Days: A Fast and Exact Discrete-Event Network Simulator on CPUs and GPUs

Days simulates packet networks exactly and in parallel. Its executor, Days AGO, compiles a TOML scenario into a simulation image, splits the network into logical processes, and advances them concurrently in rounds bounded by a safe horizon. Lean proofs (lean/DaysExecutor/) show that this round execution reaches the same result as serial execution, and every backend that runs a scenario produces byte-identical complete state.

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)

Modeled mechanisms: closed-loop TCP Reno and CUBIC, Mellanox-form DCQCN, RoCE queue pairs with Go-back-N, PFC, ECN marking on a Kmin/Kmax/Pmax ramp, strict-priority, DRR, WRR, and exact-rational WFQ scheduling; ring all-reduce, all-gather, reduce-scatter, all-to-all, and send/receive collectives over TCP or RoCE queue pairs with delay-only compute stages; fat-tree, torus, dragonfly, and Spectrum-X rail topologies; and AICB/SimAI training traces as workloads. Every mechanism Days AGO lowers runs on all four backends; a scenario outside its model is refused at lowering with a message naming the field, never with a silent fallback.

Quick start

Run from the repository root.

# Scalar reference run
cargo run --release --bin days -- \
  configs/benchmarks/tcp/fattree_k4_tcp_cubic_f16_smoke.toml --engine scalar

# Multicore CPU run
cargo run --release --bin days -- \
  configs/benchmarks/tcp/fattree_k4_tcp_cubic_f16_smoke.toml \
  --engine cpu --workers 2 --repetitions 1

# GPU run on Apple Metal
cargo run --release --features metal --bin days -- \
  configs/benchmarks/tcp/fattree_k4_tcp_cubic_f16_smoke.toml --engine metal

# GPU run on NVIDIA
cargo run --release --features cuda --bin days -- \
  configs/benchmarks/tcp/fattree_k4_tcp_cubic_f16_smoke.toml --engine cuda

Every engine ends with a record=days_result line carrying the complete-state byte length and its FNV-1a fingerprint; equal fingerprints mean byte-identical results. --engine metal or --engine cuda without its feature is an error.

Documentation

The documentation site is at days.sh/docs. To build it locally:

cd docs/
bun install
bun dev

Tests

The four-package default matrix:

cargo test -p days-executor -- --show-output
cargo test -p days --features test -- --show-output
cargo test -p days-legacy --features test -- --show-output
cargo test -p days-validation --features test -- --show-output

GPU surfaces add --features test,metal (Apple) or --features test,cuda (NVIDIA, CUDA 13 toolkit). See the testing page in the documentation for the long-running gates.

Feature flags

• cuda: the CUDA backend (requires a CUDA 13 nvcc)
• metal: the Metal backend
• test: extra assertions and test helpers
• cuda-planner-test: host-only CUDA plan-equality tests; builds without nvcc (no kernels)
• metal-test-hooks, cuda-test-hooks: device conformance hooks

The legacy crate has its own l2, l2_pfc, dcqcn, and lean features.

Legacy Days

The original actor-model simulator (Nexosim coroutines, versions up to 0.4.3) is frozen in legacy/ as the days-legacy crate and still runs:

cargo run --release -p days-legacy --bin days-legacy -- configs/simple.toml

Its last commit on main before Days AGO is tagged legacy-main-final.

Repository layout

• src/: scenario compiler, topologies, and runners
• executor/: the Days AGO executor and its four backends
• lean/: executor theorems and LeanGuard protocol checkers
• legacy/: the frozen actor-model simulator
• validation/: cross-engine tests against legacy Days
• configs/: scenarios, benchmarks, and fixtures
• docs/: the documentation site

License

AGPL-3.0-only.