I'm using Jekyll for building my site. When I write a new entry, I have executed jekyll --auto. But I noticed that ruby consumes up to 25% of CPU on my quad core PC.

This problem is caused by directory_watcher module. It executes File::Stat() on all files under the given directory every one second! It works fine when the number of files is small, but as it grow up, it begins wasting our CPU.

For example, my site has over 600+ posts by which Jekyll generates 900 files. And more, there are 5000 files under .git dir. Suprisingly, Jekyll ~0.12 watches all files under the current directory, which means that it execute stat to these 6,500 files every one second.

Jekyll 1.0's jekyll build --watch doesn't watch .git and _site directory. Despite of such improvement, ruby still uses up to 10% of CPU.

So I decided to use Grunt instead of using --watch (or --auto) option.

Grunt config files

I'm use following versions:

• Jekyll 1.0.3
• Grunt 0.4.1
  • grunt-shell-spawn plugin
  • grunt-contrib-watch plugin

At first, I used grunt-jekyll instead of grunt-shell-spawn, but grunt-jekyll plugin doesn't show Jekyll's output until it exits.

See Getting started - Grunt to setup Grunt.

Here is my package.json and Gruntfile.js.

package.json

{
  "name": "tech-ni",
  "version": "0.1.0",
  "devDependencies": {
    "grunt": "~0.4.1",
    "grunt-shell-spawn": "~0.2.4",
    "grunt-contrib-watch": "~0.4.4"
  }
}

Gruntfile.js

module.exports = function(grunt) {
  grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    shell: {
      jekyll: {
        command: 'jekyll build',
        options: {
          async: false
        }
      }
    },
    watch: {
      jekyll: {
        files: ['_posts/**/*.md', '_layout/*.html', '_includes/*.html'],
        tasks: ['shell:jekyll']
      }
    }
  });

  grunt.loadNpmTasks('grunt-shell-spawn');
  grunt.loadNpmTasks('grunt-contrib-watch');

  grunt.registerTask('default', ['shell:jekyll']);
};

(If you're using Jekyll ~0.12, replace jekyll build to jekyll)

How to Use

By entering grunt watch, it starts watching files. When some posts or HTML files are modified, grunt executes jekyll build.

But CPU usage is not 0%. It turns out that grunt-contrib-watch doesn't use native watch API -- fs.watch(). It uses gaze module which execute fs.statSync() periodically.

Oh!! Nothing changes...

It seems that fs.watch() had many problems such as not reporting filenames on mac and executing fs.stat() became popular. Oh, hell!