Set-Associative Cache

Chapter 7

The set of blocks in a cache can be configured as:◦ Direct mapped◦ N-way set associative◦ Fully-associative

Associative cache have shown to result in lowering miss rate.

As N increases, the number of index bits decreases and it reaches 0 for a fully-associative cache.

Lets look at a 4-way set associative cache fig.7.17

Direct map: one main memory has only one possible cache block map (direct map).

N-way set associative cache has N block choices per main memory block.

For example, 2-way set-associative has two possible choices for every main memory block.

See Fig.7.14 The comparator complexity increases with

N.

Processor with base CPI of 1, assuming 100% hit ratio in primary cache, and clock rate of 5Ghz.

Assume main memory access time of 100ns including all the miss handling.

Miss rate per instruction at primary cache 2%.

How mush faster will the processor be if we add a another level of cache with following characteristics?◦ Secondary cache has 5ns for hit or miss, and can

reduce miss ratio to main memory to 0.5%.

With only primary cache:◦ 1 + (2/100) X (5Ghz*100ns) = 11 CPI

With secondary cache:◦ No miss: 1 + ◦ L1 miss : (2/100) X (5Ghz *5ns) + ◦ L2 miss: (0.5/100) X (5Ghz * 100ns) = ◦ 1 + 0.5 + 2.5 = 4 CPI