noncachecoherent

Noncachecoherent refers to a memory system in which caches associated with processing elements (such as CPUs, GPUs, or I/O devices) do not automatically maintain a single, consistent view of memory. In a non-cache-coherent environment, a memory read by one processor may return stale data if another processor has written to that memory location and that update has not been propagated to caches. Hardware does not guarantee coherence, so correctness must be achieved through software or architectural design.

Non-coherence is common in architectures that separate processing from devices or where performance, power, or simplicity is prioritized over full cache coherence. It can occur with non-coherent interconnects between CPUs and accelerators, with DMA engines that bypass caches, or in specialized SoCs where only a portion of the system participates in a coherence protocol. In such systems, certain components operate with private caches, and shared memory updates may require explicit coordination.

Programming models in non-coherent environments rely on synchronization primitives, memory barriers, and explicit cache management, such as flushing or invalidating caches, or mapping memory regions as non-cacheable for device access. Without hardware coherence, data races and memory ordering issues are more likely, making debugging more challenging. Non-coherent designs can reduce coherence-related overhead and scale in some workloads, but place greater responsibility on software to ensure data consistency.

Related concepts include cache coherence protocols (such as MESI and MOESI), snooping versus directory-based approaches, and coherent versus non-coherent I/O. Many systems combine coherent CPUs with non-coherent devices, requiring careful synchronization to coordinate shared memory access.