Basic AMAT formula: AMAT = hit time + (miss rate x miss penalty) Two-level cache

Question

Basic AMAT formula: AMAT = hit time + (miss rate x miss penalty) Two-level cache AMAT formula: AMAT = Hit TimeL1 + Miss RateL1 x (Hit TimeL2 + Miss RateL2 x Miss PenaltyL2) Split cache AMAT formula: AMATSplit = AMATicache + AMATdcache A particular processor contains a split level 1 cache (data and instruction) and a unified level 2 cache. The level 1 data cache has a 92% hit rate and a 2-cycle hit latency. The level 1 instruction cache has a 90% hit rate and a 2-cycle hit latency. The cache miss penalty for both dcache and icache is 20 cycles. The unified level 2 cache has a 95% hit rate with a 5-cycle hit latency and a miss penalty of 60 cycles. Calculate the overall AMAT for this processor.

Explanation / Answer

Consider the following data,

Level 1:

Data cache hit ratio = 92%

Data cache hit latency = 2-cycle

Instruction cache hit ratio = 90%

Instruction cache hit latency = 2-cycle

Cache miss penalty = 20 cycles

Level 2:

Cache hit ratio = 95%

Hit latency = 5 cycle

Miss penalty= 60 cycles

Overall AMAT for this processor is,

= Hit TimeL1 + Miss RateL1 * (Hit TimeL2 + Miss TimeL2 * Miss PenaltyL2) …… (1)

At level 1 the AMAT,

AMAT1 = AMAT_iCache + AMAT_dCache

= (hit time + (miss rate x miss penalty)) + (hit time + (miss rate x miss penalty))

= (.92 * 2 + (.08 * 20)) + (.90 * 2 + (.10 * 20)) cycle

= (3.44 + 3.8) cycle

= 7.24 cycle

Since the hit rate of the data cache is 92% in the level 1 cache, therefore the miss ratio is 8%.

Thus, the overall AMAT,

= .92 * AMAT1 + .08 * (Hit TimeL2 + Miss TimeL2 * Miss PenaltyL2)

= .92 * 7.24 + .08 * (.95 * 5 + .05 * 60) cycles

= 7.29 cycles

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