A multi-core processor is a processing system composed of two or more independent cores (or CPUs). The cores are typically integrated onto a single integrated circuit die (known as a chip multiprocessor or CMP), or they may be integrated onto multiple dies in a single chip package. A many-core processor is one in which the number of cores is large enough that traditional multi-processor techniques are no longer efficient — this threshold is somewhere in the range of several tens of cores — and likely requires a network on chip. A dual-core processor contains two cores, and a quad-core processor contains four cores. A multi-core processor implements multiprocessing in a single physical package. Cores in a multi-core device may be coupled together tightly or loosely. For example, cores may or may not share caches, and they may implement message passing or shared memory inter-core communication methods. Common network topologies to interconnect cores include: bus, ring, 2-dimensional mesh, and crossbar. All cores are identical in homogeneous multi-core systems and they are not identical in heterogeneous multi-core systems. Just as with single-processor systems, cores in multi-core systems may implement architectures such as superscalar, VLIW, vector processing, SIMD, or multithreading. Multi-core processors are widely used across many application domains including: general-purpose, embedded, network, digital signal processing, and graphics. The amount of performance gained by the use of a multi-core processor is strongly dependent on the software algorithms and implementation. In particular, the possible gains are limited by the fraction of the software that can be "parallelized" to run on multiple cores simultaneously; this effect is described by Amdahl's law. In the best case, so-called embarrassingly parallel problems may realize speedup factors near the number of cores. Many typical applications, however, do not realize such large speedup factors and thus, the parallelization of software is a significant on-going topic of research.