To reduce the impact of the problems pointed out above a local private cache memory is attached to each processor. This results in a new problem since the sharing of global data structures can result in several copies of a shared block in one or more caches at the same time. To maintain a coherent view of the memory, these copies must be consistent. This is the cache coherent problem . In this lecture we will discuss different solutions to the cache coherence problem including Snoopy protocols, directory schemes and cache coherent architectures.
All local memories are private and accessible only by local processors.
Internode communication is carried out by passing messages through the network. Message passing demands special hardware and software support. Deadlock situations in a message passing network are examined. As a case study we will take a look at the multicomputer system IBM SP2. Low machine efficiency due to interprocessor communication and Difficulty to program .