Optimizing Heroku

2011-06-01

I spent some time recently playing with a trivial Rails 3 app using girl_friday and learned a few interesting things:

• Michael’s post on using Unicorn with Heroku is a fantastic idea and great way to improve the efficiency of your Heroku application’s web dynos.

• Heroku imposes a limit of 15 threads per Ruby process (there is an open question about this, it might just be a bug in the Heroku Cedar stack, which is still beta). If you are using thin, you get 1 process and 15 threads per dyno. If you are using Unicorn, you get 3 processes^* and 45 threads. UPDATE: I don’t believe Heroku has a thread limit, just a memory limit.

• girl_friday defaults to 5 threads per queue. With the thread limit on Heroku, you’ll want to carefully ration the size of each queue. girl_friday’s Rack status app can tell you at runtime which queues are busy and which are quiet for tuning purposes.

• girl_friday can, in many cases, replace or dramatically reduce your need for separate worker dynos, saving you money.

• Where 3 is based on the memory consumption of your application. YMMV.

Unicorn appears to work great with girl_friday and Heroku’s free service level actually becomes useful for non-trivial applications with these optimizations. Know any other tips? Let me know!