Delayed Job on Heroku does not work - ruby-on-rails-4

Question

My app runs fine on my local machine which has 16 Gig of Ram using 'heroku local' command to start both the dyno and workers using the Procfile. The background jobs queued in Delayed Job are processed one-by-one and then the table is emptied. When I run on Heroku, it fails to execute the background processing at all. It gets stuck with the following out of memory message in my logfile:
2016-04-03T23:48:06.382070+00:00 app[web.1]: Using rack adapter
2016-04-03T23:48:06.382149+00:00 app[web.1]: Thin web server (v1.6.4 codename Gob Bluth)
2016-04-03T23:48:06.382154+00:00 app[web.1]: Maximum connections set to 1024
2016-04-03T23:48:06.382155+00:00 app[web.1]: Listening on 0.0.0.0:7557, CTRL+C to stop
2016-04-03T23:48:06.711418+00:00 heroku[web.1]: State changed from starting to up
2016-04-03T23:48:37.519962+00:00 heroku[worker.1]: Process running mem=541M(105.8%)
2016-04-03T23:48:37.519962+00:00 heroku[worker.1]: Error R14 (Memory quota exceeded)
2016-04-03T23:48:59.317063+00:00 heroku[worker.1]: Process running mem=708M(138.3%)
2016-04-03T23:48:59.317063+00:00 heroku[worker.1]: Error R14 (Memory quota exceeded)
2016-04-03T23:49:21.449475+00:00 heroku[worker.1]: Error R14 (Memory quota exceeded)
2016-04-03T23:49:21.449325+00:00 heroku[worker.1]: Process running mem=829M(161.9%)
2016-04-03T23:49:24.273557+00:00 app[worker.1]: rake aborted!
2016-04-03T23:49:24.273587+00:00 app[worker.1]: Can't modify frozen hash
2016-04-03T23:49:24.274764+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/attribute_set/builder.rb:45:in `[]='
2016-04-03T23:49:24.274771+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/activerecord-4.2.6/lib/active_record/attribute_set.rb:39:in `write_from_user'
I know that R14 is out of memory error. so I have two questions:
Is there anyway that delayed job can be tuned to take less memory. There will be some disk swapping involved, but it least it will run.
Why do I keep getting rake aborted! Can't modify frozen hash error (lines 4 and 5 from bottom of the log shown below). I do not get it in my local environment. What does it mean? Is it memory related?
Thanks in advance for your time. I am running Rails 4.2.6 and delayed_job 4.1.1 as shown below:
→ gem list | grep delayed
delayed_job (4.1.1)
delayed_job_active_record (4.1.0)
delayed_job_web (1.2.10)
Bharat

Answer 1

I found the problem. I am posting my solution here for those who may be running in similar problems.
I increase the heroku worker memory to use 2 standard dynos meaning I gave it 1 Gig memory so as to remove the memory quota problem. That made R14 go away, but still I continued to get
rake aborted!
Can't modify frozen hash
error and the program will crash then. So the problem was clearly here. After much research, I found that the previous programmer had used the 'workless' gem to reduce heroku charges. Workless gem makes heroku workers go to sleep when not being used and therefore no charges are incurred when not running heroku.
What I did not post in my original question is that I have upgraded the app from Rails 3.2.9 to Rails 4.2.6. Also my research showed that the workless gem had not been upgraded in the last three years and there was no mention on rails 4 on their site. So the chances were that it may not work well with Rails 4.2.6 and Heroku.
I saw some lines in my stack trace which were related to the workless gem. This was a clue for me to see what happens if I subtract, i.e., remove this gem from production. So I removed it and redeployed.
The frozen hash error went away and my delayed_job worker ran successfully to completion on Heroku.
The lesson for me was carefully read the log and check out all the dependencies :)
Hope this helps.