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Interactive Audio Lesson
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Introduction to Hierarchical Page Tables
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Welcome, everyone! Today, we will explore hierarchical page tables, which allow us to tackle the problem of fixed-size page tables. Can someone tell me what a page table is?
A page table keeps track of where the virtual pages are located in physical memory.
Exactly! Now, what happens if our virtual address space is very large?
The page table can get very big, making it less efficient.
That's right! Hierarchical page tables help by splitting the logical address space into multiple tables. For example, in a two-level page table, how is the address divided?
It's broken down into two parts, P1 and P2. P1 points to the outer table and P2 to the inner.
Wonderful! Remember, we can think of this like a directory that leads to other directories. Let's summarize: hierarchical page tables reduce size and improve memory management by allowing independent paging. Can anyone summarize how this benefits modern systems?
It allows better sharing of main memory and makes it easier to manage larger address spaces!
Digging Deeper into Hashed Page Tables
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Now, let's move to hashed page tables. Can anyone explain what hashed page tables are used for?
They are used for larger address spaces, particularly over 32 bits.
Exactly! In this method, how do we find the corresponding physical page?
We hash the virtual page number, right?
Correct! Each entry then contains a linked list for elements that hash to the same location. Why is this structure advantageous?
It reduces memory usage by only keeping entries for the pages that are currently needed!
Great job! Remember, this approach optimizes searches through the page table. Let's recap: hashed page tables help manage large address spaces efficiently and make address resolution faster. How does this differ from traditional methods?
Traditional methods keep everything in a single table, making it bloated and slow.
Inverted Page Tables
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Finally, let's delve into inverted page tables. Who can explain what an inverted page table is?
Instead of having one page table per process, it keeps track of physical pages in memory.
Exactly! This means the table is indexed by physical memory addresses instead of virtual ones. What’s the main benefit of this approach?
It reduces the amount of memory needed for storing page tables and allows for easier management!
Great point! However, is there a trade-off with this method?
Yes, it could potentially increase the time it takes to search through the entries.
Correct! To mitigate this, we can also use a hash table. So, to wrap up, inverted page tables provide size efficiency but at the cost of search time. Can anyone think of scenarios when this would be beneficial?
When dealing with a large number of processes where memory is scarce!
Introduction & Overview
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Quick Overview
Standard
The section discusses the evolution of page tables from a two-level structure to more advanced techniques like hashed and inverted page tables. It explains how hashed page tables enable efficient searching in larger address spaces, especially in modern systems. It emphasizes how these methods improve memory usage and processing speed.
Detailed
Detailed Summary
This section focuses on advanced page table techniques, specifically hierarchical page tables and hashed page tables. It begins by explaining hierarchical page tables, where the logical address space is divided into multiple tables to address memory more efficiently. The importance of segmenting address spaces to facilitate processes growing in multiple directions is first highlighted, allowing page table size reduction.
Hierarchical Page Tables
Hierarchical page tables allow for splitting the logical address space into multiple page tables, often described in the context of a two-level page table system. In this setup, the logical address is divided into multiple segments, referred to as P1 and P2. Here, the outer page table is indexed by P1, which points to inner page tables indexed by P2. This method improves overall memory sharing since each page table can be paged independently, allowing better management of main memory.
Hashed Page Tables
The second part of the section introduces hashed page tables, a method designed to handle large address spaces that are typically over 32 bits. In this structure, the virtual page number is hashed to find the corresponding physical page. Each entry in the hashed page table contains a linked list that includes entries with the same hash. This innovation allows virtual pages that are only needed at a specific moment to reside in the hash table, reducing memory requirements significantly compared to traditional page tables.
Conclusion
The section concludes with the description of inverted page tables, which index physical memory instead of logical address spaces, thereby streamlining memory management. This collection of techniques showcases the evolution and adaptation of page tables to meet modern computing needs.
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Audio Book
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Introduction to Hashed Page Tables
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In order to control the size of another technique that is used to control the size of a page table is by using a hashed page table. It is commonly used for address spaces greater than 32 bits. So, this is more prevalent for 64 bit computers. The virtual page number is now hashed into a page table.
Detailed Explanation
Hashing helps in managing large page tables by using a hash function to index virtual page numbers. When a virtual address is needed, the virtual page number part of that address is processed through a hash function, resulting in a specific index in the page table, which is designed to take up less memory and be quicker to search than traditional page tables.
Examples & Analogies
Imagine searching for a name in a large directory: instead of looking through each name one by one (which would take a long time), you could use initials to jump to a section where names starting with those initials are stored. This is akin to how hashing works with page tables.
Structure of the Hashed Page Table
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This page table contains a linked list of elements hashing to the same location. Now, this now what we have this logical address has this offset part and this is the virtual page number, virtual page number. Now, this virtual page number part of the virtual address or logical address is used as a hash function; this virtual page number is used as a hash function and it hashes into the page table.
Detailed Explanation
Each entry in the hashed page table can point to a linked list of all the entries that have the same hash value. This is necessary because multiple virtual page numbers might hash to the same index. The elements in that linked list will then be searched for a match with the required virtual page number.
Examples & Analogies
Think of it like a basket of fruits: if you hash all apples into one basket, when you need an apple, you only check in that basket instead of searching through many types of fruits. You still need to check each apple in that basket, but it’s a smaller set to look through.
Extraction of Physical Addresses
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Each element contains what the virtual page number, the value of the mapped page frame. So, this is the frame number, this is the page number virtual page number, this is the page frame number page frame number virtual page number and the third part is a pointer to the next element.
Detailed Explanation
When a virtual address is needed, the hash table is accessed using the hashed virtual page number. Then the linked list at that index is traversed to find the specific entry that matches the requested virtual page number. If a match is found, the corresponding physical frame number is retrieved.
Examples & Analogies
Imagine you’re at a library searching for a book by title. First, you ask the librarian which shelf to go to (that's the hash function). Next, you check that shelf (that's the list of linked entries). Finally, you find a specific book on that shelf (this is your physical frame number).
Handling Physical Memory Pages
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However, however I need to keep only as many virtual pages as are needed by the program at a given time. So, if I hash to this place and these elements will only contain those element will only be there which actually contains a physical page frame number here.
Detailed Explanation
The hashed page table only maintains entries for virtual pages that are currently mapped to physical memory. This means if a certain virtual page is not in use, it will not end up in the hash table, making it more efficient and reducing the required memory space.
Examples & Analogies
This is similar to a restaurant that only lists items currently available on its menu. If a dish is out of stock, it won’t be on the menu. Similarly, only virtual pages that are actively occupying physical memory spaces are listed in the hashed page table.
Efficient Searching using Hashing
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This hashing makes it easy because hashing will actually get it through a through a through a small number through a small search I can actually get to the, get to the entry which will contain my page frame which get to the element which will contain my page frame due to the hashing that I have.
Detailed Explanation
Because the entries are indexed using a hash function, finding the right page frame does not require a full search of all entries. Hashing minimizes the search effort, allowing the system to quickly locate the needed physical memory address without unnecessary overhead.
Examples & Analogies
Consider a fast-food drive-thru where you place your order using a number. Instead of explaining your order to each person in line, using a number gets you straight to the right person preparing your meal, thereby speeding up 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: A memory management structure that improves efficiency by using multiple tables to track pages.
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Hashed Page Tables: An optimization technique for faster access in large virtual address spaces by hashing page numbers.
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Inverted Page Tables: A method for reducing memory requirements by indexing physical memory instead of virtual memory.
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 outer table indexes the base addresses of inner tables, allowing efficient access.
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In a hashed page table, when a virtual page number is hashed, it leads to a linked list, simplifying memory management and reducing space.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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In tables that are stacked, pages are tracked, to keep our memory intact!
📖 Fascinating Stories
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Imagine a library where books are not on shelves but in blocks; a hash table pulls titles into focus to find the stories we want to access quickly.
🧠 Other Memory Gems
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H for Hierarchical, H for Hash; both help with memory - not just a flash!
🎯 Super Acronyms
HPT for Hierarchical Page Tables - think of H as 'Help' in memory management!
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Hierarchical Page Tables
Definition:
A structure that divides the logical address space into multiple page tables to improve memory management.
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Term: Hashed Page Tables
Definition:
A method where the virtual page number is hashed to find the physical page, improving memory usage.
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Term: Inverted Page Table
Definition:
A page table that indexes physical memory instead of logical, simplifying memory management.
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Term: Virtual Page Number
Definition:
A number representing a page in virtual memory, used for addressing in page tables.
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Term: Page Frame Number
Definition:
A number representing a physical address, where a page is stored in memory.