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Interactive Audio Lesson
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Introduction to Page Faults
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Today, we will explore page faults. A page fault occurs when a program tries to access data not currently in physical memory.
What happens when there's a page fault?
Great question! When a page fault happens, the system checks the page table to see if the reference is valid. If the valid bit is 0, meaning the page isn't loaded, we need to bring it from disk.
How does the OS know where to find the page?
The OS keeps track of where each virtual page is located on the disk using the page table. This is how it knows where to fetch the missing data.
Handling Page Faults
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Let’s discuss the steps the OS takes when a page fault occurs. First, it finds a free physical page frame in memory.
What if all physical page frames are full? Does it just crash?
No, if memory is full, the OS must implement a replacement policy, swapping out an existing page to make space, often referred to as page replacement.
Got it! So, it brings the page in and then updates the page table, right?
Exactly! After swapping in the new page, the OS updates the page table to mark the page as valid and then it restarts the instruction that caused the fault.
Example: Intrinsity FastMATH Architecture
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Let’s look at the Intrinsity FastMATH architecture as an example. It has a 32-bit virtual address space and 4 KB pages.
How does the TLB fit into this?
Great point! The TLB, or Translation Lookaside Buffer, stores recently used translations from virtual to physical addresses to speed up memory access.
What if there’s a miss in the TLB?
If there's a TLB miss, the system must access the page table, which adds time to the memory operation. It's important to have efficient handling for these situations.
Memory Hierarchy: Hits and Misses
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In a system with a memory hierarchy, we can have various combinations of hits and misses at different levels. Can anyone list the outcomes?
You might have a TLB hit and cache hit at the same time!
Good! What about when there's a page fault?
If there's a page fault, that means there's a miss in the page table, which means the data isn't in memory!
Exactly! Understanding these combinations is key to improving system performance.
Effective Instruction Time
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Finally, let's calculate effective instruction time when page faults occur. If m is the time without a fault and n with a fault, and faults happen every j instructions...
So we would essentially average the time over a batch of instructions?
Exactly! The formula helps us understand the impact of page faults on performance. Anyone can summarize it?
The effective instruction time is then a combination of both times averaged over the instructions!
Well done! This understanding helps quantify performance impacts due to page faults.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
In this section, the phenomenon of page faults is explored, focusing on what occurs when required data is not present in memory, the role of the operating system in resolving such issues, and the processes involved in bringing data from disk to memory. The section also discusses the complexities of the TLB and its interaction with page tables.
Detailed
Page Faults
This section covers the concept of page faults extensively. A page fault occurs when a program attempts to access data that is not loaded into physical memory. The operating system must first determine if the reference is valid or invalid, based on the page table entry. If valid but not in memory, the OS must find a free physical page frame. It then retrieves the required data from disk, updates the page table, and resumes execution of the instruction that was interrupted by the page fault.
The explanation also includes an example of the Intrinsity FastMATH architecture, discussing its TLB and cache components. The architecture features a 32-bit virtual address space with a 4 KB page size, and a TLB that is fully associative. In the context of the memory hierarchy, the section elaborates on different possible combinations of TLB, page table, and cache hits and misses, explaining their implications for system performance.
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Audio Book
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Understanding Page Faults
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So, now we will take a deeper look into page faults. So, during a page fault when I do not have the required data in memory, I incur a page fault. So, at that time the page table entry corresponding to the page that I want to access shows that it is invalid. That is the valid bit is 0; that means, the corresponding physical page number is not mapped to the page table entry.
Detailed Explanation
A page fault occurs when a program attempts to access data that is not currently located in physical memory. This is indicated by an entry in the page table showing that the relevant physical page number is not mapped, signified by a valid bit of 0.
Examples & Analogies
Imagine trying to access a book from a library that isn’t located on the shelves. The library catalog shows the book's location as invalid (for example, marked as damaged or lost), so you will need to check if it's available elsewhere before you can read it.
Invalid & Valid References
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In that case what happens is that; firstly, I need to decide whether this translation that I want to do is for an invalid reference or for a valid reference. If it is for an invalid reference, that means, that this particular virtual address is not present in my virtual address space itself. This can happen for scattered virtual address spaces of a process. If this virtual address is not part of the address space of the process itself, then this is an invalid reference and we immediately abort. Otherwise we see that the valid bit is 0 and therefore, the page is just not in memory. So, therefore, it has to be brought from the disk or the secondary memory.
Detailed Explanation
When a page fault occurs, the operating system checks if the address being accessed is valid. If the virtual address is not part of the process's address space, it is marked as an invalid reference, leading to immediate termination of the process. If valid but not in memory, the OS needs to fetch it from secondary storage.
Examples & Analogies
Consider ordering a menu item at a restaurant but being told that the item isn’t currently available. If you ordered something completely off the menu, it’s invalid, and the waiter can’t process that request. But if the item is just temporarily out of stock (a valid request), they would need to check the stockroom for it.
Finding a Physical Page Frame
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So, then what happens? I have to find a physical page frame; I need to find a physical page frame. So, if you see the sequences that are happening here I try to reference in number 1, I try to reference and I see that the corresponding mapping is not there; it is not there. Therefore, I will have to trap the OS; I will have to trap the operating system to indicate that this is a page fault.
Detailed Explanation
Once it's confirmed that the page is valid but not in memory, the operating system is alerted. It must find a free physical page frame in memory to load the required page. If no frame is available, the OS may have to replace an existing page, but this is not focused on in detail at this moment.
Examples & Analogies
Imagine needing to store a new piece of furniture but realizing your garage is full. You’d need to call the moving company (the OS) to help identify which item to remove (replace) to make space for this new piece (the page).
Swapping Pages from Disk
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The operating system will first find out what kind of trap has it received. So, it will then find that this is a page fault type of a trap. And therefore, then what the OS will do? It will find a physical page frame in the physical memory, it will find a physical page frame in the physical memory and then maybe through replacement of an existing page in physical memory which we will not bother right now, it will swap page into this frame via scheduled disk operation.
Detailed Explanation
Upon recognizing the page fault, the OS seeks a free physical frame. If no free frame exists, it might replace a page currently in memory. The required page is then fetched from disk to that frame via a scheduled operation, minimizing overall access delay.
Examples & Analogies
If a moving company is delivering a new piece of furniture (the data), they might first need to load a previous piece back onto the truck (swapping out), thereby making room in the garage (physical memory) before bringing the new piece inside.
Updating the Page Table
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After I have brought in this page I know the page number. So, therefore I have to reset the tables, I have to reset the tables to indicate that the page is now in memory. So, I have to update the page table entry corresponding to this page, corresponding to this virtual page, I will update the page table entry and write where what is the current physical page number corresponding to this virtual page number. I will also set the valid bit to indicate that the virtual this virtual page is currently now in physical memory.
Detailed Explanation
Once the needed page has been loaded into a physical frame, the OS updates the page table to reflect this. The valid bit is set to 1 to indicate that the page is now valid and present in physical memory, allowing the program to access it.
Examples & Analogies
Think of this as changing the label on a storage box after unloading it from the truck. You write down what’s currently inside the box (updating the page table) so that next time you look in it, you can tell immediately that it has the new item (data) without needing to check.
Restarting the Instruction
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Then I will restart the instruction that caused the page fault. So, I will then restart the instruction. So, in a particular memory access I had a page fault, then I service the page fault, then I will restart the memory and then in the subsequent memory reference I will get the data that I sought for in the physical memory.
Detailed Explanation
After handling the page fault, the instruction that originally triggered the fault is retried. This time, the necessary data is in physical memory, allowing for successful completion of the operation.
Examples & Analogies
Imagine that after finding that a book in your hand was not available, you go retrieve the book from storage, return to your reading nook, and then attempt to read the book again, this time finding it accessible and continuing smoothly.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Page Fault: An interruption in a program's execution due to unavailability of required data in memory.
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Page Replacement: The method of selecting a page to remove from memory when needing to load a new page.
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TLB: A cache for fast access to frequently accessed page table entries.
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Memory Hierarchy: The organization of various memory types in a computing system, optimizing access times.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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When a program attempts to access a page not currently in memory, a page fault is triggered, requiring the OS to fetch the page from disk.
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In a TLB miss scenario, if the required entry is not found, the OS must reference the page table, which can add latency to memory access.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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In memory's fault, don't pout or cry, the OS will fetch your page from the sky!
📖 Fascinating Stories
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Imagine trying to retrieve a book from a library where the title is unknown. You'll check catalogs to find the right shelf—this echoes how OS searches the page table in case of faults.
🎯 Super Acronyms
Think 'FIND'—Fault, Invalid, Necessary Data—actions the OS takes during a page fault.
Remember 'PF' for Page Fault, where P is for Process needing data, and F is for Fetching from disk.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Page Fault
Definition:
An event that occurs when a program attempts to access data that is not currently in physical memory.
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Term: Page Table
Definition:
A data structure used by the operating system to maintain the mapping between virtual pages and physical frames.
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Term: TLB (Translation Lookaside Buffer)
Definition:
A memory cache that stores recent translations of virtual memory addresses to physical memory addresses.
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Term: Physical Memory
Definition:
The actual RAM that holds data and programs currently being used by the computer.
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Term: Page Replacement
Definition:
The process of swapping out a page currently in physical memory for another page that needs to be loaded.