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ser334_unit11_exercise_sample.lyx
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Arizona State University
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SER334: Operating Systems & System Programming
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\noindent
Lecturer Acuña (10), UGTA Bahremand (4), UGTA Alvaran (1)
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Revised 4/18/2022
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Unit 11 Sample Problems - Virtual Memory
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\begin_inset Formula $\;$
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(SOLN)
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In this exercise, we will review the concepts of virtual memory.
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Length: 50 minutes with discussion.
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Questions: Q1, Q3-Q4, Q6-Q10 (optional: Q2, Q5, Q11-Q14)
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\begin_layout Section
Background
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\begin_layout Enumerate
[Acuña] What is a big advantage of using virtual memory over main memory
paging? [2 points]
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Ans: [Acuña]
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(Multiple answers are possible.)
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It completely decouples logical memory from a specific physical device (i.e.
RAM) by providing an extra layer of redirection/abstraction.
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\begin_layout Enumerate
[Acuña] For the idea of
\emph on
paging
\emph default
that was used to previously implement main memory, we had a single page
table.
Does virtual memory require adding an entirely new table? [2 points]
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Ans: [Acuña]
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Conceptually, no.
Moving to a paging system simply means that we will need to store more
information.
The table will probably have extra columns to specify additional information
about where it exists on secondary storage (address, swap file name, drive
letter, etc).
\begin_inset VSpace 2cm
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\begin_layout Enumerate
[Acuña] Consider the following diagram:
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Overview of a typical virtual memory system.
(Diagram from Operating System Concepts by Silberschatz, Galvin, and Gagne.)
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Do the arrows represent addresses/pointers? Explain.
[2 points]
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Ans: [Acuña]
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No.
Notice that, for example, there is an arrow from the hard drive to the
physical memory - this would imply that at some point the hard drive would
be storing addresses.
This simply doesn't make sense in our model.
Instead, they represent the flow of data between parts of a virtual memory
system.
\end_layout
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\begin_layout Section
\series bold
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Demand Paging
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\end_deeper
\begin_layout Enumerate
[Acuña] Consider two programs: 1) A compression program that supports many
compression formats.
2) Microsoft Paint.
Which is more likely to have a higher relative number of pages in physical
memory?
\series bold
Explain.
\series default
Assume both are the same size, and have finished loading but have not been
used to do work.
[2 points]
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Ans: [Acuña]
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The Paint program.
Almost all of the functionality of the program is immediately available
to the user and is likely to be used.
Almost all the pages that store the program must be loaded to make sure
all functionality is available.
The compression program is likely to have much fewer because it supports
many compression formats.
Typically only one format is really loaded (the default such as ZIP or
RAR), and using the others requires the user to specific select it.
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\begin_layout Enumerate
[Bahremand]
\emph on
Pure swapping
\emph default
is the procedure of copying an entire process from main memory to secondary
storage.
What are the advantages and disadvantages for using
\emph on
demand paging
\emph default
over pure swapping? (Hint: page-replacement is used in both demand paging
and pure swapping.) [2 points]
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Ans: [Bahremand]
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With demand paging, the operating system copies a disk page into physical
memory only if an attempt is made to access it and that page is not already
in memory; an example instance being a page fault.
\end_layout
\begin_layout Standard
The advantage of using demand paging is that, unlike pure swapping, only
the pages needed are loaded into memory.
This allows for faster process execution as excess memory isn't allocated
for unused memory addresses.
Whereas with pure swapping, all memory for a process is swapped from secondary
storage to main memory during the process startup.
\end_layout
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The disadvantage of using demand paging arises when the scenario involves
continuously having to forcefully swap processes in and out.
The concept of lazy loading technique applies to this situation.
Processes are swapped from secondary storage to main memory repeatedly,
and this can halt execution time as we keep waiting for processes to be
loaded.
With pure swapping, everything is loaded during process startup so we have
less delay in execution.
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\begin_layout Enumerate
[Acuña] Is it possible for a virtual memory system to have zero page faults?
If so,
\series bold
explain
\series default
when.
[2 points]
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Ans: [Acuña]
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No.
Initially RAM stores nothing so the the first time any piece of a data
is needed, a page fault will be triggered.
Even loading the virtual memory system itself will cause a page fault!
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\begin_layout Enumerate
[Acuña] You are given a choice of using an array or list to store a collection
of elements.
Which is likely to result in fewer page faults?
\series bold
Explain.
\series default
[2 points]
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Ans: [Acuña]
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An array will result in fewer page faults.
Considering the following: a page contains multiple pieces of data.
In an array of integers, which is stored contiguously, this means that
a page will contain data for multiple indices.
So, if we have multiple accesses occurring, then one access will load a
page, which may then be used by future accesses (which won't require another
page fault because they can use the page already loaded).
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\series bold
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Copy-On-Write
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\begin_layout Enumerate
[Bahremand] Under Copy-On-Write (CoW), a new private copy is created only
when a forked process actually writes to memory.
\series bold
Describe
\series default
an example of CoW with respect to system calls other than fork and vfork.
[2 points]
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Ans: [Acuña]
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Consider copying files to an external device like a USB stick.
Sometimes the OS will show the file as instantly being copied, despite
the fact that they are very large.
This is a COW operation.
Only when the USB stick is ejected, does the operating system carry out
the actual write operation.
If a user later decides not to copy a file, this approach saves time because
the file won't need to be written.
This also saves write cycles on the device.
(In general, this may be seen as a lazy approach to file IO.)
\end_layout
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\begin_layout Section
\series bold
\size larger
Page Replacement
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\begin_layout Enumerate
[Acuña] Consider the following reference string for page lookups: 4, 1,
6, 3, 1, 4, 2, 1.
Compute the number of page faults that would occur with FIFO and give a
trace of the cache, assuming 3 pages may be kept in memory.
[2 point]
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Ans: [Acuña]
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