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Interactive Audio Lesson
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Introduction to Hashed Page Tables
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Today, we will delve into hashed page tables, which are increasingly relevant in systems with larger address spaces. Can anyone tell me why managing memory efficiently is crucial?
I think it's because if we waste memory, the system can become slow, right?
Exactly! Efficient memory management ensures smooth operation. Now, hashed page tables help with this by storing virtual page numbers in a way that minimizes memory usage. Can anyone remember what a hash function does?
It converts input into a fixed-size string of bytes. So, it’s like turning a long name into an ID number!
Great analogy! This ID number helps us find the corresponding memory faster. In hashed page tables, each virtual page number is hashed to find its entry.
What happens if two virtual pages hash to the same location?
Good question! This situation generates a collision, and each entry in a hashed table contains a linked list to manage these collisions. So, we can still retrieve the correct frame data efficiently.
How does this method help with memory usage?
By storing only necessary entries for processes, thus freeing up space for other processes to use. And remember, hashed tables go beyond traditional methods by reducing the search time for page lookups.
Structure of Hashed Page Tables
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Now, let’s look at the internal structure of hashed page tables. Each entry contains three key components: the virtual page number, the mapped physical frame number, and a pointer.
What’s the pointer for?
The pointer connects to the next entry in case of a collision. By chaining the entries, we ensure all entries are accessible. Can anyone recall why we need to hash virtual page numbers?
To make searching for them easier and faster, right?
Absolutely! By hashing, we avoid searching through a bulky table. Instead, we can quickly navigate to potential matches. This efficiency is critical in larger systems. Let’s think deeper about how we ensure the entries remain relevant. What do you think?
Maybe we only keep the entries that are currently in use?
Exactly! We keep only what’s needed, optimizing memory for active processes. This is why hashed page tables are so effective in modern computing environments. Who can summarize our discussion today?
Hashed page tables reduce overhead by only storing necessary entries, using a hash function for faster lookup, and managing collisions with linked lists.
Well done! That’s a perfect summary.
Practical Applications and Limitations
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In what types of systems do we think hashed page tables shine?
Probably in 64-bit systems where addressable memory is vast?
Correct! They handle much larger address spaces than traditional tables. However, there are limitations. What can you think of?
If the hashed table grows too big or has a lot of collisions, it could slow down, right?
Exactly. While they are efficient, there’s always a trade-off. Can anyone think of a solution to manage large numbers of collisions?
Maybe adjusting the hash function or the size of the table?
Nice thought! We can apply better hash algorithms or expand the table size, keeping performance in check. So, today we’ve explored the brilliance and drawbacks of hashed page tables. Who can give me a final takeaway?
Hashed page tables effectively manage large address spaces but can struggle with performance due to collisions.
Exactly! A great observation!
Introduction & Overview
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Quick Overview
Standard
This section covers the concept of hashed page tables, which streamline memory translation by using a hash function to index virtual page numbers into a compact structure. This technique is especially beneficial for processes with large address spaces, reducing the conventional page table overhead while allowing efficient lookups using linked lists for collision resolution.
Detailed
Detailed Summary
The section introduces hashed page tables, a sophisticated method used to manage memory in systems with large address spaces, particularly 64-bit architectures. Unlike traditional page tables that map virtual addresses directly, hashed page tables utilize a hash function applied to the virtual page number to index into a more compact structure. This effectively reduces the memory overhead associated with large page tables.
When a virtual address is accessed, the system hashes the virtual page number to locate a corresponding entry. Each entry in this hashed page table holds a linked list of potential matches, allowing for efficient collision resolution when multiple virtual page numbers hash to the same index. Each element in the list contains the virtual page number, its corresponding physical frame number, and a pointer to the next entry in the list.
This technique allows systems to keep only the necessary page entries for processes currently in need, optimizing memory usage. However, it also emphasizes the ease of lookup due to the indexing mechanism and linked list usage, facilitating quick and efficient access to page frame information needed for memory operations.
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Introduction to Hashed Page Tables
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In 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. The virtual page number is now hashed into a page table.
Detailed Explanation
Hashed page tables are a method used to store information about virtual memory to optimize memory usage, especially in systems with large address spaces. Unlike traditional page tables that use a fixed structure, hashed page tables use a hash function to determine the location of entries. This is particularly useful for computers with a 64-bit architecture, where the address space is significantly larger and can lead to very large page tables.
Examples & Analogies
Think of a hashed page table like a library index system. Instead of storing all books (or entries) on a shelf in order, you can use a number to categorize books by genre. When you need a book, you quickly find its genre category using the number and locate it faster than if they were all on the same shelf.
Hash Function and Linked Lists
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This page table contains a linked list of elements hashing to the same location. Now, this logical address has this offset part and this is the 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
In hashed page tables, when a virtual page number is used, it is processed through a hash function to determine where to store or find the corresponding physical page frame. Each location in the page table might point to a linked list of entries, which allows for multiple entries to exist at the same position if their hash values collide. This linked list contains the necessary information to map the virtual page number back to its physical counterpart.
Examples & Analogies
Imagine a busy restaurant with a reservation system. Each time someone makes a reservation (akin to a virtual page number), the system notes it in a specific spot based on their name (the hash function). If multiple people have similar names, their reservation slips would be kept in a stack (the linked list) at that spot, making it easy to retrieve when they show up.
Accessing Physical Memory
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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
Each entry in the linked list of a hashed page table includes the virtual page number, the corresponding physical page frame number, and a pointer to the next entry in case of collisions. When accessing memory, the system uses the virtual page number to hash into the table, retrieves the linked list, and traverses it to find the correct matching entry to retrieve the corresponding frame.
Examples & Analogies
Continuing with the restaurant analogy, if you were looking for your reservation, you would give the host your name. The host would check the reservation book (hashed page table), and if they find multiple entries (the linked list), they would look through each one until finding the reservation that matches the name (virtual page number) you provided.
Advantages of Hashed Page Tables
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Although this one is a per process; so, I have a hash table for each process here. However, however I need to keep only as many virtual pages as are needed by the program at a given time.
Detailed Explanation
Hashed page tables are context-specific since each process can have its own hash table. This means that the memory footprint can be significantly reduced as the system only retains entries for active virtual pages that are currently in use. If a virtual page is not needed, it is not stored, leading to more efficient memory usage overall.
Examples & Analogies
Consider your smartphone apps. Only the apps you frequently use are kept active and available, while others are saved in a cloud or not loaded at all. This keeps your phone's workspace clean and efficient, allowing you to access what’s relevant quickly.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Hashed Page Tables: A method to efficiently store virtual page numbers in a smaller structure using hash functions.
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Memory Optimization: The strategy of retaining only relevant entries to enhance performance and save memory.
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Collision Management: Techniques used to resolve when two inputs lead to the same hash output, typically by using linked lists.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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Example 1: In a 64-bit computing environment, each virtual page number is hashed, optimizing retrieval from a potentially vast address space.
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Example 2: A process has virtual page numbers 0, 1, and 2. If both pages 0 and 1 hash to the same index, a linked list is formed to manage these entries.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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Hashing makes it neat and compact, page info easy, no need to unpack!
📖 Fascinating Stories
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Imagine a librarian finding books: instead of checking each shelf, she has a magic card that knows exactly where each book can be found, using shortcuts represented in her system of magic keys, much like the hashed page tables streamline references.
🧠 Other Memory Gems
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HAVE: Hashing (H) Always (A) Verifies (V) Efficiency (E) in memory use.
🎯 Super Acronyms
PCT
- Page Number (P)
- Collisions (C)
- Table Management (T) together form the hashed tables.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Hashed Page Table
Definition:
A memory management structure that uses a hash function to map virtual page numbers to physical memory addresses.
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Term: Hash Function
Definition:
A function that transforms input data into a fixed-size string or value, making it easier to index and retrieve data from a database.
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Term: Virtual Page Number
Definition:
The identifier for a page of data within a virtual address space utilized by a process.
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Term: Collision
Definition:
In hashing contexts, a collision occurs when two inputs produce the same hash value.
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Term: Linked List
Definition:
A data structure consisting of a sequence of elements, where each element points to the next to facilitate dynamic data storage.