Cache Placement Policies

Last updated Nov 8, 2022 Edit Source

We need a way to decide where the data is placed when it is first copied into the cache where in the cache a copy of selected memory block will reside.

Each memory block can only be mapped to a single fixed cache line. This means that there are no sets of cache memory, rather each line is a set itself. If another block maps to this set, the previous data block is replaced. E.g. Block 0, Block 64, Block 0, Block 64 etc.

• This placement policy is power efficient as it avoids the search through all the cache lines.
• It has lower cache hit rate, as there is only one cache line available in a set. Every time a new memory is referenced to the same set, the cache line is replaced, which causes conflict miss

To increase flexibility, one way is to allow memory block to be placed anywhere in the cache. This can be framed as a single cache set holding all the cache lines. Index bits are no longer required since there is no distinguishing between sets:

• Fully associative cache structure provides us the flexibility of placing memory block in any of the cache lines and hence full utilization of the cache.
• The placement policy provides better cache hit rate.
• Offers the use of a wider variety of replacement algorithms on miss
• The placement policy is slow as it takes time to iterate through all the lines.

Trade-off between direct and fully associative cache.