Operating Systems Chapter on Paging

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Questions and Answers

Which step occurs first when a program references a page not currently in memory?

Find a free frame

Update the page and frame tables

Locate the desired page on disk

Page Fault invoked (correct)

What is the role of the 'small number of extents' method in file space allocation?

To store the link to the associated extent (correct)

To manage the indexing of all file blocks

To eliminate page faults in demand paging

To optimize the memory access time

In the context of virtual addressing and paging, how many memory accesses might a program potentially make to read from a memory cell?

Up to four (correct)

One or two

Two or three

Three or four

Which of the following is a page replacement algorithm mentioned in the lecture?

Least Recently Used (LRU) Signup and view all the answers

What happens if there is no free frame available during a page fault?

A page replacement algorithm is used Signup and view all the answers

What is a potential drawback of using the FIFO page replacement policy?

It can remove pages that are heavily used rather than infrequently used. Signup and view all the answers

Which page replacement policy is considered provably optimal?

MIN Signup and view all the answers

What is a significant challenge of implementing the LRU replacement policy?

It involves maintaining a complex data structure to track page usage. Signup and view all the answers

Why is page replacement policy particularly important in memory management?

To minimize the frequency of page faults and access times. Signup and view all the answers

Which statement best describes the RANDOM page replacement policy?

It selects pages arbitrarily, potentially making access times unpredictable. Signup and view all the answers

What initial action occurs when a page fault is detected?

Registers and active state are saved. Signup and view all the answers

Which factor influences the effective access time in demand paging?

The probability of a page fault. Signup and view all the answers

What is the informed sequence of actions following a page fault?

Save state, initiate read, check legality, restore context. Signup and view all the answers

The effective access time formula includes which of the following terms for a page fault?

The sum of page fault time and two times memory access time. Signup and view all the answers

What event triggers the need for the operating system to take control during a page fault?

An undefined address error occurs. Signup and view all the answers

Which statement correctly describes the probability 'p' in the context of demand paging?

It reflects the likelihood of a page fault occurring. Signup and view all the answers

What is typically included in the page fault handling process?

Waiting for CPU allocation after context restoration. Signup and view all the answers

How does demand paging improve system performance?

By loading only the needed pages into memory. Signup and view all the answers

What is the primary function of the Translation Lookaside Buffer (TLB) in memory management?

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

Which of the following best describes the write-back policy in memory management?

Data is written to memory and only updated to disk when necessary. Signup and view all the answers

What occurs during a page fault when all frames in memory are occupied?

One of the existing resident pages is removed to free up a frame. Signup and view all the answers

How does the modified-bit (m-bit) function in relation to pages in memory?

It shows whether the page has been modified since loading. Signup and view all the answers

Which page replacement strategy requires a detailed history of page access to function effectively?

LRU (Least Recently Used) Signup and view all the answers

What is the consequence of a modified page needing to be replaced in memory?

It necessitates moving the modified page back to the disk before removal. Signup and view all the answers

What advantage does the principle of Transparent Level of Indirection provide for user programs?

It enhances performance by hiding physical data storage details. Signup and view all the answers

Why is it necessary to minimize the number of data moves between memory and disk?

Moving pages is a time-consuming operation. Signup and view all the answers

How many page faults occur when using FIFO with the given reference stream?

7 Signup and view all the answers

What is the strategy used by the MIN page replacement algorithm?

Replacing the page that will not be used for the longest time in the future Signup and view all the answers

In the examples provided, how does LRU compare to FIFO?

LRU performs the same as FIFO in the provided examples Signup and view all the answers

When using LRU, under what circumstance does it perform poorly?

When the working set exceeds N+1 pages Signup and view all the answers

What is one shortcoming of the FIFO page replacement policy?

It can replace a page that is needed immediately Signup and view all the answers

How does the number of page faults in the MIN algorithm compare to FIFO for the given reference stream?

MIN has fewer faults than FIFO Signup and view all the answers

Which algorithm is guaranteed to always perform the best?

None of the algorithms are guaranteed to perform well Signup and view all the answers

Based on the provided examples, how does LRU handle repeated page access?

Every reference results in a page fault Signup and view all the answers

What is the main goal of a page replacement algorithm?

To minimize the number of page faults in the future Signup and view all the answers

Which page replacement algorithm is associated with Bélády’s anomaly?

First-In-First-Out (FIFO) Signup and view all the answers

What phenomenon does Bélády’s anomaly describe?

More frame allocation can lead to more page faults Signup and view all the answers

What is the expected outcome when a process has more frames assigned to it?

Decrease in page faults Signup and view all the answers

How many page faults occurred with the test reference string using 3 frames?

9 page faults Signup and view all the answers

In the FIFO page replacement algorithm, what does the pointer signify?

The next page to replace Signup and view all the answers

What happens to the number of page faults when using the FIFO algorithm with more frames in certain scenarios?

It can increase unexpectedly Signup and view all the answers

What does the term 'page reference string' refer to?

The order in which pages are referenced by processes Signup and view all the answers

Study Notes

Lecture Agenda - November 14

Reminders:
- Read chapters 20-22 (Virtual Memory Module 8)
- Project 4 - Driving Simulation (due 11/20): Demonstrates concurrency using threads.
- Project 5 - Implement a Simple File System (2nd deliverable 11/20)
Attendance: Attendance is required.
Quiz Review: Review for Quiz will be held.
Quiz: Quiz 11/20

File System Allocation Methods

Indexed Allocation (Inode Example): Linux's inode is an example of indexed file block allocation.
Small Number of Extents: The method stores the link to associated extents as the last element in a contiguous data segment.
Virtual Addressing and Paging: Virtual addressing involving paging of tables leads to potential program memory access to retrieve data from memory cells (e.g., 1, 2, 3, and 4).

Demand Paging

Basic Demand Paging Steps:
- Code references a page not in memory
- Page fault is triggered
- Find the page location on disk
- Locate a free frame: If available, use it. Otherwise, a page replacement algorithm selects a victim frame and removes its content.
- Bring the desired page into the free frame, update tables.
- Restart the process (and the faulted instruction). (Note: page not yet in the TLB).
Diagram (Page Fault → Demand Paging): The image illustrates the process, including virtual address, physical address, MMU, page table, frame number, and offset, and the role of the Operating System (OS) and Page Fault Handler in loading the page from disk.
Performance Evaluation: Effective access time is dependent on the probability of a fault. Access time is faster if no page faults. Time increases when faults occur and involves extra steps like disk reading or updating tables.
Time Consuming Acts of a Page Fault:
- Trap to OS (undefined address error)
- Save registers and active state
- Determine the interrupt cause (page fault)
- Verify page reference legality
- Obtain page location on storage device
- Issue I/O read request
- Queue for read servicing
- Wait for device seek and latency
- Begin transfer
- CPU allocated to another task; I/O progresses
- Receive interrupt from I/O done.
- Return context; update tables showing the page location.
- Wait for CPU allocation for faulted process
- Restore context and execute the instruction again
Summary of Operations: Service page fault interrupt, locate and swap in page, and restart process in 1-3 steps.
Summary of Operations (Page Fault Time): - Time for steps 1 and 3 are software controlled (5-100 microseconds). Stage 2 is affected by hardware factors like HDD seek, latency to disk, and queue times (can be over 10 milliseconds and more significant).
Demand Paging Example: Presents an example evaluating effective access time. Performance degradation factor from fault is illustrated; example provides a 10% degradation.

Page Replacement

Why Replacement is Crucial: Replacement is important in caching, particularly in page management. Cost of improper replacement is significant, requiring disk accesses. Essential to prevent tossing out important memory.
FIFO (First-In, First-Out): The oldest page is replaced first. Not efficient as heavily used pages might be replaced, but less useful ones might be kept.
RANDOM: Replaced page is chosen randomly. Simple to implement in hardware but unpredictable.
MIN (Minimum): A page that will not be referenced for the longest time is replaced. This is optimal but impossible to implement in practice, as the future is unknown.
LRU (Least Recently Used): Replaces the page that hasn't been referenced in the longest time. Good approximation of MIN since program behavior often shows locality. LRU is used as approximation of MIN in practice but can vary computationally. LRU algorithm tracks the usage order via a list or similar structure.
Page Replacement (1 & 2): When all frames are used and a fault occurs, a resident page is swapped out for a different page loaded from the disk. Modified pages are prioritized in order to reduce overhead of page replacement, as those pages need to be written back to disk.

LRU (Least Recently Used)

Concept of LRU: Replace the page least recently used (LRU) in memory since it's less likely to be needed soon.
Implementing LRU with a list: Pages are added to the head of a list whenever they're used, putting the least recently used page at the tail.

Example: FIFO

Scenario: Illustrates the workings of the FIFO algorithm with examples of page reference streams and page faults.
Result: FIFO replacement (with specifics of a page stream) is shown to result in a specific number of page faults

Example: MIN

Scenario: Demonstrates how MIN manages pages and predicts which pages won't be used soon.
Result: Illustrates the results and faults associated with replacing the page not used in the future in reference to a page stream
Using Modified Bit: Diagram of how a modified bit and its presence bit flags will help determine whether disk writes are needed or pages have been updated recently and need to be saved to disk (if modified).

How Memory Management Works

Demand Paging as Caching

Block Size and Page Location: Pages (e.g. 4KB); using TLB (Translation Lookaside Buffer) for fast access and page table traversal.
Miss and Write Actions: Misses at lower memory levels involve disk access (write-through or write-back).
Page Replacement Policies: LRU (Least Recently Used) and random are examples

Illusion of Infinite Memory

Disk's Role: Disk acts as a large supplementary memory, exceeding physical memory, allowing concurrency and virtual addresses larger than physical memory
Transparent Level of Indirection: Page tables allow data to be located anywhere and used transparently (on disk, memory) for processes.
Performance Considerations: Accessing memory is a crucial concern for program performance.

Page Faults Versus the Number of Frames

General Expectation: More frames reduce page faults. However, certain algorithms, like FIFO, exhibit Belady's anomaly. The number of faults can increase initially even if more frames are available

Page Replacement Algorithms

Goal: Minimizing page faults. Understanding algorithm evaluations with respect to page reference strings and computing faults through testing. Numerical data are needed for string examples.

FIFO Algorithm

Concept and Implementation: FIFO (First-In, First-Out) is implemented with a queue; pages are removed based on entry order.
Illustrating Belady's Anomaly: Illustrates the counterintuitive phenomenon where the number of faults can increase even with more page frames available. Includes an example reference string to illustrate FIFO's use.

What could be the best we can do in terms of an algorithm?

Min (Optimal) Page Replacement Algorithm

Optimal Replacement: Page with the longest time before it is referenced again is replaced. Ideally, this would lead to fewer faults overall. However, it's impossible to implement in practice because its use requires knowing the future. Includes an example where the algorithm is used and data is illustrated, showing how many page faults happen (a number) and why, in an example reference string

Min (Optimal) Page Replacement

Illustrative Example: An example demonstrating Min optimal behavior, showing how page accesses relate to faults in different frames, leading to fewer (9) total page faults. This demonstrates how optimal replacement makes use of page reference streams.

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Description

Test your knowledge on key concepts of virtual addressing and paging in operating systems. This quiz covers page replacement algorithms, memory access during page faults, and the implications of different paging policies. Each question will help reinforce your understanding of memory management techniques.