3. Suppose the main memory access time of 200 cycles including all the miss hand

Question

3. Suppose the main memory access time of 200 cycles including all the miss handling, the primary cache has 2 cycles access time and the miss rate per instruction of primary cache is 4%

a) What is the average memory access time (AMAT)?

b) The designer wants to add a second level cache to improve the performance. Suppose the second level cache has 20 cycles access time (including all the miss handing due to primary cache miss) and is large enough to reduce the miss rate to main memory to 0.5%, how much faster will be by adding such second level cache?

Explanation / Answer

a) main memory access time = 200 cycles

primary cache access time = 2 cycles

miss rate of primary cache = 4% = 0.04

therefore hit rate of primary cache = 0.96

AMAT = 0.96*2 + 0.04(2+200) = 0.96*2 + 0.04*202 = 1.92 + 8.08 = 10 cycles

b) second level cache access time = 20 cycles

second level cache miss rate = 0.5% = 0.005

therefore hit rate of second level cache = 0.995

AMAT = 0.96*2 + 0.04*0.995(2 + 20) + 0.04*0.005*(2 + 20 + 200) = 1.92 + 0.8756 + 0.0444 = 2.84 cycles

now 10/2.84 = 3.52

Therefore second architecture is 3.52 times faster than the first one.

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