COMP SCI 3004/7064 – Operating Systems Assignment 2
Important Notes
• Handins:
– The deadline for submission of your assignment is 23:30pm, 28ththOOctc,t,22001199.
– For undergraduate students, you may do this assignment as a team of two stu- dents and hand in one submission per team.
– For postgraduate students, you have to do this assignment individually and make individual submissions.
– All implementations have to be done in C++.
– You need to submit your source code using the web submission system. You should
attach you and your partner’s name and student number in your submission.
– Late submissions will attract a penalty: the maximum mark you can obtain will be reduced by 25% per day (or part thereof) past the due date or any extension you are granted.
• Marking scheme: 20 marks for online testing in total:
– 4 marks for FIFO (First In First Out) implementation (2 random cases, automarked)
– 4marksforLRU(LeastRecentlyUsed)implementation(2randomcases,automarked)
– 6 marks for ARB (Additional Reference Bits) implementation (3 random cases, au- tomarked)
– 6 marks for WSARB (Additional Reference Bits) implementation (3 random cases, automarked)
If you have any questions, please send them to the student discussion forum. This way you can all help each other and everyone gets to see the answers.
The assignment Introduction
The aim of this assignment is to improve your learning experience in the page replacement algorithms. Following our discussion of paging in lectures, this practical will allow you to explore how real applications respond to a variety of page replacement schemes. Since modifying a real operating system to use different page replacement algorithms can be quite a technical exercise, your task will be to implement a program that simulates the behaviour of a memory system using a variety of paging schemes.
1
Memory Traces
We provide you with some memory traces to assist you developing your simulator.
Each trace is a series of lines, each listing a hexadecimal memory address preceded by R or W to indicate a read or a write. There are also lines throughout the trace starting with a followed by a process ID and command. For example, a trace file for gcc might start like this: # /usr/bin/gcc
R 0041f7a0
R 13f5e2c0
R 05e78900
R 004758a0
W 31348900
The lines in the trace file beginning with # .
2
Simulator Requirements
Your task is to write a simulator that reads a memory trace and simulates the action of a virtual memory system with a single level page table.
Your memory system should keep track of which pages are loaded into memory.
• As it processes each memory event from the trace, it should check to see if the correspond- ing page is loaded.
• If not, it should choose a victim page in memory to replace.
• If the victim page to be replaced is dirty, it must be saved to disk before replacement.
• Finally, the new page is loaded into memory from disk, and the page table is updated.
This is just a simulation of the page table, so you do not actually need to read and write data from disk. When a simulated disk read or write occurs, simply increment a counter to keep track of disk reads and writes, respectively.
You must implement the following page replacement algorithms:
• FIFO: Replace the page that has been resident in memory longest.
• LRU: Replace the page that has been resident in memory oldest.
• ARB: Use multiple reference bits to approximate LRU. Your implementation should use an a-bit shift register and the given regular interval b. See textbook section 9.4.5.1.
– If two page’s ARB are equal, you should use FIFO (longest) to replace the frame.
• WSARB: Combine the ARB page replacement algorithm with the working set model that keeps track of the page reference frequencies over a given window size (page references) for each process (see textbook section 9.6.2). It works as follows:
– Associate each page with reference bits as the shift register (R) to track the reference pattern over the recent time intervals (for a given time interval length), and an integer counter (C) of 5 bits to record the reference frequency of pages (in the memory frames) in the current working set window. R and C are initialized to 0.
– Page replacement is done by first selecting the victim page of the smallest reference frequency (the smallest value of C), and then selecting the page that has the smallest reference pattern value in the ARB shift register (the smallest value of R).
3
Test Arguments
Your code will be compiled using following order in your SVN folder. g++ -std=c++11 PageReplacement.cpp -o PageReplacement
The simulator should accept arguments as follows:
1. The filename of the trace file
2. The page/frame size in bytes (we recommend you use 4096 bytes when testing). 3. The number of page frames in the simulated memory.
The trace provided should be opened and read as a file, **not** parsed as text input from stdin.
For example, your code might be run like this:
“‘ ./PageReplacement input.txt 4096 32 FIFO “‘ Where:
• The program being run is ‘./PageReplacement‘, • The name of the input file is ‘input.txt‘,
• A page is ‘4096‘ bytes,
• There are ‘32‘ frames in physical memory,
• The page replacement algorithm to use: FIFO / LRU / ARB / WSARB
If the page replacement algorithm is ARB , it should accept the following additional argument:
The number of a, the shift register (R) uses an a-bit shift register (1 ≤ a ≤ 8). The number of b, use an the regular interval b(1 ≤ b ≤ 10).
If the page replacement algorithm is WSARB, it should accept the following additional argument:
The number of a, the shift register (R) uses an a-bit shift register (1 ≤ a ≤ 8). The number of b, use an given regular interval b(1 ≤ b ≤ 10).
The size of the working set window δ, in page references(1 ≤ δ ≤20).
For example, your code might be run like this:
“‘
./PageReplacement input.txt 1024 16 ARB 3 3
“‘
“‘
./PageReplacement input.txt 4096 32 WSARB 8 3 11 “‘
4
Input
We will provide you with a selection of memory traces to assist you developing your sim- ulator. These will be a mix of specific test cases and real traces from running systems. Each trace is a series of lines, containing two(2) values that represent memory accesses:
1. A character ‘R‘ or ‘W‘ that represents whether the memory access is a Read or Write respectively.
2. A hexadecimal memory address.
A trace may also contain comment lines, followed by a process Name.
An example of a trace:
# chrome
R 0041f7a0 R 13f5e2c0 R 05e78900 R 004758a0 W 31348900
5
Output
The simulator should run silently with no output until the very end, at which point it prints out a summary like this:
events in trace: 1025 total disk reads: 151 total disk writes: 92 page faults: 151
Where:
• * **events in trace** is the number of memory access’ in the trace. Should be equal to number of lines in the trace file that start with R or W. Lines starting with do not count.
• * **total disk reads** is the number of times pages have to be read from disk.
• * **total disk writes** is the number of times pages have to be written back to disk.
• * **page faults** is the number of disk reads in a demand paging system. It may be same with total disk reads in this problem.
We will provide a set of expected outputs(on web-submission) to match the given memory traces.
6
Web-submission instructions
• First, type the following command, all on one line (replacing xxxxxxx with your student ID):
svn mkdir –parents -m ”OS” https://version-control.adelaide.edu.au/svn/axxxxxxx/2019/s2/os/assignment2
• Then, check out this directory and add your files:
svn co https://version-control.adelaide.edu.au/svn/axxxxxxx/2019/s2/os/assignment2 cd assignment2
svn add TicketBooker.cpp
···
svn commit -m ”assignment2 solution”
• Next, go to the web submission system at: https://cs.adelaide.edu.au/services/websubmission/
Navigate to 2019, Semester 2, Operating Systems, Assignment 1. Then, click Tab ”Make Submission” for this assignment and indicate that you agree to the declaration. The automark script will then check whether your code compiles. You can make as many resubmissions as you like. If your final solution does not compile you won’t get any marks for this solution.
• We will test your codes by the following Linux commands: g++ -std=c++11 PageReplacement.cpp -o PageReplacement ./PageReplacement input.txt 4096 32 FIFO >output.txt
7
FAQ
1. How are the page numbers and memory addresses connected? Is there a relation to page size?
This comes down to the fundamentals of paging. Be sure to read and understand section 8.5.1 of the text book before going any further. It will save you much confusion later on.
2. I’m confused by the ARB algorithm. How is it actually supposed to work?
For those of you Googling along at home you may have come across a variety of somewhat conflicting descriptions of this algorithm. For reference, we’re strictly following this one outlined in the book:
Figure 1: ARB
And we use a-bit shift register (R), instead of 8-bit shift register in book.
• That trace was run on a 64-bit system which uses a larger address space. If you’re converting these values to numbers, you may need to use the long long data type.
• In a system running this algorithm, there should never be a case where the inter- val/quantum for shifting the bits is greater than the number of frames. That behavior is undefined and we do not expect you to code for it.
• I’m confused by the working set model. How is it actually supposed to work? Please refer to 9.6.2 of pages 427-429 of the text.
8
For Working Set ARB. Some extra notes:
• We are concerned only with single-process memory allocation, so each input reference string is only for the same process.
• Ensure you keep track of the process’ working set over the given time window on the input reference string.
• All page faults should be recorded.
• The working set window δ is NOT linked to ARB shift interval b.
• For WSARB, assume that the number of frames allocated to the process is greater or equal to the size of its working set window.
• The shift register (R) and reference frequency counter (C) only count the pages which are in memory frames.
For example:
Time: 1 2 3 4 5 6 7 8 9 10…
requestpages: 2134524612…
frames size=4;
window size=4; Atendoftime4,fourframesareinatableinmemory: 2134. Time: 12 3 4 5 678910…
requestpages: 21 3 4 5 24612…
At time 5, a new page 5 need into the frames. And the current working set window is 1 3 4 5. The 5-bit reference frequency counter (C) will find that the frequency of page 2 is zero (the smallest value of C) in the current working set, and then the new page 5 will replace page 2 in the memory table.
9