Building a discrete-event model with SimPy
Discrete Event Simulation in Python
Dr Diogo Costa
Adjunct Professor, University of Saskatchewan, Canada & CEO of ImpactBLUE-Scientific
Before we build our model
Understand dynamic system
Define objective(s) of discrete-event model
Identify processes:
- running in parallel
- running in sequence
Identify appropriate SimPy resources (
resource,container,store)Define architecture of model:
- Identify state variables
- Workflow of model
- Generators
The dynamic system and objectives of the model
Understand the dynamic system
- What are the aspects that modulate its behavior?
- What processes/events need to be included?
Objectives of the model
- Often used to optimize systems
What are the aspects you want to optimize?
- More customers
- Faster processing times
- Optimize resource allocation
State-variables to monitor these objectives
Identify the appropriate SimPy resources
Shared Resources
simpy.Resources(env, capacity=4)
- Resources shared between processes
Examples
- Tables at a restaurant
- Machines in a factory
- Seats at a theatre
- Cashiers in a shop
Containers
simpy.Container(env, capacity,
init=initial_capacity)
Resources behaves like a tank or basket
Add and remove quantity:
.put() and .get()
Examples
- Fuel reserves in a gas station
- Coal in a coal-fired power plant
Identify the appropriate SimPy resources
Store
Resource like a store with multiple items
Capacity is unlimited (default)
Capacity slots for storing arbitrary objects
Add and remove items:
.put() and .get()
Examples
- Shops selling multiple items
- Storage room
- Workshop
- Household
Concatenate events using bitwise operators
SimPy uses bitwise operators to concatenate two events
Types of operators in Python
- Arithmetic, logical, and comparison operators
Bitwise operators
- Bitwise calculations on integers
- Expect two operands
- Example:
a & b,a >> b,a ^ b
- Example:
In SimPy
- Bitwise operators generate condition event
- Wait for both or one of two events/processes
Relevant bitwise operators for SimPy
- Waits for both processes:
__and__()or& - Waits for either process:
__or__()or|
Let's practice!
Discrete Event Simulation in Python
