Effective Access Time

Question

Consider a two level memory hierarchy L₁ (cache) has an accessing time of 10 nsec and main memory has accessing time 100 nsec. Assume the hit ratio read operation is 0.75 and 40% references are for write operation. The average access time for system (in nsec) if it uses write through technique ________. (Upto 1 decimal places)

Ans. 67.6

Comments

Please Help me around here:-

Please tell me where i am wrong and why i am wrong:-

Teff = 0.75 * (60 % * 10 + 40 % * 100 ) + 0.25 * ( 60 % * 110 + 40 % *100) = 61.

60 % -----> If its a read and 40% if its a write

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Made Easy Solution:- 

 

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Solution provided is wrong. You can see they have done (1-0.75)=0.35 instead of 0.25  for which the answer they obtained = 67.6

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Ohk i didn't saw that is my solution correct sir ?

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Teff = 0.75 * (60 % * 10 + 40 % * 100 ) + 0.25 * ( 60 % * 110 + 40 % *100) = 61.

I am getting the same answer but i am not sure how to interpret this line. It seems like you are doing like this ->

0.75 * (60 % * 10+ 40%*100)

When there is a hit, Read operation takes 60% time of the L1 time to access it and write ops take 40% of the memory time.

I do like ->

When there is a hit, read op takes 10ns (i.e. the time to access L1)  and when there is miss, it takes 110ns.   Hence, 0.75(10)+ 0.25(110)

For write operations, whether there is hit or miss does not matter as the effective memory access time = time to access main memory as it follows write through policy. So 100ns.

Now 40% of the ops are write and 60% are reads..So,

60%( 0.75*10+ 0.25*110 ) + 40%( 100)

 

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Well sir you are calculating seperately that's ok.

What i did is that here when there is hit. Now in hit we can have 40% of write and 60% of read and like wise in miss. Therefore i wrote:-

0.75 * (60 % * 10 + 40 % * 100 ) + 0.25 * ( 60 % * 110 + 40 % *100) = 61

By the way both are giving the same answer yours as well as mine i just wanna know that my approach is good or not :)

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Yes we will be getting same answers anyway..it was just that i wasn't getting your approach..

Now it is okay :)

And do you really think i am a teacher even after seeing this username? :P :P

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61 :/

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(1- Hit%) * (cache + main memory) , why is cache memory access time considered ?