Virtual Memory Management Concepts
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Questions and Answers

Which of the following scenarios would most likely benefit from using virtual memory?

  • A system running multiple large applications concurrently. (correct)
  • A real-time system requiring predictable execution times.
  • An embedded system with limited storage capacity.
  • A system prioritizing minimal power consumption.

A system using paging has a 32-bit virtual address space and a page size of 4KB. How many entries are needed in a single-level page table?

  • 8,192
  • 524,288
  • 2,097,152
  • 1,048,576 (correct)

Which of the following is a primary advantage of using a Translation Lookaside Buffer (TLB)?

  • Reduces the size of the physical memory required.
  • Decreases the number of disk accesses.
  • Speeds up virtual-to-physical address translation. (correct)
  • Simplifies the process scheduling algorithm.

A process has 4 frames allocated in physical memory. The process accesses pages in the following order: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5. Using the Least Recently Used (LRU) page replacement algorithm, how many page faults will occur?

<p>7 (D)</p> Signup and view all the answers

Which of the following is the most significant challenge associated with contiguous memory allocation?

<p>External fragmentation. (C)</p> Signup and view all the answers

A system uses a virtual address space of 4GB with a page size of 4KB. How many entries are required in the page table if each process has its own page table?

<p>1,048,576 (C)</p> Signup and view all the answers

Consider a virtual address of 0xABCD1234 with a page size of 4KB. What is the virtual page number?

<p>0xABCD1 (D)</p> Signup and view all the answers

A process accesses a memory location with a virtual address. The MMU translates this to a physical address. What is the primary role of the page offset in this translation?

<p>To specify the exact location within the page frame. (A)</p> Signup and view all the answers

If a system uses a two-level page table, what is the primary benefit compared to a single-level page table?

<p>Decreased physical memory usage for page tables. (B)</p> Signup and view all the answers

A system employs virtual memory with an MMU. Which of the following represents the correct sequence of actions during a memory access?

<p>CPU generates virtual address -&gt; MMU translates to physical address -&gt; Memory access. (A)</p> Signup and view all the answers

A system uses FIFO page replacement with 3 page frames. Given the reference string 'ABCD ABE ABCD EBCFBB BCDE F', how many page faults will occur?

<p>15 (C)</p> Signup and view all the answers

What is the primary goal of Belady's Optimal page replacement algorithm?

<p>Minimize the number of page replacements by predicting the future reference string. (D)</p> Signup and view all the answers

Given the page reference string 'ABCD ABE ABCD EBCFBB BCDE F', what is a key challenge in implementing Belady's Optimal algorithm in a real-world operating system?

<p>The requirement to predict the future page reference string. (A)</p> Signup and view all the answers

A system employs Belady’s Optimal page replacement algorithm with 3 page frames, using FIFO to resolve ties. Consider the page reference string 'ABCD ABE ABCD EBCFBB BCDE F'. How would the algorithm decide which page to replace when a tie occurs?

<p>It replaces the page that has been in memory the longest amount of time. (D)</p> Signup and view all the answers

Consider a system with limited RAM. How does the page size of 512 bytes influence overall system performance when using page replacement algorithms like FIFO or Belady's?

<p>The chosen page size affects the trade-off between internal fragmentation and the frequency of page faults. (B)</p> Signup and view all the answers

Consider a system using Belady's Optimal Algorithm with 3 page frames. Given the reference string 'ACDB', what is the minimum number of additional page references required to guarantee a fourth page fault?

<p>1 (C)</p> Signup and view all the answers

Suppose a system employs Belady's Optimal page replacement algorithm with a limited number of frames. Under what circumstance will Belady's algorithm result in the same number of page faults as the FIFO algorithm?

<p>When the number of page frames is equal to the number of unique pages in the reference string. (D)</p> Signup and view all the answers

A system with 4 page frames uses Belady's Optimal Algorithm. Given the reference string 'ABCDE', what is the maximum number of additional page references that can occur before a page fault is unavoidable?

<p>0 (D)</p> Signup and view all the answers

Consider a memory system with a 64-bit architecture and 64KB of RAM. If the page size is 512 bytes, how many bits are required to represent the offset within a page?

<p>9 bits (B)</p> Signup and view all the answers

In a system employing the Least Frequently Used (LFU) page replacement algorithm, what additional information is required to effectively break ties between pages with the same usage frequency?

<p>The access time of each page. (A)</p> Signup and view all the answers

Under what circumstance does an operating system typically remain idle?

<p>When all processes are waiting on I/O operations and there are no RUNNABLE processes. (A)</p> Signup and view all the answers

What is the primary goal of employing page replacement policies in paged virtual memory?

<p>To ensure the resident set closely mirrors the working set. (B)</p> Signup and view all the answers

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

<p>Belady’s Optimal (C)</p> Signup and view all the answers

What is the main function of a 'use bit' in the context of page replacement algorithms?

<p>To indicate whether a page has been referenced since the last time the bit was cleared. (B)</p> Signup and view all the answers

What is the primary advantage of paged virtual memory, which relies on the principle of locality?

<p>It enables a large program to execute, even if it only uses a small portion of its address space at any given time. (C)</p> Signup and view all the answers

Which of the following scenarios would most likely result in a process experiencing a significant interruption due to a page fault?

<p>Accessing a memory location that is not currently loaded in physical memory. (B)</p> Signup and view all the answers

Which page replacement policy replaces the page that has been referenced the fewest number of times?

<p>LFU (Least Frequently Used) (B)</p> Signup and view all the answers

How does clearing the use bit at regular intervals help approximate the Least Recently Used (LRU) page replacement policy?

<p>It identifies pages that have not been referenced recently, making them candidates for replacement. (D)</p> Signup and view all the answers

Based on the provided graphs, what general conclusion can be made about the relationship between memory allocation and task completion time when building a kernel?

<p>Increasing memory allocation consistently reduces task completion time up to a certain point, after which there is no significant improvement. (D)</p> Signup and view all the answers

What is the primary function of swapping in an operating system, and under what circumstances is it typically employed?

<p>Swapping is a memory management technique where data is transferred between memory and disk to alleviate memory pressure when physical memory is limited. (C)</p> Signup and view all the answers

According to the provided information, what is the expected performance impact of excessive swapping on a system?

<p>Excessive swapping causes a significant degradation in performance as the system spends more time transferring data between memory and disk. (A)</p> Signup and view all the answers

In a Unix-style process virtual address space, which segment is responsible for storing machine code instructions?

<p>Text segment. (C)</p> Signup and view all the answers

What is the copy-on-write (COW) optimization, and how does it affect memory usage when multiple processes access the same read-write page?

<p>COW delays the duplication of pages until a process modifies them, reducing memory usage by sharing pages initially. (D)</p> Signup and view all the answers

For a read-only segment in a virtual address space shared by multiple processes, how many copies of each page are typically kept in physical memory, and why?

<p>One copy, as the data cannot be modified, so sharing a single copy is safe and efficient. (B)</p> Signup and view all the answers

Which of the following scenarios would most likely lead to the operating system resorting to swapping?

<p>The system is running multiple memory-intensive applications, exceeding the available physical RAM. (A)</p> Signup and view all the answers

Consider a program that dynamically allocates memory using malloc() and new. Which segment of the process's address space is primarily affected by these operations?

<p>Data Segment (A)</p> Signup and view all the answers

How is the stack segment's size typically adjusted during the execution of a program?

<p>The stack segment is automatically adjusted when functions are called or return. (D)</p> Signup and view all the answers

A system is running several applications and starts experiencing significant performance slowdowns. Monitoring tools indicate that the system is spending a large amount of time swapping memory pages to disk. Which action is most likely to improve the system's performance?

<p>Add more RAM to the system to reduce the need for swapping. (D)</p> Signup and view all the answers

Flashcards

Operating System (OS)

Software that manages computer hardware and software resources, providing common services for computer programs.

Memory Management

A fundamental function of the OS that manages how computer memory is allocated and utilized.

Contiguous Memory Allocation

Allocating contiguous blocks of memory to processes; simple but can lead to external fragmentation.

Paging

Dividing memory into fixed-size blocks (frames) and processes into same-size blocks (pages); reduces external fragmentation.

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Virtual Memory

A technique where parts of a process are stored on disk and brought into memory only when needed, allowing processes larger than physical memory to run.

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Byte

A unit of digital information that consists of 8 bits.

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MMU (Memory Management Unit)

Transforms virtual addresses into physical addresses.

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Virtual Address

Logical address used by a process.

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Physical Address

The actual physical location in memory.

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Pages

Divides processes into fixed-size blocks.

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What is FIFO page replacement?

A page replacement algorithm where the oldest page in memory is replaced first.

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What is Belady's Optimal Algorithm?

Algorithm that replaces the page that will not be used for the longest time in the future. It has the fewest page faults

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What is a page fault?

An event that occurs when a program tries to access a page that is not currently in memory.

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Typical value for a page size?

4096 bytes (or 4KB).

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What is a page reference string?

The sequence of pages accessed by a program during its execution.

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Idle System State

If no process is ready to run, the system is idle, often because all processes are waiting for I/O operations to complete.

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Impact of a Fault

A fault causes a significant interruption in a process's runtime, equivalent to thousands of instructions, making fault management worthwhile.

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Locality Principle

Virtual memory effectiveness relies on the principle that programs use only a small portion of their address space at any given time.

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Working Set

The set of pages a program actively uses at a given time.

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Resident Set

The set of pages allocated page frames in physical memory.

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Page Replacement Policy

A page replacement policy determines which pages in memory should be replaced when a new page needs to be loaded.

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Belady’s Optimal Replacement

This algorithm replaces the page that will not be used for the longest time in the future.

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Use Bit

A hardware bit set when a page is accessed; cleared by software to track recent use for page replacement algorithms.

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Belady's Optimal Algorithm

An optimal page replacement algorithm that replaces the page that will not be used for the longest period of time.

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Belady's(3) in Example

The number of page faults that occur when using Belady's Optimal Algorithm with 3 page frames for a given page reference string is 11.

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Belady's(4) in Example

The number of page faults that occur when using Belady's Optimal Algorithm with 4 page frames is 8.

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System Specs

Page size is 512 bytes, RAM size is 64 kilobytes. The machine is 64-bit.

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LFU (Least Frequently Used)

A page replacement algorithm that replaces the page with the smallest use count.

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Swapping

The movement of fixed-size blocks of memory (pages) between RAM and disk.

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Performance Degradation (Swapping)

A significant decline in system performance due to excessive swapping.

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Stack (Memory)

The memory region where a program's local variables and function call information are stored.

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Data Segment

The memory segment containing initialized and uninitialized data for a program.

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Text Segment

The portion of memory where the executable instructions (machine code) of a program are stored.

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Copy on Write

A technique where a page is duplicated only when a process attempts to modify it; reduces memory usage.

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Adjusting Stack

Address space is adjusted on stack by using what?

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Adjusting Data

Address space is adjusted on data segment by using what?

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Shared segment

Shared copy of a page until modification

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Graceful Degradation

Linear degradation of performance

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Study Notes

  • A Unix-Style Process Image consists of sections including text for executable code, data for static data, and a symbol table for symbol definitions.

Process Memory

  • A unique process image contains all data.
  • Cloning a process using fork() mostly copies data, but shares pointers to shared libraries, memory, and read-only parts, like text and symbols.
  • POSIX threads share most data, copying only the stack, which affects local variable handling to avoid race conditions; different implementations give different things.
  • Fibres use co-operative multitasking instead of preemptive multitasking.

Protected Virtual Addressing

  • Programs operate with virtual addresses.
  • The kernel manages all address spaces.
  • Programs can't directly access another's memory.
  • There is a large space to store programs
  • Programs may have "empty space" between the stack and data.
  • Programs use pages; only needed ones are in real memory.

MMU and Address Translation

  • The Memory Management Unit (MMU) translates virtual addresses to physical addresses.
  • The virtual address consists of a page number and offset.
  • Processes split into pages.
  • The virtual page number finds the frame number in the page table.
  • The Physical Address is the Frame Number * Page Size + Offset.
  • The hardware page table tracks information for each page, including validity (in memory), writeability (RO or W), dirty/used bits, and the page frame number.
  • Additionally, the in-memory page table contains an associated disk address if paged out.
  • A valid bit says if a page exists in memory.
  • The bottom of the stack and top of the heap indicate if the page is legally part of the process.

Examples of Memory Translation

  • Given a page size of 512 bytes, and a virtual address 0x072E. Calculate the offset as 0x200 = 2^9 (9 bits offset). The page is 0x072E / 0x200 = 0x3, with a remainder of 0x12E.
  • Page 0x3 is the frame 0xD. The physical address is 0xD * 0x200 = 0x1A00 + 0x12E = 0x1B2E.
  • Given a page offset of 11 bits (2^11 = 0x800), the virtual address is 0x77FF. The page is 0x77FF / 0x800 = 0xE with remainder of 0x7FF.
  • Therefore, the frame is 0x11 and the final physical address is 0x8FFF.
  • If the page isn't in memory, a page fault occurs, and the required data must be read from the disk.

Page Fault Handling

  • If the address is invalid, the process terminates.
  • If a page frame is empty, use it.
  • If replacing a page, save it to the paging disk if modified.
  • Read the required page from the paging area.
  • If the page is text or initialized data read from the executable program.
  • Otherwise, initialize the page to zeros for security.
  • Update the page table to point to the new page and mark the process as runnable.

Page Faults

  • A page fault causes the running process to be blocked on I/O until th erquired page is loaded.
  • Other runnable processes may continue.
  • An idle system means all processes are waiting on I/O.
  • Faults disrupt runtime and are equivalent to thousands of instructions.
  • Paged Virtual Memory relies on the locality principle, utilizing the the working set.
  • The resident set consists of allocated frames in memory.
  • The ideal page replacement policy would achieve (resident set = working set)

Page Replacement Policies

  • Belady's Optimal Replacement replaces the page used furthest ahead in the reference string..
  • FIFO replaces the longest resident page is the oldest
  • LFU replace the page that has been referenced the fewest times
  • LRU (Least Recently Used) replaces the page whose last reference is the furthest in the past
  • A hardware use bit signifies page reference, with software resetting it; a clear bit means no reference since last clear, set to 1 upon hardware access.

Page Replacement Algorithms

  • At regular intervals, clear all reference bits. Pages with a clear bit are replaced, which serves as a crude LRU approximation. The dirty bit indicates page modification.

Global vs Local Policies

  • A global page replacement policy affects all pages.
  • A local policy applies only to the specific process pages
  • A fixed size partition policy uses a fixed frame allocation for each process.
  • A variable partition policy adjusts frame allocations.
  • Frame allocation may be adjusted; a high fault rate increases it, while a low rate decreases it, potentially multiplicatively or exponentially.

Memory

  • Swapping moves entire segments of processes between memory and disk.
  • Separate copies of read-write pages are created for each process to implement the “copy on write” unless it is is a shared segment
  • read-only segment, only 1 copy of each page must be in physical memory for all processes
  • The stack and data segments have variable sizes
  • Kernel routines like brk() and sbrk() adjust the data segment size

Dynamic Allocation

  • First Fit allocates space in the first sufficiently large block.
  • Best Fit allocates in smallest remaining block and may result in fragmentation.
  • Worst Fit allocates in largest remaining block and may result in fragmentation.
  • The Knuth Buddy System is more complicated and less popular, but used in Linux.

Memory Caches

  • Memory caches are like paging systems, but happen in hardware.
  • Access to lines 3, 7, 2, 8, 1, 9 and 4 cause memory hits.
  • Access to lines 5, 6 and 10 cause memory misses.
  • The tags are page table entries, while the lines are pages.
  • The further the memory is from the CPU, the slower and larger it is.

Hierarchical Page Table

  • A hierarchical page table breaks down the page table itself into pages
  • Each second order page table is 1 page in size
  • This allows us to page out parts of the page table that are not being used

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Operating Systems: Memory - University of Guelph - PDF

Description

This quiz covers key concepts in virtual memory management. It includes page replacement algorithms, address translation, and the benefits of using a TLB. Understanding the nuances of virtual memory is crucial for efficient system design.

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