Race Conditions

written by Dave Krahl, QMT Group

In computer science and engineering a race condition is "an undesirable situation that occurs when a device or system attempts to perform two or more operations at the same time, but because of the nature of the device or system, the operations must be done in the proper sequence to be done correctly". In discrete event simulation programs such as ExtendSim, a race condition is when multiple events happen at the same simulated time and the order of these events has an impact on the operation of the model. Because discrete event simulators process events at discrete times, race conditions are fairly common. There is no standard solution to this, and it is handled differently by the various simulation software programs. When an event occurs in a discrete event simulation model any number of actions can be triggered. The order of these actions can have a significant effect on the behavior of the model. A classic example is if a resource is released and the item (or entity) releasing that resource requests the same resource again and other items are waiting for that resource. Is the releasing item in contention for the resource?

ExtendSim has a number of features that provide additional control over the sequence and scheduling of events as well as the transmission of messages. These include:

• Scheduling a 0 time event in an equation before it is evaluated. This provides the ability to delay the calculation of the equation and any subsequent messages sent by the equation returning control to the block that initiated the calculation. This can be implemented in custom blocks as well.
• Detailed control of messages in the equation and custom blocks. Whether or not a connector responds to a message is a user-defined option.
• In custom blocks, there are message handlers for every type of interaction. These can be used to control how other messages are sent out.

While there are numerous tools for solving race condition problems, the biggest challenge is detecting them in the first place. This often requires detailed inspection of the results of an event in the simulation. Some tools available for this in ExtendSim are:

• Tracing of simulation execution
• Enabling debugging code and in particular examining the "stack" of function calls and message handlers
• Animation of item movement
• Record Message block in the ExtendSim Utilities library shows sequences of messages
• History block that shows item movement and properties.

Race conditions are an artifact of discrete event simulation as a technology. This issue is something that every experienced simulation modeler has encountered in their modeling experience. Identifying the race condition can be challenging. However, tools exist in simulation programs to address any race condition problems and control the execution of the model.