First-generation -kernels have a reputation for being too slow and lacking sufficient flexibility. To determine whether L4, a lean second-generation -kernel, has overcome these limitations, we have repeated several earlier experiments and conducted some novel ones. Moreover, we ported the Linux operating system to run on top of the L4 -kernel and compared the resulting system with both Linux running native, and MkLinux, a Linux version that executes on top of a firstgeneration Mach-derived -kernel. For L 4 Linux, the AIM benchmarks report a maximum throughput which is only 5% lower than that of native Linux. The corresponding penalty is 5 times higher for a co-located in-kernel version of MkLinux, and 7 times higher for a userlevel version of MkLinux. These numbers demonstrate both that it is possible to implement a high-performance conventional operating system personality above a -kernel, and that the performance of the -kernel is crucial to achieve this. Further experiments illus...