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What would be the answer? I am getting 100.

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85?

Let the miss penalty be p, initially the average memory access time will be:

Hit time + Miss rate * Miss Penalty = 10 + 0.4p

After optimization, the average memory access time becomes: 15 + 0.3p

There was a reduction of 10% in the average memory access time

$\implies (10+0.4p)*0.9=15+0.3p$

$\implies 9 + 0.36p=15+0.3p$

$\implies 0.06p=6$

$\implies p=100$

So, the miss penalty is $100\ ns.$

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Thanks bro. I also got the same ans, but the answer given in the test series was 60.
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@sinchit @mayureshpatle

you guys are considering that miss penalty doesn’t include the cache access time(the time wasted during the cache miss)

0.7(15)+0.3(p+15)=0.9[0.6(10)+0.4(p+10)]

so p=100ns

but i think miss penalty will implicitly include the cache access time also i.e no need to add 10 and 15 ns in the miss penalty as it already includes it in p.

cause in gate pyq’s the miss penalty in page fault includes memory excess time also so we never add it explicitly.

https://gateoverflow.in/669/gate2000-2-22?show=5230#c5230

see sachin mittal sir comment on this pyq ques

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I searched this question now, it was asked in an NPTEL assignment too (link, question no. 9).

They have also mentioned 100 as correct answer, maybe the answer given in the test series is wrong.