CMS resource utilization and limitations on the grid after the first two years of LHC collisions

After years of development, the CMS distributed computing system is now in full operation. The LHC continues to set records for instantaneous luminosity, and CMS records data at 300 Hz. Because of the intensity of the beams, there are multiple proton-proton interactions per beam crossing, leading to larger and larger event sizes and processing times. The CMS computing system has responded admirably to these challenges, but some reoptimization of the computing model has been required to maximize the physics output of the collaboration in the face of increasingly constrained computing resources. We present the current status of the system, describe the recent performance, and discuss the challenges ahead and how we intend to meet them.