Rails 3.2 -> 4.2 Emergency upgrade, memory leak on Heroku

with the May 1st deprecation of the Heroku-18 stack, I needed to update a very old Rails app to the minumum version required. It was though, but I got it working.

I have a completely new version to deploy soon based rails 6, but for the time being, I need to run this old app. I noticed the memory footprint almost doubled (on 3.2 it was avg 90-120% of 512mb ram) and my immediate stop gap solution is to update the dynos to 2x 1gb ram, however, I would like to pinpoint the isse.

2023-05-01T17:46:12.209506+00:00 heroku[web.1]: Process running mem=990M(188.0%)
2023-05-01T17:46:12.211046+00:00 heroku[web.1]: Error R14 (Memory quota exceeded)
2023-05-01T17:46:33.107262+00:00 heroku[web.1]: Process running mem=990M(188.0%)
2023-05-01T17:46:33.111875+00:00 heroku[web.1]: Error R14 (Memory quota exceeded)
2023-05-01T17:46:54.224029+00:00 heroku[web.1]: Process running mem=990M(188.0%)
2023-05-01T17:46:54.227886+00:00 heroku[web.1]: Error R14 (Memory quota exceeded)
2023-05-01T17:47:15.327098+00:00 heroku[web.1]: Process running mem=990M(188.0%)
2023-05-01T17:47:15.328612+00:00 heroku[web.1]: Error R14 (Memory quota exceeded)
2023-05-01T17:47:36.150757+00:00 heroku[web.1]: Process running mem=990M(188.0%)
2023-05-01T17:47:36.152131+00:00 heroku[web.1]: Error R14 (Memory quota exceeded)
2023-05-01T17:47:57.287322+00:00 heroku[web.1]: Process running mem=990M(188.0%)
2023-05-01T17:47:57.289241+00:00 heroku[web.1]: Error R14 (Memory quota exceeded)

The old app used memcachd, the new app is using in-memory caching, which can surely be one of the issues. Can I switch to file-base caching? Should I look into Redis caching or any other solution that I have not considered?

This app is also using Unicorn, all my newer apps are using Puma. Is this worth looking into?