Demystifying operating systems

academics computer science operating systems

I have tutored a great number of students in undergraduate operating systems. Personally, I enjoy the topic. My dissertation is in the field of distributed systems. Distributed systems is an academic offspring of operating systems research where partial failures are expected and allowed. The operating systems course is almost universally taught with a laboratory component. I have not (yet) found the perfect operating systems course. These are my observations on the various approaches that I have seen.

Production Operating System

Linux and Android are the most popular options for courses that teach real world open source operating systems. This style of teaching generally uses a virtual machine (such as VirtualBox) or an emulator (such as QEMU). The benefits of this approach are that students have exposure to real production operating systems source code. The drawbacks of this approach are that students have exposure to real production operating systems source code. Linux and Android are large complex projects. They have non-trivial build systems. They are implemented in C. For many students this is their first exposure to programming in C. This may be the student’s only exposure to C in their undergraduate curriculum. It is a challenge to teach the textbook learning objectives of the operating system course and teach the practical C programming skills necessary for Linux and Android. And teach both kinds of topics at the same time in the same course. A simple build process goes a long way. I prefer running QEMU to VirtualBox. The implement-compile-test feedback cycle is faster on QEMU. Emphasize and discourage common mistakes. For example, when running a virtual machine with an ext2 filesystem with a high probability of filesystem corruption when the OS crashes, always make a snapshot of the operating system before booting new changes. In these real-world build environments, it becomes possible to lose hours of work due to a simple workflow error. These kinds of traps are discouraging to students who are just trying to learn the material.

Research/Educational Operating System

xv6 is the most popular option for small teaching operating systems in the UNIX family that are implemented in C. QEMU is the most common execution environment. These operating systems are on the order of 10,000 lines of code. Compare that to the Linux kernel which is around 10,000,000 lines of code. The implement-compile-test feedback cycle is faster than on a production operating system. The build environment is typically disposable; the state of the virtual disk is not saved on shutdown. As an exposure to the C programming language the development workflow is dramatically simplified. Using an educational operating system has become my preferred method for operating systems labs. It is realistic enough for students to learn the concepts of operating systems and not too realistic to bog students down in implementation details. There are a few drawbacks. The x86 version of xv6 is no longer being maintained. New development has continued on the RISC-V version while schools continue to use the x86 code.

Managed Runtime Environment

In this approach a simulator of a complete operating system or some partial features of an operating system will be implemented in a language with a runtime and a garbage collector, such as Java, Python, or JavaScript. The advantage of this approach is that students are not learning C and operating systems concepts simultaneously. The labs can focus on OS concepts in a programming language that the students are already familiar with. The two downsides of this approach are the abstractions that are required to teach in an environment. If the students are implementing a simulator of one feature of an operating system (such as a process scheduler) then all the assumptions and simplifications of the simulation need to be explicitly enumerated. If it is ambiguous then the students waste a lot of time trying to determine what answer the instructor wants rather than learning the instructional concepts. The second set of abstractions are concerned with workarounds and limitations of a managed runtime. The instructions may say “we are going to use this workaround” and the students are not familiar with all the details of the Java virtual machine to understand what is a learning objective and what is a workaround. I would like to see whether the Biscuit OS could be adapted for this learning style. Biscuit is a monolithic, POSIX-subset operating system kernel in Go. Go is a compiled, memory safe programming language with a garbage collector. Go could act as a reasonable “C-lite” programming language with memory safety and garbage collection.

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