Topology Build Pipeline

Scope: Days AGO and Legacy Days. Both engines start with the shared topology builder, then diverge into different runtime representations.

1. Build the shared graph

src/topos/build.rs returns:

• an undirected petgraph::graph::UnGraph<usize, ()> of switches
• HostAttachments, which maps semantic host IDs to attachment switches
• TopologyProfile, used by topology-specific routing and lowering

Supported inputs are FatTree, Torus, Dragonfly, Spectrum-X (src/topos/rail.rs, Days AGO only), and custom edges/hosts graphs. Host IDs are not assumed to be switch IDs; all flow pairing and routing resolves them through HostAttachments.

2A. Compile a Days AGO image

compile_config in src/scenario/compile.rs validates the exact-model vocabulary and creates stable semantic IDs. It lowers:

1. one host LP for each semantic host
2. one switch LP for each directed egress queue
3. queue scheduler, admission, PFC, and in-service state
4. forward and reverse channels with rate and propagation delay bounds
5. flow generators, TCP/DCQCN state, collective generators, packets, and initial events

Exact scheduling supports FIFO, strict priority, WFQ, DRR, and WRR. The exact compiler rejects VirtualClock. Exact routing supports shortest path, FatTreeEcmp, and SimAiEcmp. FatTreeEcmp requires a canonical FatTree and derives a stable semantic flow hash rather than using construction order; SimAiEcmp requires the Spectrum-X fabric and hashes each flow's SimAI addresses and port with Murmur3, as SimAI does.

Flow sets support Random, SwitchOffsetHalf, and SameSwitchNext pairing. Days AGO collectives are RingAllReduce, AllGather, ReduceScatter, AllToAll, and SendRecv, over byte-terminated TCP or RoCE queue pairs, and [[compute]] adds delay-only compute stages; see Collectives. An AICB trace lowers through the same path from a [workload.aicb] table; see Workloads.

Callers pass the completed SimulationImage to executor::validate(image, backend). Image invariants are backend-independent; capability checks then reject mechanisms not ported to the selected device.

2B. Assemble Legacy Days actors

The legacy CLI validates the complete TOML surface before Topology::new in legacy/src/topos/topo.rs assembles Nexosim models. It:

1. configures Nexosim threading, mailbox capacity, time quantum, UI, and logger
2. creates switches and bounded mailboxes
3. creates one scheduler actor for every directed graph edge
4. creates sources, sinks, collectives, and dependency connections
5. computes routes and installs forward and reverse FIB entries
6. initializes the simulation, injects start events, runs to the stop time, and flushes enabled logs

Legacy scheduling additionally supports VirtualClock. Legacy routing supports shortest path, per-flow ECMP, explicit paths, and the shared semantic FatTreeEcmp policy, subject to each policy's validation rules. Legacy collective expansion supports Broadcast, Gather, a fan-in AllReduce (Gather's endpoint pattern: many sources, one sink), and RingAllReduce.

Host attachment and link stages

The exact image always models host injection and delivery stages. Legacy preserves its historical direct endpoint wiring unless host attachment is enabled.

model_host_attachment = true enables the matching legacy stages. Omitting that key while setting link.propagation_ns also enables them, applying propagation to host and physical links. The strict validator rejects an explicit model_host_attachment = false combined with link.propagation_ns.

For a legacy physical hop without PFC:

PacketSwitch -> Scheduler -> optional Wire -> PacketSwitch

With link.mode = "Pfc" and the legacy l2_pfc feature:

PacketSwitch -> Scheduler -> PfcEgressGate -> Link -> PfcIngressPort -> PacketSwitch

The exact compiler lowers PFC into image state, and Scalar, CPU, Metal, and CUDA execute it.