Virtual Memory Quiz - Chapters 20-22

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Questions and Answers

What advantage does FIFO page replacement take advantage of?

Instructions execute sequentially most of the time. (correct)

Programs make frequent distant past references.

Branching is a major part of most programs.

Programs often execute instructions randomly.

Which page replacement algorithm replaces the page that will not be used for the longest period of time?

Optimal Page Replacement (correct)

First In First Out (FIFO)

Least Recently Used (LRU)

Random Page Replacement

What is a main drawback of FIFO page replacement?

It requires complex implementation.

It leads to more page faults in all scenarios.

It fails to take locality into account.

It may replace pages that are still frequently accessed. (correct)

How does the Least Recently Used (LRU) replacement algorithm determine which page to remove?

By removing the page that has not been referenced for the longest time. Signup and view all the answers

What is the purpose of implementing approximate algorithms like LRU?

To mimic the optimal behavior when the true optimal algorithm cannot be implemented. Signup and view all the answers

What is the outcome when a process exceeds its Resident Set (RS) during a page fault?

A page is removed and placed in a clean or dirty list. Signup and view all the answers

Why is FIFO considered easy to implement?

It does not require any additional hardware, including reference bits. Signup and view all the answers

What occurs in a page buffer when a page is modified and needs to be removed?

The page is marked as 'dirty' in the modified page list. Signup and view all the answers

Which of the following statements characterizes LRU algorithms?

They require a high overhead for implementation and some hardware aid. Signup and view all the answers

What is a significant improvement suggested for FIFO page replacement policy?

Introducing a second chance policy. Signup and view all the answers

What is the primary characteristic of the Least Recently Used (LRU) page replacement algorithm?

Pages are replaced based on the least recently referenced page. Signup and view all the answers

In the context of LRU, what happens when a non-resident page p is referenced?

Page p is moved to the end of the queue and the least recently referenced page is removed. Signup and view all the answers

What strategy is typically employed to approximate the LRU replacement?

Implementing a time reference system to track usage. Signup and view all the answers

How many page faults occurred in the example reference string using the LRU algorithm?

12 Signup and view all the answers

Which method yields the fewest page faults compared to the LRU algorithm in the provided data?

Optimal page replacement method. Signup and view all the answers

What role does the Page Table serve in relation to the LRU algorithm?

It connects virtual addresses to physical addresses. Signup and view all the answers

Which of the following is NOT used in the process of LRU page replacement?

Total reference count for each page. Signup and view all the answers

What does a Present bit value of P==0 indicate?

Page is not in memory and invalid. Signup and view all the answers

What action is taken if the Writable bit W==0?

Page is read-only and cannot be written. Signup and view all the answers

In the context of the Second-Chance Page-Replacement Algorithm, what happens if a page has a reference bit of 1?

The page remains in memory and its reference bit is set to 0. Signup and view all the answers

What prompts the application of page replacement?

A page fault occurs and no free frames are available. Signup and view all the answers

What is the purpose of the Accessed bit A?

Records whether the page has been referenced or not. Signup and view all the answers

Which algorithm uses a circular list of pages to manage memory?

Clock Algorithm Signup and view all the answers

Which situation leads to a page being marked as Dirty?

The page has been modified since it was loaded. Signup and view all the answers

What does the Enhanced 2nd Chance algorithm prioritize for replacement?

Both referenced and modified bits. Signup and view all the answers

When is 'second chance' given to a page in the Clock replacement algorithm?

When its reference bit is 1. Signup and view all the answers

What is the main function of the 'Dirty' bit in a page table entry?

To show if a page has been modified since being loaded. Signup and view all the answers

Study Notes

Lecture Agenda - November 19

Reminders:
- Read Chapters 20-22 (Virtual Memory Module 8)
- Project 4 Demo due Wednesday, November 20 (Driving Simulation using threads and concurrency)
- Project 5 Deliverable 3 due November 20 (Implementing a Simple File System)
- Quiz tomorrow
- Attendance required

Topics Covered Last Time

Demand Paging
Page Replacement
FIFO (First-In, First-Out)
Optimal Algorithm

Topics Covered Today

Page Replacement Algorithms
- FIFO
- Optimal (MIN)
- LRU (Least Recently Used)
- LRU Approximations
Policies for managing memory allocation
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FIFO Page Replacement

Reference String: 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
Page Frames: Specific example table showing page replacements with 15 faults.

FIFO Algorithm Details

Easy to implement using single pointer to oldest page in memory.
When a page is replaced, advance pointer.
Locality Principle: A positive in FIFO. Most instructions execute sequentially. Likelihood of referencing a page from the distant past decreases with time. Large data structures often referenced sequentially.

Optimal (MIN) Algorithm

Replaces page that won't be used for longest period of time.
Difficult to implement in real time: Requires future knowledge of page references.
4 frames, example: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5, resulting in 6 page faults.

LRU (Least Recently Used) Algorithm

Replaces the least recently used page.
Implementation:
- Requires a queue (ordered list) containing page numbers.
- When a page is referenced, move it to back (end) of the queue.
- Queue always ordered (most recently used to least recently used).
- If non-resident page referenced, remove page at front of queue and place new page at the end.
Example with 4 frames: A specific example demonstrating the replacement process with 12 page faults.

LRU Implementation Details Including Alternatives

Detailed description of how LRU is implemented using a queue.
Alternative Method: Discusses using a counter to track page reference times.
- Every page entry has a related time reference
- When a page is referenced, copy the clock time to the PTE's time reference field.
- When a page needs replacing, check time references to select appropriate page.

How to Approximate LRU

Uses information collected as a process executes to approximate LRU behaviour.
Page tables: Connection between virtual addresses and physical addresses is a significant tool.

Page Table Bits

Specific PTE bits relevant for LRU algorithms (example Intel PTE)
- Present (Valid) bit
- Writeable bit
- Accessed/Used bit
- Dirty/Modified bit

When to Apply Page Replacement

Page fault occurs (no free frames in memory)
Algorithm finds page to evict.
Keeps frequently used pages in memory.

Clock Algorithms

Use Page Table Entry (PTE) bits & a circular queue of pages.
Pointer around like a clock.
Only do work at page fault time.

Second Chance Algorithm

Requires reference bit per page and organized in an ordered page list (Clock Algorithm).
If page needs replacement, and reference bit = 1, set bit to 0 and leave in memory.

Enhanced 2nd Chance

Priority-based replacement: Uses referenced and modified bits.
Different priority levels: Distinct categories based on usage. (ex. 00 least used to 11 most used)

Page Buffering

LRU algorithms perform well in memory minimizing page faults but high implementation overhead and hardware required.
FIFO is simple but it doesn't consider page reference frequency.
Ways to improve FIFO: Adding a 2nd chance policy or similar.

Page Buffering, Including Variable Allocation

Process gets maximum number of frames (Resident Set).
FIFO list is created to efficiently manage frames.
Page Fault: An evicted page is put on a clean or modified list managed by the OS.
Example of variable allocation: An excerpt from the document explaining the variable allocation (also discussed in more detail in Resident Set Management).

Fetch Policy

Demand Paging: Only brings pages into memory when referenced, many page faults at beginning. Principle of locality; more references to recently loaded pages means fewer faults.
Prepaging: Brings pages into memory that might be referenced following a page fault. Exploits the characteristic that sequential pages are loaded in secondary storage together, improves load speed.

Resident Set Management

How many pages to keep in main memory for a given process.
Trade-offs: Smaller allocation means more processes can be held but more faults.
Certain size, beyond adding more frames, won't improve fault rate.

Policies for Resident Set Size

Fixed allocation: Assigns fixed number of frames to the process.
Variable allocation: The number of page frames assigned to the processes changes over time, usually to try and hold the working set (important pages).

Replacement Scope

Global: The replacement strategy considers all unlocked pages in main memory.
Local: Only looks at resident set pages related to the process that has experienced the fault.

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Description

Test your understanding of Virtual Memory concepts as outlined in Chapters 20-22. This quiz covers page replacement algorithms, specifically FIFO, Optimal, and LRU. Make sure to review the examples and algorithms discussed in class to prepare effectively.