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Interactive Audio Lesson
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Introduction to Page Tables
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Today, we’ll dive into page tables and their sizes—vital for managing virtual memory. Can anyone tell me why we need page tables?
To map virtual addresses to physical addresses!
Exactly! Page tables help track where virtual pages reside in physical memory. Now, what happens if our page tables become too large?
They consume a lot of memory, which can slow down system performance.
Correct! This is why we need to consider methods for reducing their sizes, like hierarchical page tables.
Hierarchical Page Tables
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Hierarchical page tables break down page tables into levels. Who can explain how?
The outer table points to inner tables, and they allow us to manage larger address spaces without needing huge single tables.
Great point! This way, when we only need portions of the virtual address space, we can keep other parts off memory. What’s an example of a system that might benefit?
64-bit systems where the address space is enormous!
Exactly! Next, let’s discuss hashing techniques to optimize memory further.
Hashed Page Tables
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Hashed page tables only store entries for valid virtual addresses. Can someone explain how this helps?
It reduces memory usage by not storing all possible entries.
Exactly! And if we hash to find an entry instead of searching through the whole table, what does this do?
It speeds up the lookup process!
Yes, very good! However, an inverted page table does something even different...
Inverted Page Tables
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Now, what about inverted page tables? What’s their key feature?
They only use one page table for all physical memory, indexed by the physical frame number.
Spot on! This can save a lot of memory. Who remembers the trade-off with them?
It can take longer to search because we have to check a larger table.
Exactly! You all have a great grasp of these concepts. Let’s wrap up with a quick exercise.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
It discusses hierarchical page tables, hashing techniques, and inverted page tables as strategies to manage memory more effectively in systems with large address spaces. Each method has its benefits and trade-offs in terms of speed and efficiency.
Detailed
In this section, we explore several methods for optimizing page table sizes to manage memory efficiently. The hierarchical page table technique introduces multi-level page tables which allow the virtual address space to be broken down into smaller segments, reducing the size of individual page tables. By separating logical addresses into multiple components—outer and inner page tables—system memory can better handle scattered memory allocation.
A special focus is placed on two-level and three-level page tables, particularly their suitability for 64-bit architectures. As more address space is needed, they can lead to larger page tables. Additionally, hashed page tables utilize hashing techniques to maintain a smaller footprint in memory by only storing entries for valid logical addresses that correspond to the physical memory.
Finally, inverted page tables present an alternative where a single table is indexed by physical memory pages rather than by logical addresses, reducing memory consumption while increasing lookup time. This concludes with a practical exercise calculating page sizes for a system with specific attributes.
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Audio Book
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Hierarchical Page Tables Overview
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The next approach that is used to reduce page table sizes is by using hierarchical page tables. So firstly, what did we use? We used a page table length register. The which was without segmentation and then we said that typically the virtual address space has a stack part and a heap part to address and the page table length register only allows the page table to grow in one direction.
Detailed Explanation
This chunk introduces hierarchical page tables as a method to reduce the size of page tables. Initially, a page table length register (PTLR) was used, but it allowed growth in only one direction, making it less flexible. The text mentions that virtual address spaces typically include stacks and heaps, which underscores the need for a more dynamic and efficient organization of page tables.
Examples & Analogies
Think of a filing system in an office. Initially, you might have only one drawer for all files, leading to issues when the drawer gets full. A hierarchical filing system, where you have several drawers for different categories, helps in organizing files better and allows for easier access.
Segmentation and Multiple Page Tables
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We addressed that by having two segments; one containing possibly the stack, the other containing the heap and each of these two segments has two page tables and the so, by directions two possible directions of increase for the process becomes available. So then what happens? We came to the segmentation. So, instead of having just two segments I divided into multiple segments. So, therefore, the even if the processes address space is very scattered, so the modules tend to be clustered in their address spaces.
Detailed Explanation
The introduction of segments improves organization by allowing the virtual address space to be divided into stack and heap segments with their own page tables. The further division into multiple segments enables more efficient memory use, especially for scattered processes. This clustering of modules in address space allows for more effective access and management of memory.
Examples & Analogies
Imagine a library where books are organized not just by title but also by genre and author. This way, if someone is looking for a specific type of book, they can go directly to that section instead of searching through a single large collection.
Accessing Page Tables
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So, I have separate page tables for each of these modules or segments and therefore, the overall size of the page table reduces. And then the segment table can be in the main memory and we understand that the page tables can also be in the secondary storage as well.
Detailed Explanation
Having separate page tables for each segment contributes to reducing the overall size of the page table. It is mentioned that segment tables can reside in main memory, while page tables can also be stored in secondary storage, offering flexibility in memory management.
Examples & Analogies
Think of a stored archive with files. Instead of keeping all files in one box (main memory), you might keep some less frequently used files in a storage unit (secondary storage) while keeping your most important files readily accessible in your regular filing cabinet.
Understanding the Two-Level Page Table
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So, we break the logical address space into multiple page tables. The simplest scheme in this is a two level page table. So, as I told you the page table is also paged similar to the paging with segmentation, the second and third level page tables may or may not be in main memory.
Detailed Explanation
This chunk introduces the two-level page table structure, which breaks down the logical address space into more manageable parts. Each level in the hierarchy may be paged, meaning they can also be stored in more efficient memory. This structure enhances memory management by allowing a more granular approach to address space organization.
Examples & Analogies
Consider an organizational chart of a company. At the top level, you have different departments (first level), each with its own employees grouped by tasks (second level). If some departments are expanding, they can create teams as needed without needing to reorganize the whole company, similar to how pages are managed in memory.
Page Number Splitting
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The page number is split into two parts. One is 𝑃₁; the other is 𝑃₂; 𝑃₁ indexes the outer page table; 𝑃₂ indexes the inner page table.
Detailed Explanation
The logical address space is divided into two components: the first part (𝑃₁) points to the outer page table, while the second part (𝑃₂) points to the inner page table. This allows for efficient location of the data needed through a two-level lookup process, minimizing the search effort and potentially speeding up memory access.
Examples & Analogies
Think of searching for a document in a multi-directory filing system. First, you select the main directory (outer table) that contains several subdirectories; then you pick the exact subdirectory (inner table) where your document resides, saving you time and effort.
Challenges with Two-Level Paging
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Now, two-level paging is not always sufficient. So, even two-levels paging is not sufficient for 64 bit computers.
Detailed Explanation
The limitation of two-level paging becomes evident on 64-bit systems where the address space is enormous. Given the constraints on page sizes, the resulting page table can have an exceedingly large number of entries, making it impractical. This creates a need for additional levels of pagination to effectively manage large address spaces.
Examples & Analogies
Imagine organizing a massive library that needs more than just two levels of categorization due to an overwhelming number of books. Just having 'Fiction' and 'Non-Fiction' isn't sufficient, so you create a 'Genre' category under both to manage the multitude of books better.
Inverted Page Tables Approach
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The next approach is the use of an inverted page table. Instead of having a page table for each process and keeping track for all possible logical pages that we have, we only keep track of all physical pages.
Detailed Explanation
This chunk explains the inverted page table concept where the focus shifts from maintaining multiple page tables for each process to a single table representing all physical memory. This method indexes the table by physical page frame numbers and contains details about which virtual addresses correspond to which physical page frames.
Examples & Analogies
Imagine a classroom where instead of keeping individual attendance sheets for each class (process), you maintain one master sheet for all students (physical pages). This way, you only need to track who is present in class, significantly streamlining the process.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Hierarchical Page Tables: Multi-level structure that helps manage larger address spaces effectively.
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Hashed Page Tables: Tables that store entries for valid addresses only, using hashing for fast lookups.
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Inverted Page Tables: A single table for all physical memory, indexed by physical frames to save space.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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A two-level page table where the first level is indexed to multiple entries in the second level, allowing dynamic address allocation.
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A hashed page table where only the pages currently needed are stored, limiting waste of space for unallocated addresses.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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In layers we split, tables don't grow too lit!
📖 Fascinating Stories
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Imagine a librarian organizing books by sections and sub-sections. Instead of one oversized catalog, smaller catalogs help find books faster.
🧠 Other Memory Gems
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HITS: Hashed, Inverted, Two-level, Segmented (Four methods to handle page tables).
🎯 Super Acronyms
HPT = Hierarchical Page Tables (efficiently group details) and HPT = Hashed Page Tables (fast lookups only for current needs).
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Page Table
Definition:
A data structure used in virtual memory systems to map virtual addresses to physical addresses.
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Term: Hierarchical Page Tables
Definition:
A multi-level page table structure that divides the address space into lower levels to reduce the size of the individual tables.
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Term: Hashed Page Table
Definition:
A page table that uses a hash function to find an entry, storing only valid virtual addresses.
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Term: Inverted Page Table
Definition:
A page table where entries correspond to physical frames, reducing memory needed for storing multiple tables for each process.
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Term: Virtual Address Space
Definition:
The range of addresses that a process can use, typically larger than physical memory.