How can I find a memory leak on Heroku?

Question

I have a Rails 3.2.8 app running on Heroku Cedar with Ruby 1.9.3. The app runs fine when it launches but after a day or so of continuous use, I start to see R14 errors on my log

Answer 1

The GC should do the clean up, and probably does.

You can force the GC with GC.start; if many objects were not collected this will, but I suspect that is not the issue.

Is it possible you somehow create a bunch of objects and never release them, by keeping cached copies or something?

I'm unfamiliar with the existing tools to check this, but you may want to check which objects exist using ObjectSpace. For example:

ObjectSpace.each_object.with_object(Hash.new(0)){|obj, h| h[obj.class] +=1 }
# => a Hash with the number of objects by class

If you get an unexpected number for one of your classes, for instance, you would have a better idea of where to look for.

Answer 2

Install the New Relic add-on. It has a bunch of useful metrics that you can use to find out the source of the leak. I think its generally a better idea to try to see which part of the code takes the longest to execute and perhaps try to optimize that, rather than tweak the GC outright.

Some of the nice features New Relic includes is being able to pinpoint the source of the longest running SQL query, for example. I encourage you to give it a try.