Should I use preload_app with Puma on Heroku?

Question

I am running a small Rails app on Heroku with one dyno and several Puma workers.

The Puma docs say:

General rule is to use preload_app when your workers die often and need fast starts. If you don’t have many workers, you probably should not use preload_app.

This suggests that I should not use preload_app! in my config/puma.rb. However, I have a few unanswered questions:

• When do my workers die? Will they be reaped and re-forked after a certain number of requests? How can I monitor this?
• How do I know if my workers need fast starts?

It's clear that preload_app! should save resources when using many workers, but I don't see the disadvantage of using it even with a small number of workers.

Heroku's recommended config for "a simple Rails application" includes preload_app! but they don't offer any guidance on when not to use it.

When should preload_app! not be used, and why? (Ignoring the phased restart issue.)

Answer 1

Besides starting worker fast, preload_app! also saves memory thanks to Copy on Write feature introduced in Ruby 2.0.

You can read this great write-up from Heroku introducing the idea of copy-on-write in Ruby:

Copy on Write or COW is an optimization that can reduce the memory footprint of a Ruby process when it is copied. Instead of allocating duplicate memory when a process is forked, COW allows multiple processes to share the same memory until one of the processes needs to modify a piece of information.

Thus no matter how many works you have, it's always recommended to enable preload_app! if possible.