Only if the resources its progress needs are available independently of blocked request work. A virtual channel is a separate buffer and credit set multiplexed on one physical link. A virtual network gives one message class its own virtual channels on every link and its own endpoint queues, end to end. Different names with the same full receive queue do not break the previous lesson's cycle.
Class in this course | Examples | May wait on | Dependency to inspect |
|---|---|---|---|
Requests | GetS, GetM, PutS, PutE, PutM | Forwards, responses | Do they hold ingress while waiting for a pending entry or protected data storage? |
Forwards | Inv, Fwd-GetS, Fwd-GetM | Response egress only | Can a cache with its own request outstanding still receive and answer them? |
Responses | Inv-Ack, Data, Put-Ack | Nothing | Can they complete an existing transaction without resources held by new requests? |
These classes are the analysis the general rule needs, not a mandatory three-network implementation. PutM carries dirty data and can have different requirements from a clean release. Data can carry permission, payload or both. Inspect its role and destination before assigning it a class.
A protocol deadlock is a cycle through endpoint resources, such as pending entries and ingress queues, created by dependencies between message classes. The P/Q cycle is one. It is broken by message classes, reservation and a separate virtual network per class.
A routing deadlock is a cycle of packets each holding router buffers or channels while waiting for the next, around a loop in the topology. It is independent of what the messages mean. It is broken inside each virtual network by a deadlock-free routing function (for example dimension-order routing on a mesh) or by an escape channel.
So the P/Q fix needs more than Qresp at H: X's response also needs response virtual channels on each hop from C0 to H. If it shared a router buffer with requests on the way, a full request buffer could block it before it reached Qresp.
Keep X's dirty value 1 at C0's egress slot T0 and Y's request in H's Qreq. A useful Qresp has reserved receive space, a protected destination, response virtual channels that can reach it, and service using X's existing pending entry. Its outgoing grant must also be kept under backpressure. Independent input capacity alone is insufficient if response processing blocks on the same unavailable downstream resource.
Proposed change | Breaks the identified cycle? | Why |
|---|---|---|
Rename one message class response | No | Q is still occupied by the request. |
Two queues, no protected response destination | Not established | The data can still stall at the endpoint. |
Reserve response capacity and process under existing P | Yes for the stated two-node resource graph | The response no longer waits for Qreq or a new free P. |
Add fairness to a completely blocked cycle | No | No action is enabled to arbitrate. |
The corrected path completes under A1-A6 (assumptions), notably A5: for a full fabric the graph must include router channels, endpoint queues, transaction resources and any memory dependencies. A local acyclic graph is not automatically the global graph.
Not without checking its data obligation. PutM may carry the only current value and needs protected acceptance capacity. Its dependencies can differ from PutS even though both are releases.