In the world of Linux, "micro-kernel-like things" are done when they are beneficial. An example of this is FUSE: filesystems in user space. There is a benefit t

In the world of Linux, "micro-kernel-like things" are done when they are beneficial. An example of this is FUSE: filesystems in user space. There is a benefit to being able to run a filesystem in its own address space as a user process, so it is done. Also the model of a Unix kernel with services (daemons) in user space is kind of similar to the microkernel model. The graphical desktop is not in the kernel, the mail server isn't in the kernel, crond isn't in the kernel, the web server isn't in the kernel, the database server isn't in the kernel ... and maybe that's as close as we need to get to the microkernel. Speaking of web servers in user space: at least for serving a static page, it's faster for that to be inside the monolithic kernel, where it can access the protocol stack and filesystem code without crossing a protection boundary. Over the history of Linux, we have seen experimentation both with moving kernel things into user space (like with FUSE) as well as user things in to the kernel (TUX server, now evidently unmaintained).