While working on tweaking performance for a client, I was able to shave 7 seconds of PHP execution time time off the homepage load. The cause was eventually tracked down to calls out to TinyURL for every node being rendered. The core problem came from the service links module. We were able to fix it by disabling short URL’s in the module, but the problem has been addressed in the current pre-release 2.x branch by using caching. We might have ended up discovering this by disabling module after module one at a time, but that would have taken forever. In today’s world of API’s and social media, it’s very common for a module to make calls to outside websites. However, care should be taken by the module authors when coding in these features. In our example, each call to TinyURL was fairly fast (300 - 400ms average), but on a homepage displaying 25 nodes, that adds over 7 seconds page load time. Think of the impact if TinyURL experienced a large slowdown, or even an outage? As far as I know, Drupal doesn’t give you a way out of the box to track such requests. However, using the tcpdump binary which is available on virtually all Unix variants, we can see exactly what’s happening. Note that you need root access to run tcpdump. Let’s say that the IP address of your primary interface is 10.0.0.1. By using this tcpdump command, we can see all outbound HTTP and HTTPS requests in real time: tcpdump src host 10.0.0.1 and dst port 80 or dst port 443
If you don’t get any traffic after a few seconds, go hit your /cron.php page - this should generate some traffic like this for you to see: Here we can see that our host is making a bunch of outbound requests to master.drupal.org. This is because the “Update Status” module is checking to see what upgrades are available for us. What if you see traffic and don’t know what module is causing it? grep to the rescue! In order to find out which module was making the calls to TinyURL, we ran the following command: grep -R ‘tinyurl.com’ /path/to/drupal/sites/all/modules/*
This returned one hit, from /path/to/drupal/sites/all/modules/service_links/service_links.module. By disabling the short links feature within the module we decreased page load time by 7 seconds!