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Guido Witmond avatar

On Microkernels versus Monoliths

I like the Genode platform, it offers me a system that is much safer against all those bad things that threaten current systems. Computer viruses that cripple Mac, Windows and Linux are mostly eliminated. These become at most an annoyance, if they have any success at all. Also, the regulary patching to close security holes can become a faint memory. Here endeth the advertisement.

Right now, the Genode system is ready for system developers. End users – people who are not programmers – who want to replace their desktop with a Genode system have to wait a bit longer (for the programmers to finish). This blog is for enthousiasts who want to know more about Genode and perhaps experiment with it.

The Genode documentation describes the system from a technical point of view. In this series of posts, I'll describe the architecture of Genode from the perpective of the end user. What can Genode do that the common systems can't do. Or, in other words, why is Genode so much safer to use? And how does that make the life of an end user easier? Yes, safety and easy to use can be had together.

This blog, I focus on the difference between monoliths and microkernels. In later blogs I'll address other architecural choices and how they make life better for the end user.

But first, some technical background: an operating system consists of many components. There are components for literally every part of the system: Disk drivers, audio input, audio output, screen handling, networking. Close to one hundred components are needed to make the thing work. Each of them needs some software to make it work.

The difference between a microkernel and a monolith is that in a microkernel each component runs as a separate process, independent from the others. The (very small) kernel just provides some memory and communications so these components can do their job. Yet, if one component has a bug and crashes it certainly hinders other components – imagine you're writing a file and the disk driver crashes – but that's all the damage it could do. Just wait for it to restart and hopefully it will succeed now.

In a monolith, all parts are combined in one big process. The reason to do so is the belief that a monolith is faster. To further speed things up, components can often reach inside other components to avoid communication overhead.

But this need for speed comes at a price: Robustness suffer. Imagine 100 components, all sharing a single kernel process and all running with full supervisor rights. Each with a small number of bugs, each of them capable of changing any data structure of any component. It's recipe for disaster.

In a monolith, if one component has a bug it can literally mess up anything. Sometimes it's benign, other times fatal. If it crashes, the system can't do anything but show the famous Blue Screen of Death. Worse, a clever attacker can abuse such bugs to gain access to components he should not have access to at all.

Many components don't even need to run in the same process. An audio driver has no need to share a process with a disk driver. The music player program connects these two when the user wants to play music.

Microkernels alone do not make a computer safe against all things bad. In later blogs I'll show other security issues and what paradigms help solve them. And I'll show that program are adapting to OS'es just like dogs look like their owners.

Microkernels are not new. Long time ago, around 1991, Linus Torvalds used Minix, a microkernel with a UNIX-like approach for students to get experience with modern technology. Later he replaced the Minix parts with a monolithic kernel for speed. He traded safety for speed. And it shows: A recent paper by Simon Biggs states: "96% of critical Linux exploits would not reach critical severity in a microkernel-based system, 57% would be reduced to low severity".

We will never know if a slower but safer Linux would have become the same success had Linus used a microkernel architecture instead of the monolith it is today. But Linus never expected Linux would get big either, after all, it was just a hobby project at the start. :-)

The performance reason to choose monoliths has been debunked. While some of those old ones were very slow, modern microkernels are pretty close in performance to monoliths, often less a few percent performance loss. And for most workloads, the safety that microkernels bring should outweigh the raw performance.

And microkernel systems are everywhere even though many people never heard of them. L4 is used inside 1.5 billion smart phones for the baseband chip. Intel processors uses Minix to drive the system management component, it runs before it boots Linux or Windows on the main processor. It still runs when the main OS has Blue-screened. :-)

I think it is time to deploy microkernels for the main OS too.

Next blog: on ambient authority

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