Coherent transactions, snoop channels, shareability domains, cache maintenance, CHI topology, and fabric-level ordering.
Start with the topic hub to learn the contract.
Use mechanism and reports pages to connect rules to metrics.
Use debug and worked-example pages for interview and on-call practice.
Close with checklist and PPA impact before signing any change.
ace-coherent-transactions/ — ACE Coherent Transactions
snoop-and-cache-maintenance/ — Snoop & Cache Maintenance
chi-topology-basics/ — CHI Topology Basics
coherence-fabric-debug/ — Coherence Fabric Debug
Coherence extends memory transactions with snoop and state — traffic multiplies when software shares cache lines.
COHERENCE TRAFFIC FLOW
RN issues coherent read
-> HN looks up directory
-> snoops to sharers
-> data + state update returned
False sharing: different variables, same cache line -> coherence storm.COHERENCY TRAFFIC STACK
data fetch ████████
snoop responses ██████████████
writebacks ██████
maintenance ops ████
High snoop stack with good IPC -> suspect line sharing before faster NoC.snoop rate
intervention latency
coherency transaction mix
false sharing indicators
Benchmark IPC looked fine but system power spiked: per-core counters were on one cache line. Padding counters fixed coherency traffic without any NoC change.
If snoop latency high, check home node placement and directory policy.
If ordering bug, run litmus sequences before microarch changes.
If traffic storm, profile cache line sharing in software layout.
Ask: what is the first transaction that deviates, and which spec rule does it test?