As part of our RelEng Blogging Blitz, I'll give a quick overview of when and how builds get triggered on our main build infrastructure.
There are three ways builds can be triggered.
The first, and most common way, is when a developer pushes his or her changes to hg.mozilla.org. Our systems check for new changes every minute or so, and put new changes into a queue. Once the tree has been quiet for 3 minutes (i.e. no changes for 3 minutes), a new build request is triggered with all queued changes. If there is a free slave available, then a new build starts immediately, otherwise the build request is put in a queue.
The second way builds are triggered is via a nightly scheduler. We start triggering builds on branches at 3:02am pacific local time (some branches are triggered at 3:32am or 4:02 am). We run at 3:02am to avoid problems with daylight savings <-> standard time transitions. In the fall there are two 2:59am's when we go back to standard time, and in the spring transition there is no 2:59am. The start times are staggered to avoid slamming hg.mozilla.org, or other shared resources.
The last way builds can be triggered is manually. The Release Engineering Sheriff can trigger builds on specific revisions, or rebuild past builds pretty easily, so if you need a build triggered, contact your friendly neighbourhood RelEng Sheriff!