GATE CSE 2022 | Question: 14

Question

Let $\text{WB}$ and $\text{WT}$ be two set associative cache organizations that use $\text{LRU}$ algorithm for cache block replacement. $\text{WB}$ is a write back cache and $\text{WT}$ is a write through cache. Which of the following statements is/are $\text{FALSE}?$

• A. Each cache block in $\text{WB}$ and $\text{WT}$ has a dirty bit.
• B. Every write hit in $\text{WB}$ leads to a data transfer from cache to main memory.
• C. Eviction of a block from $\text{WT}$ will not lead to data transfer from cache to main memory.
• D. A read miss in $\text{WB}$ will never lead to eviction of a dirty block from $\text{WB}.$

Answer 1

Option: A, B, D

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WB Cache: Updates to the cache block don’t result in Updates to the Main Memory immediately. Burst Writes are preferred for higher throughput and those needing higher write performance.

WT Cache: Updates to the Cache block are reflected on Main Memory before carrying out other processes. Consistency is preferred.

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• A. A WB must necessarily have a dirty bit to avoid redundant writes to Main Memory. Whereas a WT cache needn’t as the change is reflected after a write. False
• B. A WB cache’s primary use is to increase throughput or useful work. Multiple writes to the same cache block will not be reflected immediately, avoiding unnecessary data transfer time. False
• C. WT cache’s main goal is to prefer consistency overwrite performance, and a cache block is made to reflect the current main memory. True
• D. LRU doesn’t have specific replacement strategies for dirty and regular blocks. Hence a read miss might evict a dirty block. False

Comments

References for option A to be false

http://www.cs.cornell.edu/courses/cs3410/2013sp/lecture/18-caches3-w.pdf (page 11)

http://15418.courses.cs.cmu.edu/spring2017/lecture/cachecoherence1/slide_003

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what is dirty bit?

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Aaryan_Sharma

it is maintained to keep track of the updations in a cache block if dirty bit is set it means that block is modified and hence the resulting changes needs to be reflected back into the memory .

https://en.wikipedia.org/wiki/Dirty_bit#:~:text=A%20dirty%20bit,simply%20be%20.

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@Deepak Poonia Sir, need a better explanation for this question.

Got this from PowerPoint Presentation (cornell.edu)