Cache Coherence in Multi-processor Systems

1. Cache memory keeps values that have been recently referenced, which improves performance.

2. Example states of cache items

a. Modified – it has been modified since its last load from the next lower level of memory

b. Exclusive – the value in this cell has not been modified and no other processor has this value in its cache

c. Shared – the value in this cell is valid and several other processors may have a valid value as well

d. Invalid – the value of this cell has been chanced by another processor

3. Rules of modification of a cell

a. If all processors that have a shared variable that is only being read, no conflict occurs.

b. If a processor modifies a shared variable, it must notify all other caches that this shared variable is now invalid.

c. If a cell is invalid and a “read” occurs, it must be fetched from lower level memory

4. Cache must be managed to maximize reference to valid cache values.