MESIlike

MESIlike is a theoretical or teaching-oriented cache coherence concept that mirrors the ideas of the MESI protocol while introducing variations for discussion or specialized architectures. It is not a formal standard, but it is used in computer architecture literature and coursework to illustrate how coherence can be implemented under different memory hierarchies and messaging schemes, such as bus snooping or directory-based coherence.

Core concepts and states

A typical MESIlike model uses a four-state framework similar to MESI: Modified (M), Exclusive (E), Shared (S), and Invalid (I). Each state conveys ownership and durability of a cache line:

- M: the cache line is dirty and owned by this cache; it must be written back before eviction.

- E: the cache line is clean and only present in this cache; no other cache holds a dirty copy.

- S: the cache line is clean and may be present in other caches; it can be shared.

- I: the cache line is not present in this cache.

In MESIlike, variants may introduce additional transitional states (for example Pending or Forwarded) or modify the handling of certain messages to reflect different interconnects (bus-based vs directory-based) or timing assumptions. The core mechanism involves coherence messages (read, write, invalidate, data responses) that trigger state transitions and data movement to maintain a consistent memory view across cores.

Operation and transitions

- Read miss: a processor requests a line not in its cache. Depending on other caches, the line may be supplied and caches update to S or E, with data brought from memory or a cache with a clean copy.

- Write miss: issuing a write typically drives the state to M after invalidating other copies as required by the protocol variant.

- Write or invalidate: a cache may invalidate copies in other caches, forcing them to I, while the requesting cache transitions to M or (in some variants) E before becoming M.

- Eviction: a dirty line in M generally requires a write-back.

Applications and considerations

MESIlike is used as a conceptual tool to compare coherence strategies, evaluate performance trade-offs, and study effects of interconnect topology. Variants affect coherence traffic, latency, and scalability, highlighting differences between bus-based and directory-based implementations, and between aggressive invalidation versus shared-copy coexistence approaches.