CMS reconstruction improvements for the tracking in large pile-up events

The CMS tracking code is organized in several levels, known as 'iterative steps', each optimized to reconstruct a class of particle trajectories, as the ones of particles originating from the primary vertex or displaced tracks from particles resulting from secondary vertices. Each iterative step consists of seeding, pattern recognition and fitting by a Kalman filter, and a final filtering and cleaning. Each subsequent step works on hits not yet associated to a reconstructed particle trajectory. The CMS tracking code underwent a major upgrade deployed in two phases. It was needed to make the reconstruction computing load compatible with the increasing instantaneous luminosity of LHC, resulting in a large number of primary vertices and tracks per bunch crossing. The improvements are described. Among the others, the iterative steps have been reorganized and optimized and an iterative step specialized for the reconstruction of photon conversion has been added. The overall impact on reconstruction performances is discussed and the prospects for future applications are given.