This blog post is the first of a series of posts dealing with the results of the Debian systemd survey. I intend to give a presentation at DebConf 2013, too, so you could either read my posts, or watch the talk, or both :-).
The top concern shared by most people is:
systemd has too many dependencies, or it is bloated, or it does too many things, or is too complex
Now this concern actually has a lot of different facets, and I am trying to share my opinion on each of them.
First, let’s start with “too many dependencies”, because that is easy to check and reason about. I have created a document which lists all dependencies of the systemd binary itself (pid 1) and all the binaries which are currently shipped by the systemd Debian package. If you don’t want to take my word for granted, please read that document.
Have you read the document? Very nice! As you can see, the systemd binary itself has 10 dependencies (excluding libc). Now, the question is, what is bad about dependencies? Why do people list dependencies as a top concern?
enforce_userwhich uses libcap. You’ll notice that the code is not complex and usage of the libraries is clear.
Now, let’s talk about bloat. Again, this is a point which has many facets. I’d like to quote the Wikipedia definition of software bloat:
Software bloat is a process whereby successive versions of a computer program become perceptibly slower, use more memory or processing power, or have higher hardware requirements than the previous version whilst making only dubious user-perceptible improvements.
The first part of the definition certainly does not match systemd — it is measurably faster than sysvinit. As for memory usage: systemd’s RSS is 1.8 MiB, whereas sysvinit uses 0.8 MiB. As I argued on the “More memory use” point in the dependencies section, I think the additional resource cost is well worth the benefits.
Also note that systemd’s features are NOT all implemented in the binary which is PID 1. As explained in the dependency list, systemd consists of many cleanly separated binaries. So if a new version of systemd gathers an additional feature, this does not mean that your PID 1 will be bigger.
While systemd runs on any hardware, it has an indirect hardware requirement: it requires some Linux kernel features (which are all enabled in Debian kernels). That might rule out usage of systemd on really old embedded hardware where you don’t have a chance to update the kernel. While it is sad that those machines cannot profit from systemd, switching to systemd as a default has no downside either: Debian continues to support sysvinit for quite some time, so these machines will continue to work even with upcoming Debian versions.
The Wikipedia definition continues:
[…] perceived bloat can occur from the software servicing a large, diverse marketplace with many differing requirements. Most end users will feel they only need some limited subset of the available functions and will regard the others as unnecessary bloat, even if people with different requirements do use them.
I think the last part of the Wikipedia definition applies to systemd: it does service a large and diverse “marketplace”. That marketplace is the entirety of existing software which is started by an init system. Also, systemd can be used on a wide range of hardware (embedded devices, tablets, phones, notebooks, desktops, servers) which requires different features. As an example: on a desktop system you typically don’t care strongly about a watchdog feature, but on embedded or servers that feature is very handy. Similarly, on a tablet, forward secure sealing of logfiles is not as important as on a server. Therefore, I can understand if you feel that you don’t need many of the features systemd provides. But please think of other users and maintainers who are very happy with systemd’s benefits.
Also note that while systemd supports many things (in separate binaries!), you don’t have to use them all. It still makes sense to ship them all in the same package. Take coreutils as another example in that area. The binaries belong together, even though you most likely haven’t used all of them (e.g. od, pr, ptx, … :-)).
The remaining concern is that systemd is too complex. In my experience, complexity is often inherent to a specific area and one cannot simply make it go away. Instead, there are different models of how that complexity is represented. Think of the monolithic Linux kernel versus the MINIX microkernel. The latter has a very small amount of lines of source in the kernel, but puts the complexity into userspace. The former uses a different approach with more source in the kernel. The arguments between both camps show that neither is clearly right or clearly wrong.
In a way, sysvinit represents the MINIX model: it has a small core (the init binary itself), but a lot of complexity in shell scripts and external programs. The fact that solutions are copied from one init script to another leads to lots of subtle errors and makes code reuse really hard. systemd however has more source code in the binaries, but requires only very simple, descriptive, textual configuration instead of complex init scripts. To me, it seems preferable to have the complexity in a single place instead of distributed across lots of people and projects.
In a way, you are right. systemd centralizes complexity from tons of init scripts into a single place. However, it therefore makes it very easy for maintainers to write service files (equivalent of an init script) and provides a consistent and reliable interface for service management. Furthermore, it is different than sysvinit, and different solutions often seem complex at first. While systemd consumes more resources than sysvinit, it uses them to make more information available about services; its finer-grained service management requires more state-keeping, but in turn offers you more control over your services.