Speeding Up Jekyll Builds: Exploring Multithreading Approaches

Speeding Up Jekyll Builds: Exploring Multithreading Approaches | Generated by AI

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Jekyll’s build process is single-threaded by default, which can be slow for large sites. While there’s no built-in configuration option in Jekyll to enable multithreaded builds, you can explore some approaches to achieve parallelism. Below, I outline the possibilities, challenges, and practical steps based on available information and technical considerations.

Why Jekyll Builds Are Single-Threaded

Jekyll processes pages sequentially to ensure dependencies (like site.related_posts or Liquid templates) are handled correctly. Some components, such as Liquid and certain plugins, may not be thread-safe, which complicates multithreading (). This design prioritizes correctness over speed, but for large sites, this can lead to build times of several minutes (,).

Approaches to Multithreaded Jekyll Builds

Here are potential ways to introduce parallelism to Jekyll builds, particularly in the context of a GitHub Actions workflow like the one you provided:

1. Use a Custom Plugin for Multithreaded Rendering

A proof-of-concept plugin for multithreaded rendering has been proposed (). It reduced build time from 45 seconds to 10 seconds in a test case but had issues with thread safety, leading to incorrect page content. The plugin also conflicted with plugins like jekyll-feed, which rely on sequential rendering.

Steps to Try a Custom Plugin:

Create a Plugin: Implement a Ruby plugin that extends Jekyll’s Site class to parallelize page rendering. For example, you could modify the render_pages method to use Ruby’s Thread class or a thread pool ().

module Jekyll
  module UlyssesZhan::MultithreadRendering
    def render_pages(*args)
      @rendering_threads = []
      super # Call original method
      @rendering_threads.each(&:join) # Wait for threads to complete
    end
  end
end
Jekyll::Site.prepend(Jekyll::UlyssesZhan::MultithreadRendering)

Add to Gemfile: Place the plugin in your _plugins directory and ensure it’s loaded by Jekyll.
Test for Thread Safety: Since Liquid and some plugins (e.g., jekyll-feed) may break, test thoroughly. You may need to patch Liquid or avoid multithreading for certain features ().
Integrate with GitHub Actions: Update your workflow to include the plugin in your repository. Ensure the jekyll-build-pages action uses your custom Jekyll setup: ```yaml
- name: Build with Jekyll uses: actions/jekyll-build-pages@v1 with: source: ./ destination: ./_site env: BUNDLE_GEMFILE: ./Gemfile # Ensure your custom Gemfile with the plugin is used ```

Challenges:

Thread safety issues with Liquid and plugins like jekyll-feed ().
Potential for incorrect page rendering (e.g., one page’s content appearing in another).
Requires Ruby expertise to debug and maintain.

2. Parallelize Builds with Multiple Configurations

Instead of multithreading a single build, you can split your site into smaller parts (e.g., by collection or directory) and build them in parallel using multiple Jekyll processes. This approach avoids thread-safety issues but requires more setup.

Steps:

Split the Site: Organize your site into collections (e.g., posts, pages, docs) or directories and create separate _config.yml files for each (,).

# _config_posts.yml
collections:
  posts:
    output: true
destination: ./_site/posts

# _config_pages.yml
collections:
  pages:
    output: true
destination: ./_site/pages

Update GitHub Actions Workflow: Modify your workflow to run multiple Jekyll builds in parallel, each with a different configuration file.

name: Build Jekyll Site
on:
  push:
    branches: [main]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Setup Ruby
        uses: ruby/setup-ruby@v1
        with:
          ruby-version: '3.1'
          bundler-cache: true
      - name: Build Posts
        run: bundle exec jekyll build --config _config_posts.yml
      - name: Build Pages
        run: bundle exec jekyll build --config _config_pages.yml
      - name: Combine Outputs
        run: |
          mkdir -p ./_site
          cp -r ./_site/posts/* ./_site/
          cp -r ./_site/pages/* ./_site/
      - name: Deploy
        uses: actions/upload-artifact@v4
        with:
          name: site
          path: ./_site

Combine Outputs: After parallel builds, merge the output directories into a single _site folder for deployment.

Challenges:

Managing interdependencies between collections (e.g., site.related_posts).
Increased complexity in configuration and deployment.
May not scale well for sites with tightly coupled content.

3. Use a Thread Pool for Large Sites

A pull request for the amp-jekyll plugin suggested using a thread pool to process pages, with a configurable number of threads to avoid overwhelming the system (). This approach balances performance and resource usage.

Steps:

Implement a Thread Pool: Modify or create a plugin to use Ruby’s Thread::Queue to manage a fixed number of worker threads (e.g., 4 or 8, depending on your system).

require 'thread'

module Jekyll
  module ThreadPoolRendering
    def render_pages(*args)
      queue = Queue.new
      site.pages.each { |page| queue << page }
      threads = (1..4).map do # 4 threads
        Thread.new do
          until queue.empty?
            page = queue.pop(true) rescue nil
            page&.render_with_liquid(site)
          end
        end
      end
      threads.each(&:join)
      super
    end
  end
end
Jekyll::Site.prepend(Jekyll::ThreadPoolRendering)

Add Configuration Option: Allow users to toggle multithreading or set the number of threads in _config.yml:
```
multithreading:
  enabled: true
  thread_count: 4
```
Integrate with Workflow: Ensure the plugin is included in your repository and loaded during the GitHub Actions build.

Challenges:

Similar thread-safety issues as the first approach.
Context-switching overhead for large sites with many short tasks ().
Requires testing to ensure compatibility with all plugins.

4. Optimize Without Multithreading

If multithreading proves too complex or risky, you can optimize the single-threaded build process:

Enable Incremental Builds: Use jekyll build --incremental to rebuild only changed files (,). Add to your workflow: ```yaml
- name: Build with Jekyll uses: actions/jekyll-build-pages@v1 with: source: ./ destination: ./_site incremental: true ```
Reduce Plugin Usage: Custom plugins can significantly slow builds (). Audit and remove unnecessary plugins.
Use Faster Converters: Switch from Kramdown to a faster markdown processor like CommonMark, or test Pandoc for specific use cases ().
Cache Dependencies: Ensure bundler-cache: true in your GitHub Actions workflow to avoid reinstalling gems ().

Recommendations

Start with Incremental Builds: This is the simplest way to speed up builds without risking thread-safety issues. Add --incremental to your workflow and test its impact.
Experiment with a Thread Pool Plugin: If you have Ruby expertise, try implementing a thread pool plugin with a configurable number of threads (Option 3). Start with a small site to test thread safety.
Avoid Full Multithreading for Now: Given the thread-safety concerns with Liquid and plugins (), full multithreading may require significant refactoring or a custom Liquid fork, which may not be practical unless you’re contributing to Jekyll’s core development.
Monitor Build Times: Use GitHub Actions logs to measure build times and identify bottlenecks. For example, a site taking 6 minutes to build () suggests plugins or large asset processing may be the issue.

GitHub Actions Example with Incremental Builds

Here’s an updated version of your workflow with incremental builds enabled:

name: Build Jekyll Site
on:
  push:
    branches: [main]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Setup Ruby
        uses: ruby/setup-ruby@v1
        with:
          ruby-version: '3.1'
          bundler-cache: true
      - name: Build with Jekyll
        uses: actions/jekyll-build-pages@v1
        with:
          source: ./
          destination: ./_site
          incremental: true
      - name: Deploy
        uses: actions/upload-artifact@v4
        with:
          name: site
          path: ./_site

Caveats

Thread Safety: Multithreading in Jekyll is experimental and may break plugins or produce incorrect output (). Test thoroughly on a small site first.
GitHub Actions Limits: Parallel builds (Option 2) increase resource usage, so ensure your workflow stays within GitHub’s free-tier limits.
Jekyll 5 Potential: Future versions of Jekyll may address multithreading natively, as it’s been discussed (). For now, custom solutions are needed.

If you need help implementing a specific approach (e.g., writing a plugin or splitting builds), let me know, and I can provide more detailed guidance or code!

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