A computer has a two-level cache. Suppose that 75% of the memory references hit

Question

A computer has a two-level cache. Suppose that 75% of the memory references hit on the first level cache, 15% hit on the second level, and 10% miss. The access times are 3ns (L1), 10ns (L2), and 40ns (main memory). The times for the level 2 cache and main memory start counting at the moment it is known that they are needed (e.g. level 2 cache access does not even start until level 1 cache miss occurs). What is the average access time? Would adding L3 cache improve the performance? Explain why or why not.

Explanation / Answer

Step 1:

0.75(3) + 0.15(3+10) + 0.1(3+10+40)

Step 2:

0.75(5) + 0.15(13) + 0.1(53)

Step 3:

3.75+1.95+.53 = 6.23 nsec

6.23nsec is the average access time.

Adding L3 cache won't create a high difference.

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