CacheKohärenzOverheads

Cache coherence overheads refer to the extra resources required to maintain a consistent view of memory across caches in multi-core or multi-processor systems. They arise when caches hold copies of shared data and must reflect updates made by any core. The overhead is not a single metric but a combination of latency, bandwidth, and energy consumed by coherence-related messages and state transitions within coherence protocols such as MSI, MESI, or MOESI. The interconnect or directory structure that coordinates ownership and invalidations carries a portion of these costs.

Coherence overheads are generated by several mechanisms: invalidation and update traffic, ownership transfers, and the latency of state transitions; the need to track cache line ownership across cores; and synchronization primitives that force ordering of memory accesses. Snooping protocols broadcast on a common bus, while directory-based protocols use a central or distributed directory to track sharers, with trade-offs in traffic and latency. In both cases, coherence messages compete for bandwidth and can stall cores, especially under high contention or weak memory models.

Impact on performance includes increased memory latency, reduced instruction-level parallelism, and higher energy consumption. Coherence traffic scales with the degree of data sharing and can limit scalability in many-core systems.

Mitigation techniques include designing data structures to minimize sharing and false sharing, aligning memory to reduce unnecessary invalidations, and allocating shared data with awareness of NUMA topology. Architectural approaches include hierarchical or directory-based coherence, message coalescing, and selectively disabling coherence for private data. Measuring coherence overheads uses metrics such as coherence traffic (messages per cycle), average latency of coherence operations, and invalidations per access.

Understanding cache coherence overheads helps in architectural design and software optimization for scalable parallel systems.