Switches and logical processes

Scope: Days AGO. The legacy PacketSwitch demultiplexer actor is described in Legacy switch.

Days AGO makes each directed switch egress port a logical process (LP).

LP model

Lowering assigns one LP to each host and one LP to each semantic (physical switch, directed egress link) pair. It sorts those semantic keys before assigning dense NodeId values, which removes construction order and hash-map iteration from identity.

A lowered switch LP owns:

• SwitchState.physical_switch, shared by all egress LPs for that physical switch
• one SwitchQueueState with its egress link, scheduler, admission policy, optional PFC state, waiting queue, and in-service packet
• a local child-event sequence and packet counters

The image has no forwarding-table actor. Each FlowDescriptor carries canonical forward and reverse LinkId routes. On RemoteArrival, the transition finds the packet's next directed link and therefore the destination egress LP. The owning LP performs admission, service selection, and completion without shared mutable switch state.

This ownership is the unit used by Scalar, persistent CPU workers, Metal, and CUDA. Safe-horizon rounds drain each active LP below the exclusive horizon, buffer remote events, then merge those events at the round barrier in canonical order.

Capability checks

The compiler rejects mechanisms outside the Days AGO model, such as VirtualClock and the legacy drop policies RED, RED_ECN and ECN_THRESHOLD (removed in P16; the ECN ramp replaces them). Backend validation then checks device-specific state before launch: Metal and CUDA run every lowered mechanism, including the ECN ramp, collective and compute stages, counted joins, stage notifies, PFC on switch and host links, DCQCN, and RoCE queue pairs; they reject a zero-lookahead channel, a WFQ rational beyond the device width, and a queue capacity beyond 32 bits.

Errors name the backend and direct the caller to Scalar or CPU. A capability failure returns no partial run result.