Days
Days is an exact discrete-event packet network simulator for building models and examining their behavior.
What Days is
Days is an exact discrete-event packet network simulator for building models and examining their behavior. It runs from the command line with TOML configuration files and includes runnable examples. Models can use fat-tree, torus, dragonfly, or custom graph topologies, plus TCP Reno and CUBIC, rate-paced sources, DCQCN, and CNP. Switch options include FIFO, strict priority, DRR, WRR, exact-rational WFQ, TailDrop, ECN thresholds, RED, RED with ECN, and PFC. Days also supports ring all-reduce and all-gather over TCP, with compute stages represented as delays. Events are ordered by integer-nanosecond time, phase, sending process, and sequence number, without floating-point arithmetic in simulation semantics. Scalar, multicore CPU, Apple Metal, and NVIDIA CUDA backends aim for byte-identical complete states; the project describes five Lean-proved theorems concerning parallel scheduling and checks backend conformance against serial results. RED admission, collectives, and compute stages are limited to Scalar and CPU. Days is open source under AGPL-3.0-only and runs on Linux, macOS, and Windows.
Good
- Models several network topologies and transport protocols.
- Four execution backends aim for identical states.
- Periodic CSV reports cover source, sink, and switch statistics.
- Runs on Linux, macOS, and Windows.
- Open source under AGPL-3.0-only.
Watch out
- RED, collectives, and compute stages lack Metal and CUDA support.
- CUDA builds require the CUDA 13 toolkit.
- Metal runs exclude Apple M1-family GPUs.
- Runs through a command-line program with TOML configuration.
Who it suits
Days suits people debugging network simulations, reproducing regressions across backends, or comparing other simulators' approximations with an exact reference. It is a command-line tool for users comfortable configuring simulations in TOML.
Verdict
Days combines a broad set of packet-network models with deterministic simulation goals and multiple execution backends. Check its backend limits and GPU requirements against the scenarios and hardware you plan to use.
The facts
Each one read from Days's own pages, with the date it was checked.
What it does
- Backend limits
- RED, collectives, and compute stages run only on Scalar and CPU; Metal and CUDA reject those scenarios during validation.checked 11 Oct 2026
- Collectives
- It supports ring all-reduce and all-gather over TCP, with delay-only compute stages.checked 4 Oct 2026
- Configuration
- Simulations are configured in a TOML file passed to the command-line program.checked 4 Oct 2026
- Determinism
- Events are ordered by integer-nanosecond time, phase, sending process, and sequence number, with no floating-point arithmetic in the simulation semantics.checked 11 Oct 2026
- Execution
- It provides scalar, multicore CPU, Apple Metal, and NVIDIA CUDA backends that aim to produce byte-identical complete simulation states.checked 11 Oct 2026
- GPU limitation
- RED admission, collectives, and compute stages are supported only on Scalar and CPU; Metal and CUDA reject those scenarios during validation.checked 4 Oct 2026
- GPU requirements
- CUDA builds require a CUDA 13 toolkit, and Metal runs on Apple GPUs except the M1 family.checked 11 Oct 2026
- Installation
- The installation guide instructs users to build Days from source with stable Rust and Cargo.checked 11 Oct 2026
- Intended users
- The site describes using Days to debug simulations, reproduce regressions across backends, and compare other simulators’ approximations against an exact reference.checked 4 Oct 2026
- License
- The project is licensed under AGPL-3.0-only.checked 11 Oct 2026
- Network models
- It models fat-tree, torus, dragonfly, and custom graph topologies, along with TCP Reno and CUBIC, rate-paced sources, DCQCN, and CNP.checked 4 Oct 2026
- Outputs
- By default, Days writes periodic CSV reports for source, sink, and switch statistics; additional traces depend on enabled features.checked 11 Oct 2026
- Parallel correctness
- The site describes five Lean-proved theorems about the executor’s scheduling model and says backend conformance is checked against the serial result.checked 4 Oct 2026
- Parallel execution
- The site says five Lean theorems establish that parallel execution reaches the same complete state as canonical serial execution.checked 11 Oct 2026
- Product
- Days is an exact discrete-event packet network simulator.checked 4 Oct 2026
- Programming model
- Network elements operate as isolated sequential processes with private state and communicate through timestamped messages.checked 11 Oct 2026
- Purpose
- Days is an exact discrete-event packet network simulator.checked 11 Oct 2026
- Supported models
- Days models fat-tree, torus, and dragonfly topologies, TCP Reno and CUBIC, DCQCN, PFC, several queue disciplines, and ring all-reduce and all-gather collectives.checked 11 Oct 2026
- Switch features
- Supported switch mechanisms include FIFO, strict priority, DRR, WRR, exact-rational WFQ, TailDrop, ECN thresholds, RED, RED with ECN, and PFC.checked 4 Oct 2026
- Usage
- Days runs from the command line with a TOML configuration file and includes runnable example configurations.checked 11 Oct 2026
The company
- Maker attribution
- The Days site footer credits Baochun Li.checked 11 Oct 2026
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