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ser334_unit9_exercise_sample.lyx
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\noindent
Arizona State University
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SER334: Operating Systems & System Programming
\end_layout
\begin_layout Standard
\noindent
UGTA Lisonbee (5), Lecturer Acuña (5), UGTA Alvaran (1)
\begin_inset space \hfill{}
\end_inset
Revised 8/30/2022
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\noindent
\begin_inset CommandInset line
LatexCommand rule
offset "0.5ex"
width "100col%"
height "1pt"
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\size huge
Unit 9 Sample Problems - CPU Scheduling
\end_layout
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\begin_inset VSpace bigskip
\end_inset
\end_layout
\begin_layout Standard
In this exercise, we will review the concepts of process scheduling.
\end_layout
\begin_layout Itemize
Length: 1:15 minutes with discussion.
\end_layout
\begin_layout Itemize
Questions: Q1, Q3, Q5-Q8, Q10, Q12.
\begin_inset Note Note
status collapsed
\begin_layout Itemize
50 minutes: Q1, Q3, Q5-Q7.
(optional: Q2, Q4 Q8-Q12)
\end_layout
\end_inset
\end_layout
\begin_layout Standard
\begin_inset Branch dev
inverted 0
status open
\begin_layout Itemize
Determine how CPU/IO bursts will be distributed for an application.
Q
\begin_inset CommandInset ref
LatexCommand ref
reference "enu:scenario-bursts"
\end_inset
\end_layout
\begin_layout Itemize
Select which scheduling criteria is most important for an application.
Q
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LatexCommand ref
reference "enu:criteria_selection_queue"
\end_inset
\end_layout
\begin_layout Itemize
Simulate the algorithm of basic scheduling algorithms such as FCFS, SFJ,
SFJ-L, PS, RR.
Q
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LatexCommand ref
reference "enu:simulate-sa-fcfs"
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, Q
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LatexCommand ref
reference "enu:simulate-sa-sjf"
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, Q
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LatexCommand ref
reference "enu:simulate-sa-rr"
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\begin_inset VSpace 0.5cm
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\begin_layout Section
\series bold
\size larger
Basic Concepts
\end_layout
\begin_layout Enumerate
[Lisonbee] You are editing a video that uses very high resolution, uncompressed
video files.
When you go to render the video, how will the CPU bursts for this process
look relative to the I/O bursts?
\series bold
Explain.
\series default
[2 points]
\series bold
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\series bold
Ans: [Lisonbee]
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\begin_layout Standard
Due to the size of the video files, the CPU bursts will be short relative
to the I/O bursts.
Most of the time that this process spends rendering the video will be dealing
with the large amounts of data rather than performing computations.
\end_layout
\begin_layout Standard
\begin_inset VSpace 3cm
\end_inset
\end_layout
\end_inset
\end_layout
\end_deeper
\begin_layout Enumerate
[Alvaran] You
\bar under
run
\bar default
a console-based calculator program, which
\bar under
waits
\emph on
\bar default
\emph default
for user input.
You submit an additional problem that the program calculates and outputs
to the screen.
Lastly you
\bar under
terminate
\series bold
\bar default
\series default
the program.
\series bold
Explain
\series default
which parts of the scenario correspond to scheduling events? For each event,
what type of scheduling is used (preemptive vs non-preemptive)? [2 points]
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name "enu:lo-unknown"
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\series bold
Ans: [Alvaran]
\series default
The program prompting the user for a math problem involves switching from
the ready state to the waiting state, which is non-preemptive scheduling.
Receiving then calculating the input involves a switch from the waiting
state to the ready state, which is preemptive scheduling.
The termination of the program involves non-preemptive scheduling.
\end_layout
\begin_layout Standard
\begin_inset VSpace 2cm
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Section
\series bold
\size larger
Scheduling Criteria
\end_layout
\end_deeper
\begin_layout Enumerate
[Lisonbee] Consider a scenario where all queued jobs run for a random amount
of time, but all are equally important in terms of the work they complete.
Of the five criteria for scheduling jobs, which one would be the most important
to optimize in this situation?
\series bold
Explain.
\series default
[2 points]
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Ans: [Lisonbee]
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Throughput would be the most crucial criteria for this situation as each
job is equally weighted; they should be scheduled so that the most amount
of jobs can be completed within a fixed amount of time.
\end_layout
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\begin_inset VSpace 3cm
\end_inset
\end_layout
\end_inset
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\end_deeper
\begin_layout Enumerate
[Lisonbee] If you are trying to optimize for
\bar under
waiting time
\bar default
, roughly how will the processes be ordered in terms of the time they take
to complete?
\series bold
Explain.
\series default
[2 points]
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Ans: [Lisonbee]
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\begin_layout Standard
To optimize for waiting time, generally shorter jobs will be scheduled first,
and then longer ones to ensure that the majority of jobs are not waiting
a long time in the queue.
\end_layout
\begin_layout Standard
\begin_inset VSpace 1cm
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Section
\series bold
\size larger
Scheduling Algorithms
\end_layout
\end_deeper
\begin_layout Enumerate
[Lisonbee] Consider a set of ordered pairs of process IDs and CPU time:
(P0, 12), (P1, 6), (P2, 3), (P3, 4).
\series bold
Calculate
\series default
both the average turnaround time and average waiting time of this set using
FCFS scheduling.
(You may leave the answer as a fraction.) [2 points]
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Ans: [Lisonbee]
\end_layout
\begin_layout Standard
Average completion time = (12+18+21+25)/4=19
\end_layout
\begin_layout Standard
Average waiting time = (0+12+18+21)/4=12.75
\end_layout
\begin_layout Standard
\begin_inset VSpace 3cm
\end_inset
\end_layout
\end_inset
\end_layout
\end_deeper
\begin_layout Enumerate
[Lisonbee] Consider a set of ordered pairs of process IDs and CPU time:
(P0, 12), (P1, 6), (P2, 3), (P3, 4).
\series bold
Calculate
\series default
both the average turnaround time and average waiting time of this set using
SJF scheduling.
(You may leave the answer as a fraction.) [2 points]
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\series bold
Ans: [Lisonbee]
\end_layout
\begin_layout Standard
Average completion time = (3+7+13+25)/4=12
\end_layout
\begin_layout Standard
Average waiting time = (0+3+7+13)/4=5.75
\end_layout
\begin_layout Standard
\begin_inset VSpace 3cm
\end_inset
\end_layout
\end_inset
\end_layout
\end_deeper
\begin_layout Enumerate
[Acuña] Consider a set of ordered pairs of process IDs and CPU time: (P0,
10), (P1, 6), (P2, 7), (P3, 4).
\series bold
Calculate
\series default
both the average turnaround time and average waiting time of this set using
RR scheduling with a time quantum of 5.
(You may leave the answer as a fraction.) [4 points]
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\series bold
Ans: [Acuña]
\end_layout
\begin_layout Standard
Average Completion Time: (24+25+27+19)/4=23.75
\end_layout
\begin_layout Standard
Average Waiting Time: (14+19+20+15)/4=17
\end_layout
\begin_layout Standard
\align center
\begin_inset Graphics
filename unit09/sp/sample_q7_soln.png
scale 80
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\end_layout
\end_deeper
\begin_layout Enumerate
[Silberschatz 6.4] What advantage is there in having different time-quantum
sizes at different levels of a multilevel feedback queuing system?
\series bold
Explain.
\series default
[2 points]
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\series bold
Ans: [Acuña]
\end_layout
\begin_layout Standard
It will improve the throughput of the system by reducing the amount of time
small processes spend waiting.
In the initial queue, small jobs will be able to complete immediately from
the low quantum.
Larger jobs will drop down into the next queue with a higher quantum.
Since a multilevel queue goes from top to bottom, the jobs that have consumed
the least amount of time will be processed first.
Jobs that need more time will be given a lower priority since they will
filter down to have a higher quantum.
\end_layout
\begin_layout Standard
\begin_inset VSpace 2cm
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Section
\series bold
\size larger
Thread Scheduling
\end_layout
\end_deeper
\begin_layout Enumerate
[Acuña] Would we ever want a program using 1:1 thread mapping to use PCS?
Would we ever want a program using n:m or n:1 thread mapping to use SCS?
\series bold
Explain.
\end_layout
\begin_deeper
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\end_inset
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inverted 0
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\begin_layout Standard
\series bold
Ans: [Acuña]
\end_layout
\begin_layout Standard
A program using a n:1 mapping can't use SCS because there aren't enough
kernel-level threads to do 1:1 mapping.
A n:m mapping would be possible for SCS, but might interfere with the performan
ce of other processes.
\end_layout
\begin_layout Standard
A program using 1:1 mapping could use PCS if we wanted scheduling to occur
at two levels (giving more flexibility, and hopefully performance).
\end_layout
\begin_layout Standard
\begin_inset VSpace 3cm
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Section
\series bold
Multiple-Processor Scheduling
\end_layout
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\begin_layout Enumerate
[Acuña] Consider the problem of running a large algorithm on a image (such
as in a previous homework), using threads to split work across multiple
CPU cores.
There there are two possible approaches: one is to use global shared memory
for the threads, and the second is to use geometric deposition to split
the data into a number of chunks equal to the number of threads.
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Explain
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which approach has better potential to lead to better performance and why.
If your answer uses any assumptions about the scheduler, state them.
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Ans: [Acuña]
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The second approach should better.
The issue with the first way is that there is only one memory allocation
that all threads use, meaning it must be moving around different physical
CPUs for the threads to get data.
This forms a bottleneck.
In the second, each thread has the ability to operate independently with
it's data.
However, this requires the assumption that the scheduler will try to place
threads on CPUs where their independent piece of data already exists.
If the thread is scheduled on a new CPU, then data will have to be moved
between cores and performance will be degraded.
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Real-Time CPU Scheduling
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[Acuña] Why would it be inappropriate to use something like a SJF or Priority
scheduler to handle polling sensors at regular intervals in a self-driving
car and then making adjustments to the car?
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Explain.
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In a hard real-time environment, we need to have some guarantee of the latency
between starting some event (to poll) and some action (adjusting a car
part).
This simply isn't captured by the simpler schedulers or their process models.
We need to know about process deadlines to make judgment if a process:
1) should run.
2) can run.
Although we can technical put a bunch of instances of the process into
a priority scheduler, we won't be able to determine if the processes can
run successfully in a period.
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[Acuña] Consider the processes: (P0, 30, 15), (P1, 25, 15).
The triples have the form (process, period, time).
Assume that deadline and period are identical.
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Draw
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the scheduling that would occur for these processes using Rate-Monotonic