Build Stages

For work, I've been working on revising our build system so that less of the build definition happens in Makefiles and more of it happens in Python scripts. This post is an elaboration.

I'm a complete partisan of reusing standard tools, and so moving away from Make felt like a big/hard jump. However:

1. Process creation is expensive, and every "job" starts a new shell and process, which takes time.

2. Most of the build logic was in Python anyway: over time most shell lines called Python code rather than commands directly. This seems like a common artifact of more complex build processes.

Beyond this, the generation of the Makefiles itself was encoded in Python code.

For our project, at least, we were indirecting through Make, sort of through the hell of it.

3. It turns out that multiprocessing in Python is crazy easy to use.

The transition isn't complete of course, we're still using make to handle dependencies between groups of tasks, and there's no particular rush or need to rid ourselves of Make, but the gains are huge. Things build faster one or two orders of magnitude in some cases. There's less flakiness. Rebuild times are much faster, and there are fewer moving parts.

Great win!

This has me thinking about ways of doing build systems in a generic, maintainable way without relying on something like Make. I have a prototype on my Laptop at the moment that provides a way to specify build processes concurrently. The rest of this post will be a high level overview of the design of this system. Please provide feedback and enjoy!

Build systems are basically collections of tasks expressed in a graph structure. The tools exist to enforce and encode the graph structure, or less abstractly to ensure that tasks run in the proper order. If you're paining a wall, the build system ensures that you spackle, apply the primer, and then apply the final coat, in that order.

There are, as near as I can tell, three different kinds of relationships among/between groups of tasks in a build process:

1. There are groups tasks that don't depend on each other and can run concurrently with each other.

2. There are some tasks or groups of tasks that must not run before or after another group of tasks.

3. There are sequences of tasks that must run in a specific order, but can run at the same time as other tasks or sequence of tasks.

What Make, and related systems do is provide a mechanism to specify "dependency" relationships between files (and tasks after a fashion,) or groups of files/tasks. After a fashion, Make takes the dependency information and runs tasks more or less according to one of those patterns. In many ways, my project is an experiment to see if it's possible to "outsmart Make," by generalizing the kinds of operations and forcing users to specify the concurrency constraints of the tasks explicitly, rather than letting the concurrency emerge out of the dependency graph. Thoughts:

  • This depends on users being able to intuit the actual dependencies abstractly, rather than rely on the emergence properties of Make. Arguably, Make also requires you to think abstractly about the potential concurrent modeling of the build, but allows you to avoid it in some situations.
  • If some large portion of the compilation process relies external processes, the performance gains will probably be more modest. Process creation is still expensive, but it's probably marginally cheaper to use subprocess than it is to start a full shell.

In addition to the basic machinery, I've written a few helper functions to read build definitions from a YAML file, which will produce a usable build system. I'll release this once: I've written some tests, there's better logging, and some basic README-level documentation.

Onward and Upward!

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