Sim# is a .NET port of SimPy, process-based discrete event simulation framework
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Sim# (SimSharp)

A .NET port of SimPy, process-based discrete event simulation framework

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Disclaimer: Sim# is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Sim# is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License (GPL) as published by the Free Software Foundation, either version 3 of the license, or (at your option) any later version.


Sim# aims to port the concepts used in SimPy [1] to the .NET world. Sim# is implemented in C# and is available via Nuget for .NET Framework 4.5 and is also .NET Standard 2.0 compliant. Sim# uses an efficient event queue (adapted from [3]) that allows to compute models very quickly. Simulating 10 years of the MachineShop sample [4], that uses preemptive resources, requires only 2.5s on a Core i7 2.6Ghz. This model generates more than 5 million events.

SimPy allows to model processes easily and with little boiler plate code. A process is described as a method that yields events. When an event is yielded, the process waits on it. Processes are themselves events and so it is convenient to spawn sub-processes that can either be waited upon or that run next to each other. There is no need to inherit from classes or understand a complex object oriented design.

To demonstrate how simple models can be expressed with little code, consider a model of an m/m/1 queuing system as expressed in Sim#:

TimeSpan ARRIVAL_TIME = TimeSpan.FromSeconds(...);
TimeSpan PROCESSING_TIME = TimeSpan.FromSeconds(...);
TimeSpan SIMULATION_TIME = TimeSpan.FromHours(...);

IEnumerable<Event> MM1Q(Simulation env) {
  var server = new Resource(env, capacity: 1);
  while (true) {
    yield return env.TimeoutExponential(ARRIVAL_TIME);
    env.Process(Item(env, server));
  }
}

IEnumerable<Event> Item(Simulation env, Resource server) {
  using (var s = server.Request()) {
    yield return s;
    yield return env.TimeoutExponential(PROCESSING_TIME);
    Console.WriteLine("Duration {0}", env.Now - s.Time);
  }
}

void RunSimulation() {
  var env = new Simulation(randomSeed: 42);
  env.Process(MM1Q(env));
  env.Run(SIMULATION_TIME);
}

Sim# tries to be as easy to use as SimPy, but also remains true to the .NET Framework. The most obvious difference between SimPy and Sim# is handling process interruptions. In Sim# a process that can be interrupted needs to call

if (Environment.ActiveProcess.HandleFault()) {...}

after each yield in which an interruption can occur and before continuing to yield further events. This is due to a limitation of the .Net Framework: In Python it is possible to put a try-except block around a yield statement, and an exception can be injected into the iterator. In .Net this is not possible.

Also in Sim# it was decided to base the unit for current time and delays on DateTime and TimeSpan in the simulation clock. There is however an API, called D-API (short for double-API) that allows you to use doubles as in SimPy, e.g. env.Now returns a DateTime, env.NowD returns a double, env.Timeout(delay) expects a TimeSpan as delay, env.TimeoutD(delay) expects a double, etc.. It is possible to initialize the Environment with a default timestep in case both APIs are used:

var env = new Simulation(defaultStep: TimeSpan.FromMinutes(1));

In that environment, calling env.TimeoutD(1) would be equal to calling the more elaborate normal API env.Timeout(TimeSpan.FromMinutes(1)).

References

  1. Python Simpy Package
  2. Nuget package
  3. High speed priority queue
  4. Machine Shop Example