Write Trough

Question

Do we consider hierarchical model or simultaneous access model for write through ? 

Comments

Fine. What would be the ans if WRITE BACK policy used with block size 4 words and write hit 0.6 ? Can you please let me tell.

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 Is this the case for write thruogh..??

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Yes if hit ratio for write is one it would be simultaneous mode . If nothing is given then by default take hierarchical mode

Answer 1

We will go with Hierarchical Model in Write-through,

Tavg= Avg time for Read + Avg time for Write

       =0.75(45+ 0.1*750)+ 0.25(1*750)

       =90+ 187.5

       =277.5 ns

Comments

Thank you. What would be the ans if WRITE BACK policy used with block size 4 words and write hit 0.6 ?

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I think you missed 45*0.9 in calculations.

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I have written in short form only, you can see the below calculation.Hope, it will help you

Avg Time for Read=0.9*45+0.1*(45+750)

                              =0.9*45+0.1*45+0.1*750

                              =45(0.1+0.9)+0.1*750

                              =45+0.1*750

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they have mention the hit of write though case is 1 means no page in the cache to write it back to main memory and hence we can update the cache and main memory simultaneously.

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Many people use to solve this type of question in short cut way.The problem is that they end up in trap or raised debate.Thank you for this particular solution.Logic is clear.We can easily understand that the implementation of formula.

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as we use AMAT in write through

AMAT = L1 HIT TIME + L1 MISS RATE * L1 MISS PENALTY

AS IN L1 HIT TIME = 45 NS

AND MISS RATE = 1-HIT RATE = 1-1 =0 SO WHY WE MULTIPLY WITH 750 ?

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<p>As hit ratio is one in case of write through it means updating to main memory and cache is parallel .</p>

Answer 2

= 0.75 * ( 0.9 * (12) + 0.1 * (750)) + 0.25 * ( 0.9 * 750 + 0.1 * 750)

= 86.625 + 187.5

= 274.125 ns

Answer 3

Simultaneous access is consider:

T-avg=.75(.9(45)+.1(750))+.25(750)

         =274.125