CoherencyOperationen

CoherencyOperationen is a term used in the field of computer architecture to refer to the set of operations and protocols that maintain data coherence across caches and memory replicas in multiprocessor and multi-core systems as well as in distributed shared memory environments. These operations ensure that read operations observe the most recent write and that multiple processors do not obtain stale or conflicting values. CoherencyOperationen are distinct from data consistency models; they implement the mechanisms by which the system enforces coherence at runtime.

Typical operations include:

- Invalidation: a cache line in one core is marked invalid to force re-fetch on next access.

- Update: other caches are updated with a new value.

- Fetch-and-invalidate: a cache line is retrieved while invalidating other copies.

- Write-back and write-through: cache lines are written back to memory or written through to memory on every write.

- Atomic operations: compare-and-swap, test-and-set, or fetch-and-add to ensure atomicity across cores.

Coherence is achieved through hardware protocols, commonly snooping (bus-based or shared-medium) or directory-based approaches. Well-known coherence schemes include MESI and MOESI families, MOESI variants, and directory-based protocols. The design must consider memory consistency models, latency, bandwidth, false sharing, and cache hierarchy.

In practice, coherency operations are visible to software mainly through memory fences and atomic instructions across languages and runtimes, which synchronize threads and establish ordering guarantees.

See also: cache coherence, memory consistency model, MESI protocol, directory-based coherence, distributed shared memory.