Tuning and optimization of the CMS simulation software

The CMS simulation has been operational within the new CMS software framework for more than 3 years. While the description of the detector, in particular in the forward region, is being completed, during the last year the emphasis of the work has been put on fine tuning of the physics output. The existing test beam data for the different components of the calorimetric system have been exploited to adjust different parts of the Geant4 models for hadronic and electromagnetic showers, as well as the CMS custom code used, in close collaboration with Geant4 developers. Significant improvements have been achieved in describing the data, albeit at the cost of a notable increase in cpu time. A big effort has therefore been undertaken to put in place a suite of performance analysis tools and a number of optimisations have now been introduced that allow the code to fit the resource constraints posed by the CMS computing model.