Jochen Liedtke

Jochen Liedtke (26 May 1953 – 10 June 2001) was a German computer scientist, noted for his work on microkernels, especially the creation of the L4 microkernel family.

In the mid-1970s Liedtke studied for a diploma degree in mathematics at the University of Bielefeld. His thesis project was to build a compiler for the ELAN programming language, which had been launched for teaching programming in German schools; the compiler was written in Elan itself. After his graduation in 1977, he remained at Bielefeld and worked on an Elan environment for the Zilog Z80 microprocessor. This required a run-time environment, which he called Eumel (“Extendable Multiuser Microprocessor ELAN-System”, but also a colloquial north-German term for a likeable fool). Eumel grew into a complete multi-tasking, multi-user operating system supporting orthogonal persistence, which started shipping in 1980 and was later ported to Zilog Z8000, Motorola 68000 and Intel 8086 processors. As these processors lacked memory protection, Eumel implemented a virtual machine which added the features missing from the hardware. More than 2000 Eumel systems shipped, mostly to schools but also to legal practices as a text-processing platform.

In 1984, he joined the GMD (Gesellschaft für Mathematik und Datenverarbeitung, the German National Research Center for Computer Science, which is now a part of the Fraunhofer Society), where he continued his work on Eumel. In 1987, when microprocessors supporting virtual memory became widely available in the form of the Intel 80386, Liedtke started to design a new operating system to succeed Eumel, which he called L3 (“Liedtke's 3rd system”, after Eumel and the Algol 60 interpreter he had written in High School). L3 was designed to achieve better performance by using the latest hardware features, and was implemented from scratch. It was mostly backward-compatible with Eumel, thus benefiting from the existing Eumel ecosystem. L3 started to ship in 1989, with total deployment of at least 500.

Both Eumel and L3 were microkernel systems, a popular design in the 1980s. However, by the early 1990s, microkernels had received a bad reputation, as systems built on top were performing poorly, culminating in the billion-dollar failure of the IBM Workplace OS. The reason was claimed to be inherent in the operating-system structure imposed by microkernels. Liedtke, however, observed that the message-passing operation (IPC), which is fundamentally important for microkernel performance, was slow in all existing microkernels, including his own L3 system. His conclusion was that radical re-design was required. He did this by re-implementing L3 from scratch, dramatically simplifying the kernel, resulting in an order-of-magnitude decrease in IPC cost. The resulting kernel was later renamed “L4”. Conceptually, the main novelty of L4 was its complete reliance on external pagers (page fault handlers), and the recursive construction of address spaces. This led to a complete family of microkernels, with many independent implementations of the same principles.

Liedtke also worked on computer architecture, inventing guarded page tables as a means of implementing a sparsely-mapped 64-bit address space. In 1996, Liedtke completed a PhD on guarded page tables at the Technical University of Berlin.

In the same year he joined the Thomas J. Watson Research Center, where he continued to work on L4 (for political reason called the “Lava Nucleus” or short “LN”, microkernels were not fashionable at IBM after the Workplace OS disaster). The main project during his IBM time was the Saw Mill project, which attempted to turn Linux into an L4-based multi-server OS.

In April 1999 he took up the System Architecture Chair at the University of Karlsruhe. In Karlsruhe he continued to collaborate with IBM on Saw Mill, but at the same time worked on a new generation of L4 (“Version 4”). Several experimental kernels were developed during that time, including Hazelnut, the first L4 kernel that was ported (as opposed to re-implemented) to a different architecture (from x86 to ARM). Work on the new version was completed after his death by Liedtke's students Volkmar Uhlig, Uwe Dannowski and Espen Skoglund. It was released under the name “Pistachio” in 2002.