Days
Days AGO, an exact safe-horizon network simulator whose Scalar, CPU, Metal, and CUDA backends produce byte-identical complete state.
Days simulates packet networks exactly and in parallel. Its executor, Days AGO (Adaptive Guaranteed Ordering), compiles a TOML scenario into a SimulationImage, splits the network into logical processes (LPs), and advances them concurrently in rounds bounded by a safe horizon. Lean proofs in lean/DaysExecutor/ show that round execution reaches the same result as serial execution in canonical event order.
Four backends run the same image:
| Backend | Where it runs |
|---|---|
| Scalar | one CPU thread; the reference oracle |
| CPU | a multicore worker pool |
| Metal | Apple GPUs (not the M1 family) |
| CUDA | NVIDIA GPUs (built for sm_86, sm_89 and sm_121) |
Every backend that accepts a scenario produces byte-identical complete state. A backend that cannot run a mechanism rejects the scenario at validation with a message naming the backend; it never falls back silently.
What Days AGO models
- Topologies: fat-tree, torus, dragonfly, custom graphs, and the Spectrum-X rail fabric of SimAI, with delay-only NVLink inside a server and SimAI's Murmur3 ECMP.
- Transport: deterministic packet-distribution sources, closed-loop TCP Reno and CUBIC, Mellanox-form DCQCN (the controller of HPCC's ns-3 model and SimAI) on unreliable flows, and RoCE queue pairs with Go-back-N reliability and ECN echo.
- Switches: FIFO, strict-priority, DRR, WRR, and exact-rational WFQ scheduling; TailDrop admission and ECN marking on a Kmin/Kmax/Pmax ramp; PFC.
- Collectives: ring all-reduce, all-gather, reduce-scatter, all-to-all (uniform or seeded imbalanced), and send/receive over TCP or RoCE queue pairs, with delay-only compute stages and counted joins between them.
- Workloads: AICB/SimAI training traces, lowered onto the Spectrum-X fabric with the fabric parameters taken from
SimAI.conf.
DCQCN, RoCE queue pairs, PFC, the ECN ramp, collectives, compute stages, and stage notifies run on all four backends.
What makes it exact
- Integer time and canonical order. Events carry integer-nanosecond time and a canonical key
(time_ns, phase, origin_node, origin_seq); no floating point appears in simulation semantics. - Byte identity across backends. The CPU pool and each compiled device backend must reproduce the Scalar oracle's complete result byte for byte. Device backends omit only diagnostic-only observation planes, which the comparison excludes.
- Refuse-or-run device capacity. The device planner derives every arena capacity from the image before execution. An attempt that runs out of capacity is discarded and retried with larger floors; nothing is dropped silently. The largest committed fixture,
configs/benchmarks/lookahead/rq9_frontier_closed_k32.toml, has 262,144 closed-loop TCP flows.
Quick start
From the repository root, run the Scalar oracle on the smoke fixture. The last line it prints, record=days_result, fingerprints the complete result. Every other engine that accepts the scenario must print the same fingerprint; the TCP corpus gate checks this fixture on each available backend.
cargo run --release --bin days -- --engine scalar \
configs/benchmarks/tcp/fattree_k4_tcp_cubic_f16_smoke.tomlThe quickstart covers the CPU, Metal, and CUDA engines.
Legacy Days
The original actor-model simulator on the Nexosim runtime is frozen in legacy/ as the days-legacy crate and still runs. It is documented in its own section: Legacy Days.
Where to go next
- Run simulations: Quickstart and Examples
- Every
daysoption and output record:daysCLI - The scenario format: Configuration
- The execution model and backends: Architecture
- Proofs and trace certificates: Verification and research