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Interactive Audio Lesson
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Understanding Page Faults
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Today, we're going to learn about page faults, which are significant in memory management. Can anyone tell me what a page fault is?
Is it when the data we need isn't currently in memory?
Exactly! When we try to access data that isn't in memory, we incur a page fault. This prompts the operating system to find the required data.
What does the operating system do in that case?
Great question! The OS identifies whether the reference is valid or invalid. If invalid, it aborts the process; if valid but not in memory, it locates the required data in secondary storage.
So what happens next?
The OS needs to find a physical page frame to load the required data, which involves several steps. Let's summarize what we've learned about page faults.
Navigating the Page Fault Process
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After a page fault is triggered, what's the first step the OS takes?
It checks if the reference is invalid or valid?
Correct! If the reference is valid and the data is missing, the next step is to find a free physical page frame.
How does the OS find the physical page frame?
The OS will perform a check to see if any frames are free, and it may also replace an existing page. Then, it retrieves the required page from the disk.
What if the disk is busy?
Good point! The OS follows a queue to manage disk operations, which can incur seek time and latency. Remember, managing these processes efficiently is key to optimizing performance.
Effective Instruction Time Calculation
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Now let's discuss how page faults affect instruction time. What's the basic idea behind effective instruction time?
It’s the average time that includes both hits and misses, right?
That's exactly right! If we consider that a page fault occurs every 'j' instructions, we can calculate the effective instruction time with a formula.
What does that formula look like?
For j-1 instructions, we add the time taken for hits and then add the time for the instruction causing the fault. Thus, the total effective instruction time combines both cases.
So, how do we express that mathematically?
"The effective instruction time can be calculated using:
Introduction & Overview
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Quick Overview
Standard
In this section, we delve into page faults, their handling by the operating system, and how to calculate effective instruction time in the presence of page faults. Key concepts such as TLBs and memory access patterns are highlighted in the context of performance evaluation.
Detailed
Detailed Summary
This section explores the intricacies of page faults and their implications in memory management. A page fault occurs when a required data is not present in memory, leading to the operating system's intervention. The valid bit in a page table entry indicates whether a virtual address corresponds to a valid physical address. If invalid, the OS aborts the reference; if valid but not in memory, the OS must load it from the secondary storage.
The process of handling a page fault includes steps such as trapping the OS, identifying free physical frames, and scheduling disk operations to retrieve the required page. Additionally, the section introduces the concept of TLBs (Translation Lookaside Buffers), their structure, and how they enhance memory access by caching page table entries.
Finally, the effective instruction time is discussed, explaining how the occurrence of page faults affects performance by calculating average instruction time in the presence of hits and misses.
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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 the requested data is not currently in the computer's memory. The page table keeps track of which pages are in memory. If a page is not in memory, its entry in the page table has a 'valid bit' set to 0, indicating that this page can't be accessed.
Examples & Analogies
Think of the page table like a library catalog that tells you where books are stored. If the catalog shows that a book is not in the library (valid bit is 0), you need to go find it in a remote storage, much like requesting a book from another library.
Invalid vs. 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 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.
Detailed Explanation
Before addressing the page fault, the operating system decides if the reference is invalid or valid. If the address isn't found in the process's virtual address space, it's an invalid reference causing the operation to abort. However, if the valid bit indicates the page exists but isn't in memory, this leads to further actions to load it.
Examples & Analogies
Imagine trying to access your friend's home address that you wrote down incorrectly. You would realize the address (virtual address) doesn't exist, causing you to cancel your trip. If the address did exist but you simply couldn't find it because they weren't home (the page isn't in memory), you'd need to look for them elsewhere.
Operating System's Role in Page Faults
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So, therefore, it has to be brought from the disk or the secondary memory. So, then what happens? In 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
When the system identifies a page fault, it must locate a physical page frame in memory where the required page can be loaded. The operating system is notified of the fault to manage this process properly.
Examples & Analogies
Think of the operating system as a librarian. When a book is not found on the shelf (page fault), the librarian checks to see if a shelf is empty (finding a page frame) and then requests the book from storage (loading the page from disk).
Retrieving the Required Page
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Now, when I have found a free page frame in physical memory I will need to bring the page that I require from the backing store or the secondary memory into the physical memory. And I said that how do we get that? In a earlier class we said that in addition to the mapping of a virtual memory corresponding to virtual memory and a physical memory, physical page number the I also the page table additionally logically maybe in a different physical table. It also contains where in the backing store I have this page.
Detailed Explanation
Once a free page frame is found in physical memory, the required page is retrieved from secondary memory. The page table not only maps virtual memory pages but also keeps track of where each page resides in secondary storage.
Examples & Analogies
Continuing with the library metaphor, once the librarian finds an empty shelf (page frame), they then know which storage room (secondary memory) to retrieve the missing book (page) from.
Handling Disk Operations
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So, when I get a trap the OS finds out firstly, the physical frame where which is free and where I can give get the page into the physical memory where I can bring it and then it finds out where in the in the secondary memory the page is corresponding to this virtual page number. Then it gets the page from the secondary memory through a scheduled disk operation.
Detailed Explanation
The operating system orchestrates the process of identifying the free physical frame and locating the page in secondary memory. The retrieval occurs as a scheduled disk operation, which may involve waiting due to disk latency.
Examples & Analogies
Imagine organizing a retrieval system where books have to be fetched from a remote library (disk). The librarian schedules time to gather these books, but they may face delays due to travel time (disk latency).
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 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 page is retrieved and loaded into the physical memory, the page table must be updated to reflect that the page is now valid and its position in memory. This update ensures future references can access the page correctly without causing another fault.
Examples & Analogies
After the librarian retrieves a book, they must update the library catalog to show this book is now on the shelf and ready for readers, ensuring no more requests for that book lead to confusion.
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 the necessary page has been loaded into memory and the page table updated, the instruction that caused the fault can be retried. The data should now be accessible without causing another page fault.
Examples & Analogies
Once the book is returned to the shelf and the catalog is updated, the patron can go back and check out the book without any issues, just as the program resumes smoothly after handling a page fault.
Effective Instruction Time Calculation
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If an instruction takes time 𝑚 if there is no page fault and time 𝑛 if there is a page fault, what is the effective instruction time if page faults occur every 𝑗 instructions? So, we see that time 𝑚 when page hit, I have a page hit and time 𝑛 when I have a page fault ok. So now, I am saying that in a group of 𝑗 instructions I have one miss and the all other are hits.
Detailed Explanation
This chunk explains how to calculate the average instruction time, considering page faults. When j instructions are processed, if one instruction leads to a page fault (time n) and the others do not (time m), the average instruction time is calculated based on the ratio of hits to faults.
Examples & Analogies
Imagine you're watching a movie (processing instructions) where every once in a while you need to pause it to get snacks (page fault). If most of the time goes smoothly except for those pause moments, the average time spent on watching the movie with pauses can be compared to the time only watching the movie without interruptions.
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 event leading to OS intervention when data is not in memory.
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TLB: A cache that speeds up memory access by storing recent translations.
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Valid Bit: Indicates if a virtual page is present in memory.
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Effective Instruction Time: Average time taken per instruction considering page faults.
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 data from a page that is not loaded in physical memory, a page fault occurs, prompting the OS to locate and load the required data.
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In a system with an instruction that takes 5 ms to execute without failures and incurs a page fault every 20 instructions, the effective instruction time can be calculated using the given formula.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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Page faults can lead to a halt, when needed data won't be found in vault.
📖 Fascinating Stories
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Imagine a library where a book is checked out. When someone requests it, the librarian has to go find it in another building. This process is similar to handling a page fault.
🧠 Other Memory Gems
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Remember P.T. for Page Table, V.B. for Valid Bit to recall key concepts.
🎯 Super Acronyms
P.F. for Page Fault
- P: = Physical frame needed
- F: = Find on 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 a page that is not currently mapped to physical memory.
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Term: TLB
Definition:
Translation Lookaside Buffer, a memory cache that stores recent virtual-to-physical address translations.
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Term: Physical Page Frame
Definition:
A block of physical memory where a page can be loaded for accessing data.
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Term: Valid Bit
Definition:
A flag in the page table that indicates whether a page is present in physical memory.