Tuning Legacy vcs-sync for 2x profit!

One of the challenges of maintaining a legacy system is deciding how much effort should be invested in improvements. Since modern vcs-sync is “right around the corner”, I have been avoiding looking at improvements to legacy (which is still the production version for all build farm use cases).

While adding another gaia branch, I noticed that the conversion path for active branches was both highly variable and frustratingly long. It usually took 40 minutes for a commit to an active branch to trigger a build farm build. And worse, that time could easily be 60 minutes if the stars didn’t align properly. (Actually, that’s the conversion time for git -> hg. There’s an additional 5-7 minutes, worst case, for b2g_bumper to generate the trigger.)

The full details are in bug 1226805, but a simple rearrangement of the jobs removed the 50% variability in the times and cut the average time by 50% as well. That’s a savings of 20-40 minutes per gaia push!

Moral: don’t take your eye off the legacy systems – there still can be some gold waiting to be found!