Computer Memory Management Quiz

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Questions and Answers

Which page replacement policy is characterized by replacing the page that will not be used for the longest period of time?

LRU Approximations

FIFO

Second Chance

Optimal (correct)

What strategy is used for managing the resident set in memory to ensure that the most frequently used pages are kept in memory?

Local Scope Management

Variable Allocation

Global Scope Management

Working Set Strategy (correct)

Which type of storage allocation is more likely to result in wasted space when dealing with small files in a multi-level indexing file system?

Multi-level Indexing (correct)

Indirect Addressing

Block Allocation

Contiguous Allocation

What aspect does the size of a FAT entry depend on in a FAT file system?

Size of each data block Signup and view all the answers

What is the purpose of the present bit in a page table entry?

To indicate if the page is valid in the virtual address space Signup and view all the answers

What is the main advantage of demand paging?

It loads pages into memory only when referenced. Signup and view all the answers

Which statement accurately describes prepaging?

It can lead to many unnecessary page loads. Signup and view all the answers

What is a potential trade-off of allocating a smaller number of pages to processes?

Increased page faults when more processes are introduced. Signup and view all the answers

What is the key characteristic that differentiates global replacement from local replacement?

Local replacement is restricted to the page frames of a specific process. Signup and view all the answers

Which method allows for flexibility in the number of page frames allocated to a process over its lifetime?

Variable-allocation Signup and view all the answers

What is a consequence of fixed allocation being too small for a process?

High page fault rate Signup and view all the answers

What happens when the processor's allocation in fixed allocation is too large?

Increased time spent in swapping Signup and view all the answers

Which method of page replacement does global variable allocation utilize?

OS maintains a list of free frames Signup and view all the answers

In which scenario does a process's resident set grow in size?

When a page fault occurs Signup and view all the answers

Which aspect of variable allocation contributes to its ease of implementation?

Ability to adapt the resident set size dynamically Signup and view all the answers

Study Notes

Agenda November 21 - Lecture 25

Reminders: Read chapters 20-22 (Virtual Memory Module 8), chapters 36 and 37 (Devices).
Project 5: Deliverable for Project 5 - implementing a simple file system is due December 4th. The last demonstration is December 9th.
Attendance: No class next week.
Quiz: A quiz is scheduled for December 4th.
Final Exam: The final exam is scheduled for Thursday, December 12th, from 10:30 AM to 12:30 PM in Beury 160.
Thanksgiving Holiday: Enjoy the Thanksgiving holiday.

Last Time

Page Replacement: Covered FIFO, Optimal, and LRU algorithms.
LRU Approximations: LRU approximations were also discussed.
Memory Allocation Policies: Policies for managing memory allocation were reviewed.

Today

Page Replacement: Page replacement policies for managing memory allocation.
Fetch: Discussed fetch policies related to memory allocation.
Resident Set Management: Resident set management including fixed/variable allocation with local/global scope.
Working Set Strategy: Working set strategy for managing memory allocation.
Memory Mapped Files: Memory mapped files for efficient data transfer.
Device Management: Device management for interaction with hardware devices.

Quiz 9

(Graph showing distribution of answers)

True/False Questions

Storage Block Allocation: Multi-level indexing wastes space for small files. (True or False)
FAT Implementation: A FAT entry size is dependent on the number of bytes in file system blocks. (True or False)
Clock Algorithm: A clock algorithm records reference times for determining the LRU page. (True or False)
Presence Bit: The presence bit indicates if a page is valid in a process's virtual address space. (True or False)

Fetch Policy

Demand Paging: Brings pages into memory only when referenced. Causes a high number of page faults initially but then usually decreases.
Prepaging: Brings in multiple pages at once to improve efficiency. This is sometimes effective if pages are arranged contiguously.

Resident Set Management

Trade-offs: Memory allocation to processes has trade-offs. Smaller allocation leads to more processes, but increases page faults. Larger allocation leads to fewer page faults but reduces the number of processes.
Resident Set: Resident set comprises a process's currently loaded memory pages.

Policies for Resident Set Size

Fixed Allocation: Allocates a fixed number of frames for a process; the process will suffer if there's a page fault.
Variable Allocation: Allows the allocated frames to adjust dynamically over the process lifetime.

Replacement Scope

Local Scope: Replacement choices affect only the process generating the page fault.
Global Scope: Consideration expands to all unlocked pages in the memory.

Fixed Allocation, Local Scope

Page fault rate: High page fault rate if allocation of frames is too small.
Processor idle time & swapping: Increased times if allocation is too large.

Variable Allocation, Global Scope

Implementation: Easier to implement compared to local scope in a variable allocation, as allocation can be dynamically modified.
Free frames: OS manages a list of free frames, assigning frames to active processes when a page fault occurs.
Growing resident sizes: Causes other processes to suffer if frames are unavailable.
Page buffering: Page buffering helps to mitigate problems associated with poor allocation choice.

Variable Allocation, Local Scope

New Process allocation: Allocates a certain number of page frames for a new process.
Page fault: Page to replace is selected from the resident set.
Evaluation & performance: Processes are constantly evaluated and re-allocated based on their locality of reference characteristics.

Variable Allocation, Local Scope (Key Elements)

Assessment: Resident set size changes are based on anticipated future requirements for the currently active processes.
Criteria: Criteria for determining the resident set size are crucial for efficiency.
Timing: The timing of when changes in resident set size are made is essential for the optimal performance of the OS

Working Set Concept

Required pages: A program requires a set of pages to reside in memory for smooth operation.
Page fault rate & allocation: Page fault rate increases as a program does not have enough frames.
Optimal Working Set: The optimal working set contains pages highly likely to be used in the near future.
Program Behavior & Variation: Working set sizes change greatly depending on the type of locality exhibited by the program.

Locality of Reference

Working Set

Window: A window (of size Δ) is used to determine the pages that make up the working set.
Page Reference String: The page reference string dictates to which pages to look to when a page fault occurs in order to avoid wasting potentially valuable processing time.
Working Set Composition: The working set changes over time as the window slides through the page reference string.

Window Size (Width)

Program Behavior: Program behavior dictates the size of the window, which must be large enough to capture periods of heightened activity but not so large as to become inefficient.

Working Set Algorithm

Memory Reference: Pages used in the most recent k memory references make up the working set at time t.
Size Function w(k,t): Defines the size of the working set at a specific point in time or over a specific interval.

Working Set Strategy

Variable Allocation, Local Scope: Working set of a process is tracked, and the resident set size adjusts based on the pages in its working set.
Virtual Time: Virtual time is monitored to reflect the changes in a process's working set

Working-Set Model

Parameters: working-set window - fixing amount of page references, interval △ - (# pages referenced in each interval determines working set size)
Total Demand Frames (D): The total number of frames that are needed in the set to fit all the application's work in memory.
Thrashing: D > m (where m is total frames available), meaning that there aren't enough frames to satisfy the total demand of the processes in the system
Process Suspend: The operating system must suspend one of the processes to ensure system stability.

Thrashing

Swapping: A state where a system spends most of its time swapping process portions rather than executing instructions.
Guessing: The operating system guesses and bases allocation off of the recent history to decide which pieces of information are least likely to be used in the near future.

Multiprogramming Effects

Thrashing: Threshold where processor utilization decreases significantly due to extensive swapping.

Page Replacement using the Working Set Strategy

Hardware Bits (R&M): Hardware sets bits (R and M) in the page table.
Timer Interrupt: The timer interrupt periodically resets referenced pages (R bit)
Page Fault: When a page fault happens, the OS checks if the page is found in the working set. If not, it evicts a page.

Working Set of Processes Determined by Window Size

(Data representing working sets of processes based on different window sizes over time)

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Description

Prepare for the upcoming quiz on memory management principles, specifically focusing on page replacement algorithms, fetch policies, and resident set management. This quiz covers essential concepts from chapters 20-22 and 36-37. Stay sharp and review your knowledge on FIFO, Optimal, and LRU algorithms.