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15.2.3. Disadvantages of Physically Indexed Physically Tagged Cache
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Today, we're starting with the physically indexed physically tagged cache, or PIPT. Can anyone tell me what physical indexing means?
I think it means using actual physical addresses to access cache data.
Correct! In PIPT, we first translate virtual addresses to physical addresses using the TLB before accessing the cache. This allows for consistent updates if the page table remains unchanged.
"Doesn't that create a delay, though?
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Now, the first major disadvantage is increased latency. Can someone explain why this occurs?
Because we need to check the TLB first before accessing the cache.
Exactly! This means that even if the data is in the cache, we must still go through the TLB. What happens if we encounter a TLB miss?
Then we have to fetch the page table entry from main memory, which takes even longer.
Right. This serialization of access can significantly impact performance.
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Create a free accountMoving forward, let's discuss how the TLB being on the critical path affects cache access times. Why is this an issue?
Because it means that even when the data is in the cache, we still have delays.
Exactly! These delays exist even during cache hits, making the system less efficient overall. Moreover, what does it mean to have a valid cache entry under certain conditions?
It means that the data is still useful and accurate as long as the page table isn't modified.
Correct! However, should there be a modification to the page table, it would invalidate the cache and create potential access issues.
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Create a free accountIn conclusion, the PIPT cache design has both its pros and cons. Can anyone summarize the disadvantages we've discussed?
Increased latency due to TLB lookups and the potential for cache invalidation due to page table changes.
Exactly! To address these issues, architects have looked into virtually indexed caches and other structures. Can anyone name one?
The virtually indexed virtually tagged cache?
Precisely! These alternatives aim to solve the drawbacks we discussed today.
Overview
Short Summary
This section explores the disadvantages of physically indexed physically tagged caches, focusing on TLB access delays and potential memory inconsistency.
Medium Summary
In this section, we delve into the challenges posed by physically indexed physically tagged caches, specifically highlighting how TLB access introduces delays in data retrieval, even when the data is present in the cache. We also explore how these issues can lead to inefficiencies and the necessity for subsequent design solutions.
Detailed Summary
Disadvantages of Physically Indexed Physically Tagged Cache
The physically indexed physically tagged cache (PIPT) architecture operates by first translating virtual addresses to physical addresses via the Translation Lookaside Buffer (TLB) before accessing the cache. This design allows for the caching of data based on physical addresses rather than virtual addresses, ensuring that cache content is valid as long as the page table remains unchanged.
However, this architecture presents significant disadvantages:
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Increased Latency Due to TLB Lookup: The access to the cache becomes serialized with TLB lookups, resulting in additional cycles for data retrieval especially if there is a TLB miss, where the physical page number must be fetched from main memory. This increases overall access time, even in scenarios where the data is available in the cache.
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Critical Path with TLB: When the TLB is part of the critical path, even a cache hit can lead to delays due to the mandatory TLB validation, thereby reducing cache efficiency.
Despite these drawbacks, these caches offer the benefit of maintaining cache contents' validity under stable page table conditions. The need to improve efficiency led to alternative caching architectures, such as virtually indexed virtually tagged caches and subsequently, virtually indexed physically tagged caches, which strive to overcome the TLB serialization issue while addressing potential memory inconsistencies. This section underscores the importance of evaluating the cache access methodology's impact on overall system performance.
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Audio Book
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Create a free accountThe only problem with this architecture is that the TLB comes into the critical path of data access. So, suppose even if I have the data in cache, I have to go through the TLB and then obtain the physical address and then be able to access the cache, if the page is not present in the TLB, if there is a TLB miss, then we have to go to the main memory to fetch the page table entry required page table entry and get the physical address.
Detailed Explanation
In a physically indexed physically tagged cache, accessing data is slow due to the need to check the Translation Lookaside Buffer (TLB) first. Even if the data we want is in the cache, we still need to look up the physical address in the TLB. If there's a TLB miss (meaning the physical address isn't in the TLB), we have to access main memory to retrieve the page table entry. This extra step can slow down the entire process significantly, as cache access is normally much faster.
Examples & Analogies
Think of the TLB as a front desk at a library where you have to check in before going to the book section. Even if you know the book (data) is on the shelf (in the cache), you must ask the librarian (TLB) for the shelf number (physical address). If the librarian doesn’t know, you have to first go to the main archive (main memory) and consult the card catalog (page table entry) to find where the book is located.
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Create a free accountSo, even if the data is present in cache we may need to go into memory, because the page table entry corresponding to this data is not present in the TLB. So, there is, so the caches. So, this is the disadvantage of TLB lookup TLB lookup and cache access gets serialized and the cache takes greater than one cycle time and it may take multiple cycles, because if there is a TLB miss I need to go into the main memory assuming that the page table is stored in main memory in this architecture.
Detailed Explanation
If the required page table entry isn’t in the TLB, the whole process of accessing the cache is stalled because we can't obtain the needed physical address without first consulting the TLB. This results in a serialized lookup sequence where we have to wait longer, potentially causing multiple delays, particularly if we have TLB misses, meaning back-and-forth trips to the memory to get that needed information.
Examples & Analogies
Imagine you are trying to order a special dish at a restaurant where a waiter needs to check with the kitchen for a specific recipe. If the dish is not in the waiter’s list (TLB), they may need to run back to consult a lengthy recipe book (main memory), leading to longer wait times for your food (data).
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Create a free accountHowever, the advantage of this scheme is that cache contents remain valid. So, long as the page table is not modified, we will be able to appreciate this advantage a bit later when we go into seeing how this problem of this TLB being within the critical path of data access is solved.
Detailed Explanation
One of the benefits of using a physically indexed physically tagged cache is that as long as the page table is unchanged, the validity of the cache contents is maintained. This means that once data is placed in cache, there are fewer concerns about the data becoming outdated or invalid, which is advantageous for speed and efficiency in certain operating conditions.
Examples & Analogies
Consider a storage room where certain tools (data) can be stored neatly in boxes (cache). As long as the layout of the room (page table) remains the same, you can easily find any tool without worrying it's been moved or lost. However, if the room layout changes (modifying the page table), then you might have trouble locating your tools efficiently.
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
PIPT Cache: Explanation of the physically indexed physically tagged cache.
TLB Access: The process required to translate virtual addresses to physical ones.
Latency Issues: The problems caused by serialization of cache accesses.
Cache Invalidation: The necessity and consequences of flushing the cache.
Examples
Memory Aids
Interactive tools to help you remember key concepts
Stories
Flash Cards
Glossary
Physically Indexed Physically Tagged Cache (PIPT)
A cache where data is indexed using physical addresses, and the tags also correspond to physical addresses. Accessing cache requires prior translation from virtual to physical addresses.
Translation Lookaside Buffer (TLB)
A memory cache that stores recent translations of virtual memory to physical memory addresses to speed up address translation.
Cache Latency
The time delay in accessing data from the cache.
Cache Invalidation
The process of marking cache entries as invalid, typically due to changes in the underlying data structure.