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12.2.2. Structure of a Hashed Page Table
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Let's start with hierarchical page tables. They allow us to efficiently manage memory by breaking down the virtual address space into smaller, manageable parts. Why do you think breaking addresses into smaller segments could help?
Maybe it makes it easier to find where everything is in memory?
Exactly! By managing addresses in smaller segments, we can have more flexibility. For instance, two segments can be for a stack and a heap. Remember this: 'Segments add structure to chaos!'
How do you actually access these segments?
Good question! Each segment has its own page table. You can reference them separately. Let's think of them like drawers in a filing cabinet.
So if I want something, I just go to the right drawer?
Precisely! And if it’s not in memory? What do we do?
We’d have to fetch it from secondary storage?
Correct! We’ll summarize that hierarchical page tables allow for better organization and efficiency in accessing larger memory spaces.
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Now let's discuss hashed page tables. Why do you think they are particularly useful for 64-bit systems?
Could it be because they can handle larger address spaces more efficiently?
Absolutely! In hashed page tables, we use a hash function to index the virtual page numbers. What does this mean for the size of our page tables?
It should make them smaller since we only keep what we need?
Exactly! This is crucial because it reduces memory overhead. The entry will link to the page frame number and also connect to the next element in the chain if there’s a hash collision.
What happens when we have to look something up in this table?
Great question! When you access a virtual address, the system hashes the page number to locate the correct entry rapidly. Remember: 'Hashing is fast, finding is easy!'
So we don’t waste space on unused pages?
Exactly! When pages are not in use, they don’t take up space in our hash table. Let’s recap: hashed page tables optimize our memory access significantly!
Overview
Short Summary
This section discusses hierarchical page tables and introduces the concept of hashed page tables as a mechanism to manage large address spaces effectively.
Medium Summary
The section explains how hierarchical page tables can optimize memory management by breaking down logical addresses into smaller parts. It introduces hashed page tables for 64-bit systems, detailing how they work through hashing virtual page numbers into a compact table structure for efficient memory access.
Detailed Summary
Detailed Summary
In this section, we delve into the structure of hashed page tables as a means to efficiently manage page tables in larger address spaces, specifically in 64-bit computing systems. We start by discussing hierarchical page tables, wherein the logical address space is divided into multiple smaller page tables to optimize the size and management of the page table. This organization allows for expandable logical address spaces by breaking down addresses into parts that correspond to various levels of page tables. For instance, in a two-level page table structure, logical addresses are split into a page number that indexes the outer page table and an inner page that specifies a frame number.
As systems evolve, a two-level structure becomes inadequate for vastly larger address spaces, such as those seen in 64-bit architectures, requiring even more hierarchical levels. The text also introduces hashed page tables, where virtual page numbers are hashed into a page table structure that contains a linked list of page entries. This method allows the table to keep just those pages currently in use, reducing memory overhead considerably. Each entry within a hashed page table links to its virtual page number, the corresponding mapped page frame, and connects to the next entry, facilitating efficient memory access and management.
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Audio Book
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Create a free accountIn order to control the size of a page table, another technique that is used 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.
Detailed Explanation
Hashed page tables are a solution used to manage page tables efficiently, especially in systems with large address spaces. Instead of a large table containing entries for every virtual page, a hashed table allows you to effectively reduce the amount of memory used for storing these entries by only creating entries for physical pages that are currently in use. This method is particularly beneficial for 64-bit architecture, where the address space can be extremely large.
Examples & Analogies
Imagine a large library with thousands of books (representing virtual addresses). Instead of maintaining a detailed card catalog (large page table) for every book that might be put into storage (possibly never accessed), the library uses a hash table to keep a smaller number of cards (hashed page table) for only the books that are currently in circulation. This keeps the catalog manageable and easy to search.
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Create a free accountThe virtual page number is now hashed into a page table. This page table contains a linked list of elements hashing to the same location.
Detailed Explanation
In a hashed page table, the system takes the virtual page number from a logical address and applies a hash function to it. This hash function determines where in the hashed page table the entry for that page will reside. Each entry in the hashed page table may contain a linked list to handle collisions, which happen when multiple virtual page numbers hash to the same table entry.
Examples & Analogies
Think of a vending machine with several buttons. If you press a button (hash function), it may lead to a slot (table entry). However, if multiple people press the same button at the same time (collisions), the machine has to provide a queue (linked list) to serve everyone one by one.
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Create a free accountAt the location in the hash table, there is a linked list of elements that hash to the same location. Each element contains the virtual page number, the value of the mapped page frame, and a pointer to the next element.
Detailed Explanation
Each element in the linked list inside the hashed page table contains the virtual page number, the corresponding page frame number, and a pointer to the next element in the list. When a virtual address is accessed, the system hashes the virtual page number to find the entry, then it traverses the linked list to find a match. Once a match for the virtual page number is found, it retrieves the mapped physical memory frame number to read the required data.
Examples & Analogies
Consider a treasure map where 'X' marks the spot (the hashed entry). Each X can be linked to different treasure chests (elements in the linked list). If you want to find a treasure (data request), you first go to the region marked X (hashed entry) and then check each chest in that area to find the right one (matching elements in the linked list).
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Create a free accountUsing this hashing method allows the table to be smaller than traditional page tables. It keeps only those elements for which there are valid physical page frames.
Detailed Explanation
One of the main advantages of using hashed page tables is that they can significantly decrease the memory usage required for storing page table entries. Unlike traditional page tables, where every potential virtual page has an entry that may or may not be valid, hashed page tables only store entries for those pages currently mapped to physical memory, allowing for efficient use of resources without excessive overhead.
Examples & Analogies
Think of a bus service that only lists bus stops for routes currently in operation (hashed page table) instead of listing every stop in the city (traditional page table). This makes it easier for passengers to find routes and avoids cluttering the map with irrelevant information.
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
Hierarchical Page Tables: A method to efficiently manage memory through multiple levels of page tables.
Hashed Page Tables: A specialized page table structure using a hash function for quick access and reduced memory usage.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
Using hierarchical page tables helps expand the process's address space effectively when the logical address grows.
In a hashed page table, the mapping of virtual pages cuts down memory by only storing actively used pages.
Memory Aids
Interactive tools to help you remember key concepts