L4 is a family of microkernels with a namesake kernel interface. μ-kernel (minix3, L4/Fiasco et al) are extremely lean by design and unlike monolithic (Linux, Solaris, Darwin et al), leave device drivers, filesystem, and a lot of non-hardware related stuff to the userspace. This project is basically a port of linux to an L4 Hypervisor that can Para-Virtualize Linux.

In short, with this project you can run a User Space Linux along with your other user space programs in L4 kernels.

The idea was to have something you would call microservice architecture today, just for OS functions. An ethernet driver server application would talk to an IP server, which would talk to a TCP server that distributes messages to the applications. The kernel would just isolate all those applications and enable communication between them. If the ethernet driver daemon crashed you would just restart it.

The communication overhead remained a problem for microkernels, despite L4's achievements in that area. The advantages were somewhat overstated. The complexity merely moved from the kernel into the applications. A security bug in the IP driver daemon can still compromise the whole system, just like a driver in a monolithic kernel.

Microkernels and hypervisors are very closely related in the view of some. Paravirtualization, like L4 Linux, is when the guest is aware of being hosted and, at one time - maybe still, could outperform guest OSes on fully virtualized hardware. I'm not sure if that's all still true since Intel and AMD implemented VTX.

If you want to see a neat demo of this stuff - check out the DROPs demo or TUD:OS http://demo.tudos.org/.

Screenshots of a whole bunch of linuxes (linii?) running at once: http://demo.tudos.org/l4lx_screenshot.png

I think this is from 2003...

> linii?

Linuces.