU.
If a request (which can be from the primary user or an external user) arrives when the
CPU is working on a primary user’s request, the arriving request will be rejected. This
is because there are no queueing spaces in the CPU.
If a request from the primary user arrives when the CPU is working on an external
user’s request, the external user’s work will be terminated immediately and the primary
user’s request will be admitted to the CPU immediately. In this case, the external
user loses their work and its remaining work will not be resumed. Therefore, you can
consider that the external user has left permanently.
If a request from an external user arrives when the CPU is working on another external
user’s request, the newly arriving request will be rejected.
The inter-arrival times for the primary user’s requests are exponentially distributed with
mean arrival rate λp; those for the external users’ requests are exponentially distributed
with mean arrival rate λe.
The service times of the primary user’s requests are exponentially distributed with
a mean service time of 1μp ; those for the external users’ requests are exponentially
distributed with mean 1μe .
The four probability distributions mentioned in the last two dot points are independent
of each other.
Answer the following questions. You are expected to express your answers in terms of
these rate parameters: λp, λe, μp and μe.
(a) Let us assume that at time t, you observe that the request at the CPU belongs to an
external user. What is the probability that this observed request will still be in the
CPU at time (t + ?t) where ?t is an infinitesimal time change? You should express
this probability in terms of ?t and any of the appropriate rate parameters.
(b) Formulate a continuous-time Markov chain for the CPU. Your formulation should in-
clude the definition of the states and the transition rates between states.
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(c) Write down the balance equations for the continuous-time Markov chain that you have
formulated.
(d) Derive the expressions for the steady state probabilities of the continuous-time Markov
chain that you have formulated. You should be able to solve for the steady state
probabilities analytical and provide answers in terms of λp, λe, μp and μe.
(e) What is the probability that a request from the primary user will be admitted? Why
is this probability independent of the rate parameters of the external users?
(f) What is the probability that a request from an external user will be admitted?
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Question 3 (8 marks)
This question is based on the system illustrated in Figure 1. The system consists of a database
server and an external queue. The database server consists of a front-end server and a back-
end server; each server has its own queue. Each of the three queues in this system (i.e.,
external, front-end, back-end) has the capacity to hold only one request.
Incoming
requests
Departing
requests
Database server
External
queue Front-end Back-end
Figure 1
The mode of operation for the system in Figure 1 is as follows:
The total number of requests in the database server (i.e., the two servers and two queues)
must be two or less.
If an incoming request arrives when there are a total of 2 requests in the database server,
then the incoming request will join the external queue if it is empty; otherwise it will
be rejected if the external queue is already occupied.
If an incoming request arrives when there are no requests in the database server, then
the incoming request will be sent to the front-end server.
If there is one request in the database server, then an incoming request will be sent to
the front-end server if it is idle or it will be placed in the front-end queue if the front-end
server is busy.
After the front-end server has finished processing a request, there is a probability of
p that the front-end server will send the request to the back-end server for further
processing, and a probability of (1 ? p) that the request will leave the database server
(hence the system) permanently.
After a request has been processed by the back-end server, the request will always be
sent back to the front-end for further processing; this request will need to join the queue
if the front-end server is busy.
If there is a request waiting in the external queue at the time a request is leaving the
database server permanently, then the request in the external queue will be admitted to
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the database server. There are two scenarios depending on whether the front-end queue
is occupied at the time when the permanent departure takes place. If the front-end
queue is occupied, then upon the permanent departure, the request in the front-end
queue will move to the server and the request in the external queue will move to the
front-end queue. If the front-end queue is unoccupied, then the request in the external
queue will go to the front-end server.
You can assume the following for the workload:
The incoming requests are Poisson distributed with a mean arrival rate of λ requests
per unit time.
The service time (i.e., per visit) to the front-end is exponentially distributed with a
mean of 1μf .
The service time (i.e., per visit) to the back-end is exponentially distributed with a
mean of 1μb .
All the service times and inter-arrival times are independent of each other.
(a) Formulate a continuous-time Markov chain for the system. Your formulation should in-
clude the definition of the states and the transition rates between states. The transition
rates should be expressed in terms λ, μf , μb and p.
(b) Assuming that λ = 1.4, μf = 2.1, μb = 1.8 and p = 0.3.
(i) Determine the steady state probabilities of the state of the continuous-time Markov
chain that you have specified in Part (a).
(ii) Determine the throughput of the database server.
(iii) Determine the mean response time of the database server.
Reminder: If you use a computer program to derive your numerical answers, you must
include your computer program in your submission. Do not forget to show us your steps to
obtain your answer.
End of assignment
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