What Happens When Running a Simulation?

Here are the steps that occur when you start a run via the start_run script:

• If required, it copies all of the tables from the baseyear database specified by the configuration into the baseyear cache and then creates a set of pre-baseyear gridcells tables by unrolling the develoment_event_history data.

  The unrolling processes all records in the development_event_history table. It starts with the most current year of records, and moves backward in time. For this description, assume that the baseyear is 2000. The unrolling process first loads the gridcell dataset for year 2000. For each develoment_event_history record with scheduled_year for the prior year (e.g. for 1999) it substracts from the gridcells any development by that event (e.g. residential_units). Values that go negative are set to zero. Once all events for this year have been ``undone'', the unrolling process writes the modified gridcell dataset to the prior year, e.g. to 1999. This process is repreated for each year with data in develoment_event_history.

  If you only want 10 years of data to be unrolled, only put those years of data into your develoment_event_history table.

• Alternatively, it copies files from a previously used baseyear cache into the baseyear cache for the current run (including the

• It adds a row to the run_activity table in the services database, using a new run_id value unique to this run. In order to help match runs with their cache directory, the name of the cache directory begins with the run_id value, e.g. run_342.2006_04_25_09_40.

• For each year in the set of years specified in the configuration:
  • Stores to the cache a pickled version of the configurations for that year.
  • Forks a new process to run the set of UrbanSim models for this year. The set of models to be run is specified by the configuration. Using a separate process helps reduce memory usage, and helps reduce the impact of problems such as memory leaks. This process writes a log file named, e.g., year_2003_log.txt.
  • Run the EMME/2 travel model for this year, if specified by the travel model's configuration. The set of steps to run for the travel model is fully specified by the 'models' section of the 'travel_model_configuration'. Typically, it includes a preparation step that prepares travel model inputs from the UrbanSim data, a step that actually runs the travel model, and one or more steps to puts the results of the travel model into the travel_data dataset for the current year, so the data is visible to the UrbanSim models in that and following years. Each model step is done in a separate process. The log for the running of the model is written to a file named, e.g., emme2_2005_log.txt.
• The run activity table includes status information about the run. If the simulation succeeds, it will add another row to the run activity indicating it is done. If the simulation fails, it will add a row indicating that. The run activity also contains a copy of the configuration, which is used when restarting a run.
• Whenever a row is added to the run_activity table, a row is either added or updated in the available_runs table in the services database. This table has a single row per run and records the row's current state and information.