Understanding the detailed behavior of an operating system is crucial for making informed design decisions. But such an understanding is very hard to achieve, due to the increasing complexity of such systems and the fact that they are implemented and maintained by large and diverse groups of developers. Tools like KLogger — presented in this paper — can help by enabling fine-grained logging of system events and the sharing of a logging infrastructure between multiple developers and researchers, facilitating a methodology where design evaluation can be an integral part of kernel development. We demonstrate the need for such methodology by a host of case studies, using KLogger to better understand various subsystems in the Linux kernel, and pinpointing overheads and problems therein. Categories and Subject Descriptors D.4.8 [Performance]: Measurements; C.4 [Performance of Systems]: Design studies General Terms Measurement,Performance,Experimentation Keywords operating systems, Linu...