Description

CSCI 3753 – Operating Systems
Spring 2012
By Andy Sayler and Junho Ahn and Richard Han
1 Assignment Introduction
All modern operating systems use virtual memory and paging in order to effectively utilize the computer’s memory hierarchy. Paging is an effective means of providing memory space protection to processes, of enabling the system to utilize secondary storage for additional memory space, and of avoiding the need to allocate memory sequentially for each process.
We have studied how virtual memory systems are structured and how the MMU converts virtual memory addresses to physical memory addresses by means of a page table and a translation lookaside buffer (TLB). When a page has a valid mapping from VM address to physical address, we say the page is “swapped in”. When no valid mapping is available, the page is either invalid (a segmentation fault), or more likely, “swapped out”. When the MMU determines that a memory request requires access to a page that is currently swapped out, it calls the operating system’s page-fault handler. This handler must swap-in the necessary page, possibly evicting another page to secondary memory in the process. It then retries the offending memory access and hands control back to the MMU.
As you might imagine, how the OS chooses which page to evict when it has reached the limit of available physical pages (sometime called frames) can have a major effect on the performance of the memory access on a given system. In this assignment, we will look at various strategies for managing the system page table and controlling when pages are paged in and when they are paged out.
2 Your Task
The goal of this assignment is to implement a paging strategy that maximizes the performance of the memory access in a set of predefined programs. You will accomplish this task by using a paging simulator that has already been created for you. Your job is to write the paging strategy that the simulator utilizes (roughly equivalent to the role the page fault handler plays in a real OS). Your initial goal will be to create a Least Recently Used paging implementation. You will then need to implement some form of predictive page algorithm to increase the performance of your solution. You will be graded on the throughput of your solution (the ratio of time spent doing useful work vs time spent waiting on the necessary paging to occur).
2.1 The Simulator Environment
The simulator has been provided for you. You have access to the source code if you wish to review it (simulator.c and simulator.h), but you should not need to modify this code for the sake of this assignment. You will be graded using the stock simulator, so any enhancements to the simulator program made with the intention of improving your performance will be for naught.
The values of the constants mentioned above are available in the simulator.h file. For the purposes of grading your assignment, the default values will be used:
• 20 virtual pages per process (MAXPROCPAGES)
• 100 physical pages (frames) total (PHYSICALPAGES)
• 20 simultaneous processes competing for pages (MAXPROCESSES)
• 128 memory unit page size (PAGESIZE)
• 100 tick delay to swap a page in or out (PAGEWAIT)
In addition, swapping a page in or out is an expensive operation, requiring 100 ticks to complete. A tick is the minimum time measurement unit in the simulator. Each instruction or step in the simulated programs requires 1 tick to complete. Thus, in the worst case where every instruction is a page miss (requiring a swap-in), you will spend 100 ticks of paging overhead for every 1 tick of useful work. If all physical pages are in use, this turns into 200 ticks per page miss since you must also spend 100 ticks swapping a page out in order to make room for the required page to be swapped in. This leads to an “overhead to useful work” ratio of 200 to 1: very, very, poor performance. Your goal is to implement a system that does much better than this worst case scenario.
2.2 The Simulator Interface
The simulator exports three functions through which you will interact with it. The first function is called pageit. This is the core paging function. It is roughly equivalent to the page-fault handler in your operating system. The simulator calls pageit anytime something interesting happens (memory access, page fault, process completion, etc). It passes the function a page map for each process, as well as the current value of the program counter for each process. See simulator.h for details. You will implement your paging strategy in the body of this function.
The pageit function is passed an array of pentry structs, one per process. This struct contains a copy of all of the necessary memory information that the simulator maintains for each process. You will need the information contained in this struct to make intelligent paging decisions. It is the simulator’s job to maintain this information. You should just read it as necessary. The struct contains:
long active
A flag indicating whether or not the process has completed. 1 if running, 0 if exited.
long pc
The value of the program counter for the process. The current page can be calculated as page = pc/PAGESIZE.
long npages
The number of pages in the processes memory space. If the process is active (running), this will be equal to MAXPROCPAGES. If the process has exited, this will be 0.
long pages[MAXPROCPAGES]
A bitmap array representing the page map for a given process. If pages[X] is 0, page X is swapped out, swapping out, or swapping in. If pages[X] is 1, page X is currently swapped in.
Figure 1 shows the possible states that a virtual page can occupy, as well as the possible transitions between these states. Note that the page map values alone do not define all

Figure 1: Possible Page States and Transitions
possible page states. We must also account for the possible operations currently underway on a page to fully define its state. While the page map for each process can be obtained from the pageit input array of structs, there is no interface to directly reveal any operations underway on a given page. If knowing whether or not a paging operation is underway on a given page (and thus knowing the full state of a page) is necessary for your pageit implementation, you must maintain this data yourself.
2.3 The Simulated Programs
The simulator populates its 20 processes by randomly selecting processes from a collection of 5 simulated “programs”. Pseudo code for each of the possible 5 programs is provided in
Listings 1 through 5.
1# loop with inner branch
2for 10 30
3run 500
4if .4
5run 900
6else
7run 131
8endif
9end 10exit
11endprog
Listing 1: Test Program 1 – A loop with an inner branch
2
3
4
5
Listing 2: Test Program 2 – Single loop
2
3
4
5
6
7
8
Listing 3: Test Program 3 – Double Nested Loop
2
3
Listing 4: Test Program 4 – Linear
2
4
5
6
7
8
9
Listing 5: Test Program 5 – Probabilistic backward branch
This simple pseudo code notation shows you what will happen in each process:
• for X Y: A “for” loop with between X and Y iterations (chosen randomly)
• run Z: Run Z (unspecified) instructions in sequence
• if P: Run next clause with probability P, run else clause (if any) with probability (1-P). • goto label: Jump to “label”
1. Profile processes and know which kind of programs each is an instance of.
2. Use this knowledge to predict what pages a process will need in the future with rather high accuracy.
3 Some Implementation Ideas
In general, your pageit() implementation will need to follow the basic flow presented in Figure 2. You will probably spend most of your time deciding how to implement the “Select a Page to Evict” element.
Simulator calls pageit()

Select a Process

Determine
Current Page
(pc / PAGESIZE)

Figure 2: Basic Reactive pageit() Flow
A basic “one-process-at-a-time” implementation is provided for you. This implementation never actually ends up having to swap out any pages. Since only one process is allocated pages at a time, no more than 20 pages are ever in use. When each process completes, it releases all of its pages and the next process is allowed to allocate pages and run. This is a very simple solution, and as you might expect, does not provide very good performance. Still, it provides a simple starting point that demonstrates the simulator API. See pager-basic.c for more information.
To really do well on this assignment, you must create some form of predictive paging algorithm. A predictive algorithm attempts to predict what pages each process will require in the future and then swaps these pages in before they are needed. Thus, when these pages are needed, they are already swapped in and ready to go. The process need not block to wait for the required pages to be swapped in, greatly increasing performance. . Figure 3 shows a modified version of the Figure 2 flowchart for a predictive implementation of pageit. As for the LRU implementation, a simple predictive stub has been created for you in the pager-predict.c file.

Figure 3: Basic Predictive pageit() Flow
There are effectively two approaches to predictive algorithms. The first approach is
to leverage your knowledge of the possible program types (see previous section). In this approach, one generally attempts to heuristically determine which program each process is an instance of by tracking the movement of the process’s program counter (PC). Once each process is classified, you can use further PC heuristics to determine where in its execution the process is, and then implement a paging strategy that attempts to swap in pages required by upcoming program actions before they occur. Since the programs all have probabilistic elements, this approach will never be perfect, but it can do very well.
The second approach to predictive algorithms is to ignore the knowledge you have been given regarding the various program types. Instead, you might track each process’s program counter to try to detect various common patterns (loops, jumps to specific locations, etc). If you detect a pattern, you assume that the pattern will continue to repeat and attempt to swap in the necessary pages touched during the execution of the pattern before the process needs them. Working set algorithms are a subset of this approach.
Note that in any predictive operation, you ideally wish to stay 100-200 ticks ahead of the execution of each process. This is the necessary predictive lead time in which you must make paging decisions in order to insure that the necessary pages are available when the process reaches them and that no blocking time is required. As Figure 3 shows, in addition to swapping in pages predicatively, you must still handle the case where your prediction has failed and are thus forced to reactively swap in the necessary page. This is referred to as a predictive miss. A good predictive algorithm will minimize misses, but still must handle them when they occur. In other words, you can not assume that your predictions will always works and that every currently needed page is already available. Doing so will most likely lead to deadlock.
There are a number of additional predictive notions that might prove useful involving state-space analysis [4], Markov chains [5], and similar techniques. We will leave such solutions to the student to investigate if she wishes. Please see the references section for additional information and ideas.
4 What’s Included
We provide some code to help get you started. Feel free to use it as a jumping off point
(appropriately cited).
1. Makefile A GNU Make makefile to build all the code listed here.
2. README As the title so eloquently instructs: read it.
3. simulator.c The core simulator source code. For reference only.
4. simulator.h The simulator header file including the simulator API.
5. programs.c Struct representing simulated programs for use by simulator. For reference and use by simulator code only.
6. pager-basic.c A basic paging implementation that only runs one process at a time.
7. pager-lru.c A stub for your LRU paging implementation.
8. pager-predict.c A stub for your predictive paging implementation.
10. test-* Executable test programs. Runs the simulator using your pager-*.c strategy. Built using Makefile. The simulator provides a lot of tools to help you analyze your program. Run ./test-* -help for information on available options. It also responds to various signals by printing the current page table and process execution state to the screen (try ctrl-c while simulator is executing).
11. test-api An API test program. See api-test.c.
12. see.R An R script for displaying a visualization of the process run/block activity in a simulation. You must first run ./test-* -csv to generate the necessary trace files. To run visualization, lunch R in windowed graphics mode (in Linux: R -g Tk & at the command prompt) from the directory containing the trace files (or use setwd to set your working directory to the directory containing the trace files). Then run source(‘‘see.r’’) at the R command prompt to lunch the visualization.
5 What You Must Provide
When you submit your assignment, you must provide the following as a single archive file:
1. A copy of your LRU paging implementation
2. A copy of your best predictive paging implementation
3. A makefile that builds any necessary code
4. A README explaining how to build and run your code
6 Grading
40% of you grade will be based on the performance of the best pager implementation that you provide. The following simulation scores will earn the corresponding number of points:
• Code does not compile without errors : 0 Points
• score >= 1.28 : 5 Points
• 0.64 <= score < 1.28 : 10 Points
• 0.32 <= score < 0.64 : 15 Points (Basic LRU implementation)
• 0.16 <= score < 0.32 : 20 Points
• 0.08 <= score < 0.16 : 25 Points
• 0.04 <= score < 0.08 : 30 Points
• 0.02 <= score < 0.04 : 35 Points
• 0.01 <= score < 0.02 : 40 Points (Good predictive implementation)
• 0.005 <= score < 0.01 : 40 Points + 5 Points EC
• score < 0.005 : 40 Points + 10 Points EC (Excellent predictive implementation)
• Meet all requirements elicited in this document
• Code must adhere to good coding practices.
The other 60% of your grade will be determined via your grading interview where you will be expected to explain your work and answer questions regarding it and any concepts related to this assignment.
7 Obtaining Code
8 Resources
Refer to your textbook and class notes on the Moodle for an overview of OS paging policies and implementations.
If you require a good C language reference, consult K&R[2].
The Internet[3] is also a good resource for finding information related to solving this assignment.
The most recent version of the assignment from which this assignment was adopted is available at [1].
• man 1 make References
[2] Kernighan, Brian and Dennis, Ritchie. The C Programming Language. Second Edition: 2009. Prentice Hall: New Jersey.
[3] Stevens, Ted. Speech on Net Neutrality Bill. 2006. http://youtu.be/f99PcP0aFNE.
[4] Wikipedia. “State space (dynamical system)”. Wikipedia, The Free Encyclopedia.