We use parallel_tests to speed up our test suite. It works by splitting the tests and running them in concurrent processes. This can cause some issues with resource limits in OSX.
The main culprit is MYSQL. By default each table is stored in it's own file. This is great when backing up, but can cause you to run into the OSX limit on open files.
The simplest approach I found to fixing this is configuring mysql to store all tables in one file. This can be set in your my.cnf file. If MYSQL was installed via homebrew, you should create this file `/usr/local/etc/my.cnf'. The key setting is: innodb_file_per_table = OFF. You can just copy my my.cnf fille here: https://gist.github.com/gmanley/e9c9d0e5f7c3f94eb9a9#file-my-cnf
The rest of the settings are just optimizations and aren't needed to fix this issue. I recommend using them though.
TL;DR:
curl -o /usr/local/etc/my.cnf https://gist.githubusercontent.com/gmanley/e9c9d0e5f7c3f94eb9a9/raw/792189338679592e7e9559eb1c70b11cae75629b/my.cnf
You will need to drop your databases and recreate them for this change to take effect.
It's also a good idea to up the file open limit on OSX. It is no longer needed for mysql, but running many rails processes can open quite a bit files itself, and cause another issue. You can configure it like so:
launchctl limit maxfiles 4096 4096
And to persist this change through reboot, do the following:
echo "launchctl limit maxfiles 4096 4096" | sudo tee -a /etc/launchd.conf
Then just change the num_processes variable in build.sh to a higher limit. I usually run around 6, but higher isn't always faster. My build times take about 10m.