In the simplified coherent path here, a Request Node initiates accesses, a coherent Home Node coordinates coherence for its assigned addresses, and a Subordinate Node provides downstream service. CHI defines different coherent and non-coherent node types; these three labels alone do not specify every capability. Arm IHI 0050 Issue H B1.6 permits an HN-F to include a directory or snoop filter rather than requiring one.
CHI separates REQ (request), RSP (response), DAT (data) and SNP (snoop) roles. That separation supports concurrency, but the protocol, network and buffering design must still preserve their dependency rules. Separate channels alone do not prove absence of deadlock, livelock or starvation.
CHI TOPOLOGY (simplified)
RN (CPU0) ----\
RN (CPU1) -----+--> HN (coherent home) ----> SN (DRAM)
RN (GPU) ----/ |
SNP network to peer RNsA directory or snoop filter can reduce unnecessary snoops using conservative knowledge of which caches may hold a line. Its representation, capacity and handling of missing information are implementation choices. Do not equate CHI with mandatory exact-directory tracking or assume ACE always broadcasts. There is no universal eight-core or 32-core protocol scaling threshold; compare the actual traffic and resources.
In AMBA CHI Issue H request retry, a Completer that cannot accept an eligible first request can return RetryAck and later PCrdGrant for the required Protocol Credit. The Requester must wait for both before resending with AllowRetry deasserted and the granted PCrdType; the credited retry is guaranteed acceptance under the protocol rules. A high RetryAck rate shows admission/resource pressure at a Completer or path, but does not by itself identify a home-node compute bottleneck.
Home placement and address distribution affect locality and hotspots; evaluate the selected topology.
Directory representation trades storage, filtering precision and fallback traffic; bit-vector width is not the only possible encoding.
Distinguish CHI channel/link flow control from implementation-specific NoC virtual channels and buffer pools.
Home placement and address distribution affect locality and hotspots; evaluate the selected topology.
Directory representation trades storage, filtering precision and fallback traffic; bit-vector width is not the only possible encoding.
Distinguish CHI channel/link flow control from implementation-specific NoC virtual channels and buffer pools.