Question

For a special computer system with 3 levels of cache, here are the hit times and miss % for the different levels of cache

Hit Miss% L1 Cache 1 Cycle 5% L2 Cache 5 Cycles 10% L3 Cache 20 Cycles 15% Main Memory 100 Cycles 20%

Please calculate the access time for this computer system.

Answer 1

first the system will look into cache L1, if not found then in cache L2, then in cache L3 and if not found in all of these it will look into main memory

So the average access time would take into consideration success in L1, failure in L1 but success in L2, failure in L1 also failure in L2 but success in L3, failure in all three cache and success in main memory

Access time= [h1 * t1] + [ m1 * h2 * t2 ] + [m1 * m2 * h3 * t3] + [m1 * m2 * m3 * hm * tm]

hi = hit rate i.e. 1 - mi(miss rate)

ti = time cycle

hm = hit rate of memory

=> [0.95 * 1] + [0.05 * 0.9 * 5] + [0.05 * 0.1 * 0.85 * 20] + [0.05 * 0.1 * 0.15 * 0.8 * 100]

=> 0.95 + 0.225 + 0.085 + 0.06

=> 1.32 ns