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Introduction to Hierarchical Page Tables
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Today, we’ll delve into how hierarchical page tables help manage memory more efficiently. Can anyone tell me what a page table is?
A page table keeps track of where data is stored in virtual memory, right?
Exactly! Now, what happens when we need more address space than our current page table can handle?
We might need to use hierarchical page tables?
Correct! Hierarchical page tables break down the address space into manageable parts. Think of it like a multi-level directory, where each level can lead you to the next. To remember this, remember the acronym 'HPT'—Hierarchical Page Tables!
How do we access these multiple levels?
Great question! We use parts of the logical address; the first part points to the outer table, and the second to the inner table. Let's sum up: hierarchical page tables minimize space and prevent memory overflow.
Two-Level Paging
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Now, let's explore what two-level paging entails. What can anyone tell me about its structure?
Isn't it where the page table is divided into two separate levels?
Yes! Specifically, we split the page number into two parts: the first for the outer page table and the second for the inner page table. To visualize this, remember '2-PAGE'—Two Levels of Paging Accessed with Granularity of Entries.
And how does this help with large virtual address spaces?
That’s key! With larger spaces, like 64-bit addresses, two levels might not suffice. So, we expand to three levels. This adjustment ensures that even with expansion, we manage memory effectively.
So, more levels mean we can handle more addresses?
Exactly! Remember, more layers lead to better management. Let’s summarize: two-level paging enhances access to larger spaces while ensuring efficient use of memory.
The Role of Hashed Page Tables
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Next, let’s discuss hashed page tables. Who can explain what they do?
They use a hash function to store virtual pages efficiently, right?
Precisely! This method allows us to keep fewer entries, which leads to less memory usage. What’s crucial here is that each entry can link to multiple items when addresses collide, forming a chain. To remember this, think 'HASH-LINK'—Hashing allows Storage and Handling Like In a Network of Keys.
What if a page isn’t in memory? How does that work?
That's an insight! If a virtual page isn’t in memory, it won't even appear in the table. Remember, a dynamic space varies based on necessity, and efficiency is the goal here. Let’s sum it up: hashed page tables offer efficient space usage with linked chains for collisions.
Understanding Inverted Page Tables
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Finally, let’s explore inverted page tables. What differentiates this from traditional page tables?
Inverted page tables keep track of real pages rather than virtual ones?
Correct! This optimization means that we have one table for all physical pages that store virtual addresses linked to process IDs. We can remember this as 'INVERT'—Indexing Real Pages with Virtual Table Entries and References.
So, does it make searching faster or slower?
Good observation! It can slow down search times since we have to match both virtual addresses and process IDs. However, if we combine it with a hash table, we improve that speed significantly. To close out, remember: inverted page tables are efficient in space but require careful access management.
Introduction & Overview
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Quick Overview
Standard
The text covers the mechanisms of hierarchical page tables, including two-level paging and the necessity of adapting to larger virtual address spaces. It delves into the use of hashed page tables, inverted page tables, and how these techniques optimize memory management on modern architectures.
Detailed
Final Calculation of Page Size
In this section, we explore various strategies to minimize page table sizes in virtual memory management, particularly through hierarchical and inverted page tables.
Hierarchical Page Tables
Initially, traditional page tables faced limitations when managing large address spaces. To enhance efficiency, hierarchical page tables were introduced, breaking logical addresses into multiple levels. This method effectively accommodates larger address spaces by using two-level page tables as a simple implementation. The virtual address is segmented into parts that index outer and inner page tables, allowing direct access to the physical memory through computed offsets.
Two-Level Paging
In two-level paging, the logical address space is divided into two components, each directing the CPU to necessary page tables stored in memory. However, for systems with 64-bit addresses, a mere two levels may not suffice due to the sheer size of the page tables needed. For instance, with a 4 KB page size, a 64-bit architecture could produce a massive amount of entries, hence the necessity for a three-level page table organization to further optimize memory management.
Hashed Page Tables
Another method to manage page tables effectively for large address spaces is by employing hashed page tables. This technique uses a hashing function to minimize memory requirements while enhancing retrieval efficiency. The hashed pages store pointers in linked lists to accommodate collisions, facilitating management of virtual addresses with fewer entries in memory.
Inverted Page Tables
Finally, inverted page tables offer a comprehensive solution by indexing physical memory rather than maintaining separate tables for each process. This method decreases the memory footprint of page tables but may increase search time when accessing logical addresses since a match must be found for both the virtual address and the process ID.
Through these methods, each page management strategy attempts to balance memory usage and access efficiency, essential for modern computing systems.
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Introduction to Hierarchical Page Tables
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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 go grow in one direction.
Detailed Explanation
Hierarchical page tables are introduced to optimize the memory management of virtual address spaces. Initially, a simple page table length register was used, but this only allowed for a one-directional growth of the page table, making it inflexible. As a result, to accommodate both the stack and heap areas effectively, a more complex structure known as hierarchical page tables is needed, allowing greater flexibility and efficiency in memory allocation.
Examples & Analogies
Think of this like a multi-level organization within a company. Initially, you could only have employees report to one single manager (a single page table), which limits how many can be managed. By creating several layers of management (hierarchical page tables), employees can be organized more efficiently into teams, making collaboration easier and management more fluid.
Segmentation in Address Space
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So, 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.
Detailed Explanation
To manage the stack and heap effectively, the address space was divided into two distinct segments. Each segment has its own page table, which allows for growth in two directions—upwards for the stack and downwards for the heap. This approach significantly improves the flexibility of memory allocation for processes.
Examples & Analogies
Imagine a filing cabinet where one drawer is dedicated to ongoing projects (the stack) and another to archived documents (the heap). By organizing them separately, you can simultaneously add new files to both drawers without worrying about one overflowing or taking up the space of the other.
Expanding with Multiple Segments
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So, then what happens? We came to the segmentation. So, instead of having just two segments I divided into multiple segments. So, therefore even if the processes address space is very scattered, so the modules tend to be clustered in their address spaces.
Detailed Explanation
The segmentation was further refined to involve multiple segments. This further division allows for scattered series of modules within the process address space to be grouped or clustered logically. Each cluster can access a separate page table, leading to a more manageable and compact page table size overall.
Examples & Analogies
Picture a library that has not only fiction and non-fiction sections but also sub-sections for different genres, authors, or topics. By having these multiple segments, you can find a specific book much faster, just as individual segments allow software to manage its memory more efficiently.
Understanding Hierarchical Page Tables
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Then we come to hierarchical page tables. So, hierarchical page tables the simple so the here we do not go into segmentation, we don’t have segmentation here and but we have multiple page table levels; hierarchical page tables or multiple page tables multiple multi-level page tables.
Detailed Explanation
Hierarchical page tables use a structured approach with multiple layers of page tables instead of a single table. This system allows the division of the logical address space into smaller, more manageable parts without relying on segmentation, simplifying memory management. Multiple levels mean that each logical address can be resolved more efficiently, as the address is broken down into components that index various page tables.
Examples & Analogies
Think about how a mall is organized. Instead of having one giant store (a single level page table), you have different stores on different floors (multilevel page tables), with directories (page tables) at each level to help you navigate to the specific shop you're looking for.
Page Number Partitioning
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So, what happens? The logical address space is now divided into two parts; P1, P2 and this is the offset part. So, from P1 I go to the outer page table ok. So, the page table the my page table base register tells me the position in memory of the start of the outer page table.
Detailed Explanation
In this structure, the logical address is split into three parts: two page numbers (P1 and P2) and an offset. P1 indexes into the outer page table, while P2 indexes into an inner page table. The base register helps in identifying the starting location of these tables in memory. This method allows rapid access and more efficient reference to the necessary pages.
Examples & Analogies
Imagine you need to find a specific book in a multi-floor library. First, you refer to the directory to find out which floor (page number) a certain section (P1) is on. Then, once you get to that floor, there is another directory (P2) that helps you find the aisle and shelf (offset) where your book is located, allowing for quick and organized retrieval.
Challenges with Page Tables
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Now, two-level paging is not always sufficient. So, even two-levels paging is not sufficient for 64 bit computers. So, what do we do? And why is that so? Let us say if a page is of size 4 KB as we had previously, then in 64 bit computers the page table will have 252 entries.
Detailed Explanation
As system requirements have evolved, the limitations of a two-level page table become apparent, particularly with 64-bit architectures that can support extensive virtual address spaces. For example, if each page is 4 KB, then more entries are needed than what a two-level structure can accommodate, leading to the proposal of a three-level or even multi-level page tables to handle these demands.
Examples & Analogies
It's like a massive warehouse that has grown so large that your initial two business floors can't cover all your merchandise. You'd have to add multiple floors or sections (multi-level page tables) to effectively manage the overwhelming amount of items (address spaces) without getting lost.
Inverted Page Table Concept
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The next approach is the use of an inverted page table. Now, the in the main concept in inverted page table is the following: 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
Inverted page tables streamline memory-management by eliminating individual page tables for each process. Instead, there is a single table that tracks all physical memory pages, pointing out which virtual addresses correspond to which physical pages. This setup conserves memory space needed for page tables and minimizes fragmentation.
Examples & Analogies
Think of a master checklist of all the tools and equipment in a garage. Instead of keeping separate lists for each project (like a separate page table for each process), there is one comprehensive list that tells you where everything is located within the garage itself (an inverted page table), making it easier to find and use what you need.
Challenges of Inverted Page Table
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So, what is the advantage? It decreases memory needed to store page table, but increases time needed to search the page table when a page reference is made.
Detailed Explanation
While inverted page tables reduce the memory footprint by requiring fewer resources to maintain a single table, they introduce complexity in searching for the page frame number. Each memory reference requires a search through this larger table, which can slow down performance compared to direct indexing in traditional page tables.
Examples & Analogies
Imagine you have a single giant book where every list of contacts for many friends and family resides. While it takes less space than individual small address books for each friend, finding a specific contact now requires flipping through many pages instead of just looking them up in their dedicated book.
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: Efficiently manage memory through multiple levels of page tables.
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Two-Level Paging: Enhances memory management by breaking addresses into levels.
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Hashed Page Tables: Optimize address storage and retrieval using hash functions.
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Inverted Page Tables: Reduce memory usage by tracking physical memory pages.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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In a two-level page table, the first part of the address may point to the outer table, while the second part points to an inner table.
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An example of hashing: If a virtual page number hashes to an index in a hashed page table, it may point to a linked list of entries for collision resolution.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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Hierarchical trees help address space; with nested roots, no room to waste.
📖 Fascinating Stories
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Imagine navigating a library, where each section leads to subsections. This structure represents hierarchical page tables guiding us through complex addresses.
🧠 Other Memory Gems
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HPT - Hierarchical Page Tables compress Larger Address Spaces.
🎯 Super Acronyms
HASH - Hashed Allocation Simplifying Handling to enhance retrieval.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Hierarchical Page Table
Definition:
A method of page table organization that uses multiple levels of tables to manage virtual memory.
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Term: TwoLevel Paging
Definition:
A paging mechanism where the logical address is divided into two parts to access multiple page tables.
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Term: Hashed Page Table
Definition:
A page table that utilizes a hash function to store virtual page information to optimize memory use.
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Term: Inverted Page Table
Definition:
A page table structure that maps physical memory pages instead of creating separate tables for each process.