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In a two-level cache system, the access times of L1 and L2 are 3, 10 clock cycles respectively. The miss penalty from the L2 cache to main memory is 20 clock cycles. The miss rate of L1 cache is thrice that of L2. The average memory access time(AMAT) of this cache system is 4 cycles. The miss rates of L1 and L2 respectively are:
asked in CO & Architecture by (69 points) | 98 views
You formed wrong equation as miss rate can never be negative

1 Answer

+1 vote
L2 miss rate= 0.0247

L1 miss rate=3*0.0247
answered by Active (3k points)
edited by
can you elaborate how you come up with answer
average memory access time = L1 cache always accessed+ Probability of miss in cache L1 * time to access L2+ Probability of miss in L1 cache * Probability of miss in L2 cache * time to access main memory

@Anshu Can you tell me the correct answer, please.
@sonveer I am getting a different answer. ML1= 0.705 ML2=0.235

Will you please share your equation

Hierarchical Access should be assumed.

AMAT = T1 + M1* Miss Penalty1
Miss Penalty1 = H2.T2 + M2* Miss Penalty2
Miss Penalty2 = T3

H1 = Hit rate of Level 1 cache.
H2 = Hit rate of Level 2 cache.
M1 =Miss rate of Level 1 cache.
M2 =Miss rate of Level 2 cache.
T1= Access Time of Level 1 cache.  
T2= Access Time of Level 2 cache.
T3= Miss Penalty of Level 2 cache


@Soumya , your equation is correct but I m getting L2 miss rate = 0.0313



I have used :-

AMAT = T1 + M1(T2 +M2*Miss penalty of L2)


There is a slight difference in our formula. Check again. 
I assumed simultaneous access and you assumed hierarchical. 

yeah ,you are right ..I have used hierarchical memory access..

why u used simulteneous access here?

ankit , srestha   Hierarchical access should be assumed here.
Reference -


yes ..but still I don't know when to use which memory access..  if someone write  some points about how to identify which memory access we have to use from the question then it will be good for us. I asked this question previously but sometimes I get from the question and sometimes I don't identify which memory access we should have to use..


What I observed in previous year questions is -

1.If only "Access Times"are given like in this question - then hierarchical access is considered.
2. If hit time, miss time terminologies are used  - then simultaneous access is considered.
3. It "Ignore the search time" like this is mentioned in the question then simultaneous access is considered irrespective of the terminologies(access time, hit time, miss time etc). 

yes u r right..thank you !
It's quite convincing too... :)
Access time belongs to the single type of memory only.
So hierarchical access makes sense.
First access level 1 cache. If found - access it, If data not found then access level 2 cache and so on.

If hit time or miss time is given - means we don't have to worry about the type of memory access.
It's simply - hit rate * hit time + miss rate * miss time

If given that search time is ignored. This implies simultaneous access.

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