Customization lite

There has been a great deal of interest in recent years in developing operating systems that can be customized to meet the performance and functionality needs of particular applications while being extensible to support new policies, new implementations and new interfaces. We describe a structuring technique, called building-block composition, that we are employing for this purpose. The customizability this technique provides to untrusted applications is, while large, less than that provided by some other techniques. However, it results in a more extensible and maintainable system, is easier for untrusted applications to use, and provides a better foundation for overall improved performance, particularly for multiprocessors.