We present a performance analysis and benchmarking study of the OP2 "active" library, which provides an abstraction framework for the solution of parallel unstructured mesh applications. OP2 aims to decouple the scientific specification of the application from its parallel implementation, achieving code longevity and near-optimal performance through re-targeting the back-end to different hardware. Runtime performance results are presented for a representative unstructured mesh application written using OP2 on a variety of many-core processor systems, including the traditional X86 architectures from Intel (Xeon based on the older Penryn and current Nehalem micro-architectures) and GPU offerings from NVIDIA (GTX260, Tesla C2050). Our analysis demonstrates the contrasting performance between the use of CPU (OpenMP) and GPU (CUDA) parallel implementations for the solution on an industrial sized unstructured mesh consisting of about 1.5 million edges. Results show the significance of choosing the correct partition and thread-block configuration, the factors limiting the GPU performance and insights into optimizations for improved performance.