Speed Up Your NixOS Config: Faster Evaluations!

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Speed Up Your NixOS Config: Faster Evaluations!

Hey there, fellow NixOS enthusiasts! 👋 Ever feel like your NixOS configuration evaluations are taking an eternity? You're not alone! I recently ran into the same issue and wanted to share some insights and tips on how to significantly speed up those evaluations. Let's dive in and get your configurations evaluating lightning-fast!

The Problem: Slow NixOS Configuration Evaluation

So, the scenario is this: you're making changes to your NixOS configuration, and you want to see if everything's shipshape. You run nix eval, and… you wait. And wait. And maybe go grab a coffee while you wait. Sound familiar? In my case, evaluating a single host configuration was taking around a minute – definitely not ideal when you're trying to iterate quickly. The original poster observed the same slowness and was looking for a way to fix it. Here's the situation:

[lewdo@sforza:~/Code/nixos]$ time nix eval .#nixosConfigurations.sforza.config.system.build.toplevel \
  --substituters "" \
  --option eval-cache false \
  --raw
/nix/store/15x5kc2zs3k0l6admyhpx5ijmc4f0kjb-nixos-system-sforza-25.11.20251026.d8cc103

real    1m4.644s
user    0m46.472s
sys     0m5.970s

As you can see, a full evaluation clocking in at over a minute is something we can definitely improve. The goal? Bring those evaluation times down, making your workflow smoother and more efficient. Ideally, we want to get the evaluation time down to under 20 seconds, and even better, closer to 5-15 seconds. Let's see how we can achieve that!

Understanding the Bottlenecks: Why Is Nix Evaluation Slow?

Before we jump into solutions, let's understand why Nix evaluation can be slow. Several factors can contribute to this sluggishness. Identifying these bottlenecks is the first step toward optimization.

  • Complex Configurations: The more intricate your configuration, the longer it will take to evaluate. Complex configurations involve more packages, services, and system settings, leading to a greater number of dependencies and evaluations.
  • Inefficient Code: Poorly written Nix code can also be a significant factor. If your Nix expressions are not optimized or contain unnecessary computations, they can slow down the evaluation process. This includes things like inefficient use of let and in blocks, redundant calculations, or overusing built-in functions that are computationally expensive.
  • External Dependencies: Nix often fetches dependencies from the internet. If your network connection is slow or the servers hosting the dependencies are experiencing issues, evaluation times can increase dramatically. Caching mechanisms can help here, but initial evaluations can still suffer.
  • Lack of Caching: Nix uses a caching mechanism to speed up evaluations. However, if the cache is disabled or not effectively utilized, the system has to re-evaluate the same expressions over and over again.
  • Hardware Limitations: The speed of your CPU, the amount of RAM you have, and the speed of your storage (especially if you're working with a large Nix store) can all influence evaluation times. Slow hardware naturally leads to slower evaluations.
  • Overuse of builtins.fetchurl: Repeatedly fetching URLs can significantly slow down the evaluation process. Each fetch involves network requests, which can be time-consuming. Try to cache downloads or use local files whenever possible.

Optimization Strategies: Speeding Up Your NixOS Evaluation

Now, let's explore some strategies to optimize your NixOS configuration and reduce evaluation times. Remember, there's no one-size-fits-all solution, and the best approach may involve a combination of these techniques.

1. Optimize Your Nix Code

  • Profiling: Start by profiling your configuration to identify the most time-consuming parts. Tools like nix-instantiate --eval-profile can help pinpoint performance bottlenecks. This generates a profile that shows you where the time is spent during evaluation.
  • Simplify Expressions: Review your Nix expressions and simplify them whenever possible. Avoid unnecessary computations, redundant code, and complex logic that could be simplified.
  • Use let and in Effectively: Organize your code using let and in blocks to avoid redundant calculations. This allows you to define variables once and reuse them throughout your configuration.
  • Lazy Evaluation: Understand how Nix's lazy evaluation works. Avoid forcing the evaluation of expressions that aren't immediately needed. This can save significant time, especially with complex configurations.
  • Avoid Deep Nesting: Minimize the depth of nested expressions. Deeply nested structures can make evaluation slower and harder to understand.

2. Leverage Caching and Substituters

  • Enable the Eval Cache: Make sure the evaluation cache is enabled. This stores the results of previous evaluations, allowing Nix to reuse them in subsequent evaluations. You can enable it with the command: nix eval --option eval-cache true. Double-check that your nix.conf or user-specific configuration (e.g., ~/.config/nix/nix.conf) has the eval-cache-size option set to an appropriate value (e.g., 1024MB or larger).
  • Configure Substituters: Use Nix substituters to download pre-built packages from a binary cache instead of building them from source. This can dramatically reduce evaluation and build times. Common substituters include NixOS's official cache (cache.nixos.org) and caches provided by community projects.
  • Use a Local Cache: For even faster evaluations, consider setting up a local cache. This can be especially beneficial if you have multiple machines using the same Nix configuration.

3. Optimize Dependencies and Downloads

  • Minimize External Fetches: Reduce the number of external fetches (e.g., builtins.fetchurl) in your configuration. If possible, download files locally and reference them from your configuration. Consider caching the results.
  • Network Optimization: Ensure you have a stable and fast internet connection. Poor network conditions can significantly increase evaluation times, especially if you rely heavily on external dependencies.

4. Improve Configuration Structure

  • Modularize Your Configuration: Break down your configuration into smaller, more manageable modules. This makes your configuration easier to understand and maintain, and can also improve evaluation times. Smaller modules evaluate faster.
  • Avoid Unnecessary Packages: Review the packages you're installing and remove any that are not essential. The fewer packages, the faster the evaluation will be. Unused packages add to the configuration size and evaluation time.
  • Use lib.mkDefault and Similar Functions: Utilize functions like lib.mkDefault and lib.optional to avoid evaluating parts of your configuration that aren't needed under certain conditions. This can prevent unnecessary computations.

5. Hardware Considerations

  • Increase RAM: More RAM can significantly improve performance, especially when dealing with large configurations. Ensure your system has sufficient RAM to handle the evaluation process.
  • Use SSD Storage: An SSD is essential for fast storage access. It will dramatically improve the performance of read/write operations during evaluation and build processes. Avoid using HDDs, if possible.
  • CPU Performance: A faster CPU will improve the speed of evaluations, especially for complex configurations involving many calculations and dependencies. Consider upgrading your CPU if you are frequently dealing with slow evaluations.

Practical Tips and Tricks

  • Incremental Changes: Make small, incremental changes to your configuration and evaluate frequently. This helps you quickly identify any performance regressions and pinpoint the cause of slowdowns.
  • Use nix-env for Testing: When testing changes, consider using nix-env to install packages temporarily. This avoids having to rebuild your entire system configuration every time you want to test a single package.
  • Regularly Update Nix: Keep your Nix and Nixpkgs up to date. Newer versions often include performance improvements and bug fixes that can impact evaluation times.
  • Monitor Evaluation Time: Use the time command (as shown in the original post) to measure evaluation times and track your progress as you implement optimizations.
  • Use Pre-built Binaries: When possible, use pre-built binaries instead of building from source. This significantly speeds up the build process.

Conclusion: Faster NixOS Evaluations for a Better Workflow

Optimizing your NixOS configuration evaluation is a worthwhile endeavor. By understanding the bottlenecks, implementing optimization strategies, and using practical tips, you can dramatically improve the speed of your evaluations. This leads to a smoother, more efficient workflow, allowing you to iterate faster and enjoy the benefits of NixOS without the frustration of long wait times. Remember to profile your configuration, simplify your expressions, leverage caching and substituters, and structure your configuration effectively. Happy Nixing, and may your evaluations always be swift!

I hope this comprehensive guide has given you a solid foundation for speeding up your NixOS configuration evaluations. If you have any questions or want to share your own optimization tips, feel free to drop a comment below. Let's make NixOS even better together!