Unacademy Practice Question

Question

Consider a 4 way set associative cache of size 16 KB organized into 4 words block. Cache memory is designed with the write back protocol having the miss ratio of read and write operations as 30% and 40% respectively. The tine taken by CPU to access data from cache memory is 10 ns and Main memory access time is 460 ns. Processor generates 50% read requests and 50% write requests. What is the average memory access time considering both read and write operations ?

Comments

Is the answer 171nsec?

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@Sunnidhya Roy I honestly don't know the answer myself. The question was for write throgh cache originally but i edited the question and replaced the word "write throgh" with "write back" because i was curious what will happen if it was write back cache

Please share the solution if you have got the answer.

Answer 1

Miss rate of Read = 0.3 and Hit rate of Read = 0.7

Miss rate of Write = 0.4 and Hit rate of Write = 0.6

50% are read requests and rest 50% are write requests.

Cache Memory Access time = 10ns and MM access time = 460ns.

Average Memory Access time when using Write Through protocol

$= 0.5(0.7(10) + 0.3(10 + 460))ns + 0.5(0.6(460) + 0.4(460))ns = 304ns$

Average Memory Access time when using Write Back protocol

 $= 0.5(0.7(10) + 0.3(10 + 460))ns + 0.5(0.6(10) + 0.4(10+460))ns = 171ns$

 

Comments

@Sunnidhya Roy correct ur solution incase of write through protocol in case of write the  effective access time is T(write)=Mainmemory access time 

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@Chandrabhan Vishwa 1 Thanks for pointing it out, totally overlooked the parallel access in case of write through. Is it correct now?

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@Sunnidhya Roy  yes right