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A short class with Sarah and her students. The other ways are under More.
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Today, we’re going to learn about page faults. Can anyone tell me what a page fault is?
Isn’t it when a program tries to access a page in memory that isn’t loaded?
Exactly! So when a virtual address is requested and the corresponding page isn’t present in physical memory, we have a page fault.
What happens after that?
The operating system must bring that page from secondary storage into physical memory. This process is relatively slow, because accessing secondary storage takes millions of nanoseconds.
And how does that affect performance?
Great question! The delay caused by page faults can substantially slow down system performance.
To remember this, think of the acronym 'FLAT'—Fault, Load, Access, Time—highlighting the process when a page fault occurs.
In summary, page faults disrupt the flow of data access and can lead to performance bottlenecks in computing systems.
Now that we understand page faults, let’s discuss page sizes. Why do you think larger page sizes are beneficial?
Maybe because we can bring more data at once from secondary storage to avoid multiple accesses?
Exactly! Larger page sizes help to amortize the high cost of accessing secondary storage, as they bring in more data at once, thus reducing the chances of a page fault.
What about embedded systems? Do they use larger pages too?
Good question! Embedded systems usually use smaller page sizes, around 1 KB. This helps manage their limited memory resources while avoiding excessive internal fragmentation from larger pages.
Remember the mnemonic 'SPLASH'—Smaller Pages for Limited Access in Storage & Hardware—to relate page sizes to their application contexts.
In summary, selecting the right page size is crucial for optimal memory management.
Next, let's explore how system organizations can reduce page fault rates specifically in virtual memory.
What’s one common method?
We often use fully associative memory placement. This allows any virtual page to occupy any physical page frame, which minimizes page faults.
I see, but what’s the downside?
Great observation! While searching for where a virtual page resides becomes more complex and requires more hardware, it’s still less costly than handling page faults.
For a mnemonic, think 'FAST'—Fully Associative Search Technique for memory organization.
To summarize, efficient memory placement strategies like fully associative mapping play a key role in reducing page faults.
Lastly, let’s delve into page tables. What do you know about them?
They map virtual pages to physical pages, right?
Correct! Each process has its own page table that keeps track of this mapping.
What happens during a context switch?
During a context switch, the page table register is populated with the starting address of the new process's page table, enabling the CPU to access the right memory.
Remember the acronym 'MAPS'—Mapping Addresses in Pages Structure— to reinforce the function of page tables.
In summary, page tables are crucial for managing virtual memory, enabling processes to efficiently interact with physical memory.
Occurs when a requested page is not in physical memory.
Slower memory option where data is retrieved during page faults.
Larger sizes can reduce the frequency of page faults.
Maps virtual pages to physical pages for effective memory management.
Reduces write times by postponing updates to secondary storage.
When a page is far away, a fault we must convey, from disk to RAM, our data’s way!
Imagine a busy library, where books are our data. If a book isn’t on the shelf (physical memory), a librarian (OS) must fetch it from a storage room (secondary storage), taking time and thus causing a delay.
FLAT—Fault, Load, Access, Time—to remember the steps when handling a page fault.
MAPS—Mapping Addresses in Pages Structure relates to how page tables link virtual to physical addresses.
Term
Page Fault
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Term
Secondary Storage
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Term
Page Size
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Term
Write-Back Cache
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Term
Page Table
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An event that occurs when a program accesses a virtual memory page that is not currently mapped to physical memory.
The actual hardware memory (RAM) where data is temporarily stored for rapid access.
A memory management capability that provides an 'idealized abstraction' of the storage resources that are perceived by users.
A data structure used to map virtual address space to physical addresses.
The wasted space within an allocated page that is not used by the process.
An optimization strategy that postpones writing modified data back to storage.
A memory organization technique where any block can be loaded into any cache line.
Page faults occur when a virtual page number does not have a corresponding physical page, requiring data retrieval from secondary storage.
This section explains what page faults are, their impact on system performance, and strategies to minimize their occurrence. It covers aspects like page size, the penalty of page faults due to storage access times, and the organization of memory to reduce fault rates.
Page faults occur when a virtual address requests information that is not currently loaded in physical memory. The system must access slower secondary storage to retrieve the necessary data, resulting in significant delay. The access time to physical memory is in the range of 50-70 nanoseconds, while fetching from secondary storage can take millions of nanoseconds, leading to a considerable page fault penalty.
To mitigate these high penalties, larger page sizes (commonly between 4 KB and 64 KB) are used to reduce the frequency of page faults by increasing the locality of reference. In embedded systems, smaller page sizes (around 1 KB) are utilized to manage resource constraints.
The section also elaborates on memory organization, emphasizing the advantages of a fully associative placement of pages to minimize misses, the management of page tables linked to each process, and efficient replacement algorithms to reduce page faults. Techniques such as write-back caching are also discussed as cost-effective methods of memory management. Lastly, the section introduces page tables as the data structure responsible for mapping virtual pages to physical pages and conducting memory management during context switches.