A cache system has the following characteristics: cache access time = 10 ns; mem

Question

A cache system has the following characteristics: cache access time = 10 ns; memory access time = 70 ns; cache hit percentage = 90%

a.Determine the effective memory access time.

b.Suppose a L2 cache with 40 ns access time and 95% hit percentage is introduced between the L1 cache described above and the memory, what is the effective memory access time? Recognize that you have to consider three possibilities: needed information is found in L1 cache, information is not found in L1 cache but found in L2 cache, and information is not found in either L1 or L2 caches but found in memory. Determine the probability for each case and weight the access times.

Explanation / Answer

a).

Tc = 10ns
Tm = 70ns
H = 90%

EAT = H*Tc + (1-H)*(Tm+Tc)
= 0.9*10 + 0.1*(70+10)
= 9 + 8
= 17 ns

b).
Tc1 = 10ns
Tc2 = 40ns
Tm = 70ns
H1 = 90%
H2 = 95%

EAT = H1*Tc1 + (1-H1)*(H2)*(Tc1+Tc2) + (1-H1)*(1-H2)*(Tc1+Tc2+Tm)
= 0.9*10 + 0.1*0.95*50 + 0.1*0.05*120
= 9 + 4.75 + 0.6
= 14.35 ns

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