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A short class with Sarah and her students. The other ways are under More.
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Today, we'll discuss page faults. Can anyone explain what a page fault is?
Is it when the data we need isn't in memory?
Exactly! When data required isn't found in memory, we encounter a page fault. What happens next?
The operating system needs to check if the reference is valid or invalid.
Absolutely! If it's an invalid reference, we abort. If it's valid but not in memory, we need to load it from disk. This process can slow down our systems significantly.
How does the OS know where to find the data on the disk?
Great question! The page table contains the address of the required data in secondary memory. This is crucial for bringing the page into memory.
So there are multiple steps for handling a page fault?
Exactly! First, we identify a free page frame, then fetch the required page, update the page table, and finally restart the instruction.
In summary, page faults require a structured response from the OS involving various steps to ensure we access the needed data.
Let's dive deeper into the Intrinsity FastMATH architecture. Can anyone share the page size and address space it uses?
It has a 4 KB page size and a 32-bit virtual address space.
Correct! The architecture has a fully associative TLB. Why do you think this is important?
It helps in quickly accessing both instructions and data without checking multiple places.
Exactly! During a TLB hit, we can avoid looking into the page table, saving time. Can you explain how the TLB works?
The TLB matches the virtual page number with its entry to find the corresponding physical page number.
Right! Once matched, the TLB helps generate a physical address used to access the cache directly. Remember, accessing the cache effectively is key to performance.
In essence, the TLB and cache work together, reducing access latencies significantly when functioning correctly.
Now, let’s explore hits and misses across our memory hierarchy. What can you tell me about the types of hits and misses?
There are TLB hits, page table hits, and cache hits, right?
Exactly! And what happens when one of them misses?
If the TLB misses, we check the page table for the needed data.
And if the page table misses too, we have to load the page from secondary storage.
Correct! It's essential to remember that if there's a miss higher up, it can lead to further misses downstream. What combination scenarios can you identify?
We could have a TLB miss and a page hit with a cache hit.
Great observation! Understanding these combinations is crucial for anticipating performance bottlenecks in systems.
To summarize, knowing how to handle hits and misses across TLB, page table, and cache effectively is vital for system performance.
An essential event signaling the need to load a page into memory.
Differentiates between legitimate access attempts versus non-existent memory references.
Understanding how these components speed up memory access.
Recognizing the combinations that can arise in memory requests.
If a page fault comes to strike, the OS quickly takes a hike. Valid or not, it must check, bringing data back into check.
Imagine a librarian (the OS) finding a book (data) in a large warehouse (disk) when it is not on the shelf (memory). It quickly checks the catalog (page table) to find its location, retrieves it, and puts it back on the shelf.
Remember TLB as 'Translation Light Buffer' – it brightens the way for memory access!
Remember 'PCT' for Page fault, Cache hit, and TLB miss to track memory events!
Term
What triggers a page fault?
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Term
What is the function of a TLB?
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An event that occurs when a program tries to access a page not currently mapped in physical memory.
A cache that stores the recent translations of virtual memory to physical memory to speed up access.
The actual RAM where data is stored and accessed by the CPU.
A data structure that maps virtual addresses to physical addresses.
An event where the required data is found in the cache, resulting in faster access.
An event where the required data is not found in the cache, necessitating access to slower memory levels.
The section discusses the occurrence of page faults and the types of hits and misses in a memory hierarchy.
This section delves into page faults, differentiating between valid and invalid references, and explains the system's response to such faults. It elaborates on the memory hierarchy's functionality, emphasizing hits and misses across different layers, such as TLB, page table, and cache.
In this section, we analyze the concept of page faults, which occur when the required data is not present in memory. A page fault triggers the operating system to determine whether the access attempt was valid or invalid. A valid access request with a valid bit set to zero indicates that the page must be loaded from secondary storage. This process involves several steps, including finding an available physical page frame, accessing page table entries, and handling disk operations, all of which contribute to the latency of accessing memory.
Following this, the section presents practical examples, specifically focusing on the Intrinsity FastMATH architecture. The architecture uses a 4 KB page size with a 32-bit virtual address space and includes a fully associative Translation Lookaside Buffer (TLB) shared between instructions and data. The mechanics of cache access upon a TLB hit are explained, detailing how the physical address generated can be used to index the cache effectively.
Finally, the section covers the different types of hits and misses that can occur within a memory hierarchy involving TLB, page table, and cache, including valid combinations of these hits and misses. The overall timing impact of these events is also discussed with calculations for effective instruction time considering page faults.