# Comparing Page Replacement Algorithms via Simulation in Python

### tl;dr

While studying Operating Systems, I decided to experiment with 4 of the most common Page Replacement algorithms, via simulation, using Python. Input data for my simulations were two memory traces named “swim.trace” and “gcc.trace” (provided by my CS department). All algorithms are run within a page table implemented for a 32-bit address space; all pages in this page table are 4kb in size.

I chose to implement these algorithms and the page table in Python. The final source code can be found here, on Github. The full data set that I collected can be found at the end of this document. Illustrative graphs are interspersed throughout, wherever they are necessary for explaining and documenting decisions.

In this essay itself, I will first outline the algorithms implemented, noting design decisions I made during my implementations. I will then compare and contrast the results of each algorithm when run on the two provided memory traces. Finally, I will conclude with my decision about which algorithm would be best to run in a real Operating System.

Please note, to run the algorithms themselves, you must run:

• python vmsim.py –n -a <opt|clock|aging|lru> [-r ]

### The Algorithms

The algorithms we have been asked to implement are:

• Opt– Simulate what the optimal page replacement algorithm would choose if it had perfect knowledge
• EXAMPLE RUN: python vmsim.py –n 8 –a opt gcc.trace
• Clock– Use the better implementation of the second-chance algorithm
• EXAMPLE RUN: python vmsim.py –n 16 –a clock swim.trace
• Aging– Implement the aging algorithm that approximates LRU with an 8-bit counter
• EXAMPLE RUN: python vmsim.py –n 32 –a aging –r 1 gcc.trace
• LRU– Do exact LRU.
• EXAMPLE RUN: python vmsim.py –n 64 –a lru swim.trace

### Implementation Notes

The main entry point for the program is the file named vmsim.py. This is the file which must be invoked from the command line to run the algorithms. Because this is a python file, it must be called with “python vmsim.py … etc.”, instead of “./vmsim”, as a C program would. Please make sure the selected trace file is in the same directory as vmsim.py.

All algorithms run within a page table, the implementation of which can be found in the file named pageTable.py.

Upon program start, the trace file is parsed by the class in the file parseInput.py, and each memory lookup is stored in a list of tuples, in this format: [(MEM_ADDRESS_00, R/W), [(MEM_ADDRESS_01, R/W), … [(MEM_ADDRESS_N, R/W)]. This list is then passed to whichever algorithm is invoked, so it can run the chosen algorithm on the items in the list, element by element.

The OPT algorithm can be found in the file opt.py. My implementation of OPT works by first preprocessing all of the memory addresses in our Trace, and creating a HashTable where the key is our VPN, and the value is a python list containing each of the address numbers at which those VPNs are loaded. Each time a VPN is loaded, the element at index 0 in that list is discarded. This way, we only have to iterate through the full trace once, and from there on out we just need to hash into a list and take the next element, whenever we want to know how far into the future that VPN is next used.

The Clock Algorithm is implemented in the files clock.py and circularQueue.py. My implementation uses the second chance algorithm with a Circular Queue. Of importance: whenever we need to make an eviction, but fail to find ANY pages that are clean, we then run a ‘swap daemon’ which writes out ALL dirty pages to disk at that time. This helps me get fewer page faults, at the expense of more disk writes. That’s a calculated decision for this particular algorithm, since the project description says we should use page faults as our judgment criterion for each algorithm’s effectiveness.

The LRU Algorithm can be found in the file lru.py. For LRU, each time a page is Read, I mark the memory address number at which this happens in the frame itself. Then, in the future—whenever I need to make an eviction—I have easy access to see which frame was used the longest time in the past, and no difficult calculations are needed. This was the simplest algorithm to implement.

The Aging Algorithm is likely the most complex, and its source code can be found in the file named aging.py. Aging works by keeping an 8 bit counter and marking whether each page in the page table was used during the last ‘tick’ a time period of evaluation which must be passed in by the user as a ‘refresh rate’, whenever the Aging Algorithm is selected. All refresh rates are in milliseconds on my system, but this relies on the implementation of Python’s “time” module, so it’s possible that this could vary on other systems. For aging, I suggest a refresh rate of 0.01 milliseconds, passed in on the command line as “-r 0.01”, in the 2nd to last position in the arg list. This minimizes the values in my testing, and going lower does not positively affect anything. In the next section, I will show my rationale for selecting 0.01 as my refresh rate.

### Aging Algorithm Refresh Rate

In order to find a refresh rate that would work well, I decided to start at 1ms and move 5 orders of magnitude in each direction, from 0.00001ms to 100000ms.

To ensure that the results were not biased toward being optimized for a single trace, I tried both to confirm that the refresh rate would work will for all inputs.

The graphs below show the Page Faults and Disk Writes I found during each test.

For all tests, I chose a frame size of 8, since this small frame size is most sensitive to the algorithm used. At higher frame sizes, all of the algorithms tend to perform better, across the board. So I wanted to focus on testing at the smallest possible size, preparing for a ‘worst case’ scenario.

### Aging Algorithm – Page Faults Over Refresh Rate

X-axis:   Refresh rate in milliseconds

Y-axis:   Total Page faults

GCC. TRACE reaches its minimum for page faults at 0.0001ms and SWIM.TRACE reaches its own minimum at 0.01ms. The lines cross at around 0.01ms.

### Total Page Faults For SWIM.TRACE – Detail

X-axis: Page Faults

Y-Axis: Refresh Rates

X-axis: Page Faults

Y-Axis: Refresh Rates

Additionally, 0.01ms seems to achieve the best balance, in my opinion, if we need to select a single time for BOTH algorithms.

### Aging Algorithm – Disk Writes Over Refresh Rate

X-axis:   Refresh rate in milliseconds

Y-axis:   Total Disk Writes

The number of disk writes also bottoms out at 0.01ms from SWIM.TRACE. It is relatively constant for GCC.TRACE across all of the different timing options.

Because of this, I suggest 0.01ms as the ideal refresh rate. This is because it is optimal for SWIM.TRACE. For GCC.TRACE, it is not the absolute best option, but it is still acceptable, and so I think this selection will achieve a good balance.

### Results & Decisions

With the algorithms all implemented, my next step was to collect data for each algorithm at all frame sizes, 8, 16, 32, and 64. OPT always performed best, and thus it was used as our baseline.

In the graphs below, I show how each algorithm performed, both in terms of total page faults and total disk writes.

### SWIM.TRACE – Page Faults Over Frame Size

X-axis:   Frame Size

Y-axis:   Page Faults

Data for all algorithms processing swim.trace

### SWIM.TRACE – Disk Writes Over Frame Size

X-axis:   Frame Size

Y-axis:   Disk Writes

Data for all algorithms processing swim.trace

### GCC.TRACE – Page Faults Over Frame Size

X-axis:   Frame Size

Y-axis:   Page Faults

Data for all algorithms processing gcc.trace

### GCC.TRACE – Disk Writes Over Frame Size

X-axis:   Frame Size

Y-axis:   Disk Writes

Data for all algorithms processing gcc.trace

Given this data, I was next tasked with choosing which algorithm is most appropriate for an actual operating system.

In order to determine which algorithm would be best, I decided to use an algorithm. I’ll call it the ‘Decision Matrix’, and here are the steps.

DECISION MATRIX:

1. RANK ALGORITHMS FOR EACH CATEGORY FROM 1=BEST to 4=WORST,
2. SELECT ALGORITHM WITH LOWEST OVERALL SCORE
 ALGORITHM SWIM – Page Faults SWIM – Disk Writes GCC – Page Faults GCC – Disk Writes TOTAL (Lowest is Best) OPT 1 1 1 1 4 CLOCK 2 4 2 4 12 AGING 4 2 4 2 12 LRU 3 3 3 3 12

Ranking:             1=Best;

4=Worst

NOTE:

• Wherever lines cross each other in our graphs, the algorithm ranked as “better” is the one with MORE total low data points. Ties were broken by my own personal judgment.

### Decision:

• Of course, using this set of judgment criteria, OPT is by far the winner.
• However OPT isn’t an option in a real OS, since it requires perfect knowledge of the future, which is impossible in practice.
• So we want to pick between the 3-way tie for CLOCK and LRU and AGING.
• Of these three:
• AGING does well on disk writes, but generally has the most page faults.
• Conversely, CLOCK does well on page faults, but often has the most disk writes.
• LRU achieves a balance.
• Therefore, My pick goes to LRU, because where clock does beat LRU, on page faults it does so only by a narrow margin.
• But where LRU beats clock—on disk writes—it does so by a large amount. This means LRU is better overall, if both page faults and disk writes are equally weighted for judgment purposes.

Therefore, I would select LRU for my own operating system.

### Data Set

Figure 1.1 – Full Data Set

 ALGORITHM NUMBER OF FRAMES TOTAL MEMORY ACCESSES TOTAL PAGE FAULTS TOTAL WRITES TO DISK TRACE REFRESH RATE OPT 8 1000000 236350 51162 swim.trace N/A OPT 16 1000000 127252 27503 swim.trace N/A OPT 32 1000000 52176 11706 swim.trace N/A OPT 64 1000000 24344 6316 swim.trace N/A OPT 8 1000000 169669 29609 gcc.trace N/A OPT 16 1000000 118226 20257 gcc.trace N/A OPT 32 1000000 83827 14159 gcc.trace N/A OPT 64 1000000 58468 9916 gcc.trace N/A CLOCK 8 1000000 265691 55664 swim.trace N/A CLOCK 16 1000000 136154 52104 swim.trace N/A CLOCK 32 1000000 73924 45872 swim.trace N/A CLOCK 64 1000000 56974 43965 swim.trace N/A CLOCK 8 1000000 178111 38992 gcc.trace N/A CLOCK 16 1000000 122579 26633 gcc.trace N/A CLOCK 32 1000000 88457 20193 gcc.trace N/A CLOCK 64 1000000 61832 15840 gcc.trace N/A AGING 8 1000000 257952 52664 swim.trace 0.01ms AGING 16 1000000 143989 41902 swim.trace 0.01ms AGING 32 1000000 91852 29993 swim.trace 0.01ms AGING 64 1000000 82288 27601 swim.trace 0.01ms AGING 8 1000000 244951 31227 gcc.trace 0.01ms AGING 16 1000000 187385 22721 gcc.trace 0.01ms AGING 32 1000000 161117 19519 gcc.trace 0.01ms AGING 64 1000000 149414 16800 gcc.trace 0.01ms LRU 8 1000000 274323 55138 swim.trace N/A LRU 16 1000000 143477 47598 swim.trace N/A LRU 32 1000000 75235 43950 swim.trace N/A LRU 64 1000000 57180 43026 swim.trace N/A LRU 8 1000000 181950 37239 gcc.trace N/A LRU 16 1000000 124267 23639 gcc.trace N/A LRU 32 1000000 88992 17107 gcc.trace N/A LRU 64 1000000 63443 13702 gcc.trace N/A

Figure 1.2 – GCC.TRACE – Refresh Rate Testing – 8 Frames

 ALGORITHM NUMBER OF FRAMES TOTAL MEMORY ACCESSES TOTAL PAGE FAULTS TOTAL WRITES TO DISK TRACE REFRESH RATE AGING 8 1000000 192916 33295 gcc.trace 0.00001ms AGING 8 1000000 192238 33176 gcc.trace 0.0001ms AGING 8 1000000 197848 31375 gcc.trace 0.001ms AGING 8 1000000 244951 31227 gcc.trace 0.01ms AGING 8 1000000 339636 140763 gcc.trace 0.1ms

Figure 1.3 –SWIM.TRACE – Refresh Rate Testing – 8 Frames

 ALGORITHM NUMBER OF FRAMES TOTAL MEMORY ACCESSES TOTAL PAGE FAULTS TOTAL WRITES TO DISK TRACE REFRESH RATE AGING 8 1000000 275329 53883 swim.trace 0.00001ms AGING 8 1000000 274915 53882 swim.trace 0.0001ms AGING 8 1000000 268775 53540 swim.trace 0.001ms AGING 8 1000000 257952 52664 swim.trace 0.01ms AGING 8 1000000 278471 56527 swim.trace 0.1ms