title: Doxygen is Slow
body: <p>Doxygen takes about 12 hours to run on our code base. This is primarily because there is a lot of code to process (~1.5M lines). However, it's very quickly approaching the point at which we can't do nightly documentation updates because they take too long. We've already had to reduce the graph depth to get it down to 12 hours.</p>
  
  <p>I've tried the standard approaches, but I really do need high quality output, and this includes graphs and SEARCH_INCLUDES. I have a fairly good machine to run Doxygen on, but Doxygen doesn't take advantage of its many cores. (It pegs a single CPU on the build server, but is only 4% of the available system.) Having a multithreaded Dot build is handy, though that's only half or so of the build time.</p>
  
  <p>Are there any techniques I can use to run doxygen via multiple processes and manually shard the task? I've seen some talk about creating tag files, but I don't understand enough about them to know if they'd do what I want. What I'm looking for is something like:</p>
  
  <pre><code>doxygen Doxyfile-folder1
  doxygen Doxyfile-folder2
  doxygen Doxyfile-folder3
  doxygen Doxyfile-folder4
  doxygen-join output/folder1/html output/folder2/html output/folder3/html output/folder4/html
  </code></pre>
  
  <p>Of course, I'm just making stuff up, but that's an idea of what I'm looking for. Also, I'd use a lot more than 4 processes.</p>