How to install jemalloc and save RAM on your Mastodon instance

mastodon-install-jemalloc.txt

### If you set up your Mastodon instance before 9th of June 2018, you probably
### might not have jemalloc active on your Ruby setup. Jemalloc changes the way Ruby 
### allocates memory and saves up RAM. On my 140 users instance RAM usage dropped by 300 MB just by using jemalloc.
### This is how to set it up on Debian 9 Stretch:

### As root, install libjemalloc-dev:
  
  apt install libjemalloc-dev

### Change to mastodon user:
  
  su - mastodon

### Check your active ruby version and if there maybe is a new version recommended in the Mastodon Production Guide:
### https://github.com/tootsuite/documentation/blob/master/Running-Mastodon/Production-guide.md#dependencies-that-need-to-be-added-as-a-non-root-user
### (see rbenv install command). At the time of writing my server was running on Ruby 2.5.0, but version 2.5.1 
### was recommended. Therefore I did not recompile the old Ruby version, but the new one instead:

  RUBY_CONFIGURE_OPTS=--with-jemalloc rbenv install 2.5.1

### Make sure you're on the latest Ruby version:

  rbenv global 2.5.1
  
### Leave mastodon user and restart all the services:

  exit
  systemctl restart mastodon-sidekiq
  systemctl restart mastodon-streaming
  systemctl restart mastodon-web
  
### DONE!