Why would a rails 5 upgrade drastically increase database CPU usage?

Question

I upgraded a production app from rails 4 to rails 5 a few days ago, and I have seen a big increase in CPU utilization on my RDS postgresql instance. There does not seem to be any associated increase in database requests or decrease in speed.

What could cause that? What can I do to bring CPU utilization back down while still using Rails 5?

The image below shows CPU utilization on the RDS instance. The time of the upgrade is just to the left of the 11/09 line.

Hirsute

Is the system using prepared statements? The SQL in the log file would have $1, $2 etc and a series of bound parameters. – Jegar

yes, we are using prepared statements – Hirsute

Could this be related? github.com/rails/rails/commit/… – Hirsute

You can see at the tail end of this graph an immediate drop in CPU usage variance that happened as soon as we switched the server back to Rails 4. – Alignment

We reverted the postgresql_adapter.rb changes from the above commit and have also tried explicitly disabling prepared_statements in our database.yml, both without improvement. This leads me to believe that the Rails 5 DB CPU load uptick is not related to prepared statement management. – Tailwind

Answer 1

We eventually tracked this down to an issue in activerecord: https://github.com/rails/rails/issues/27201

Basically, since changed? was sometimes erroneously set to true, we were doing more queries in rails 5 than in rails 4, and we were mis-reading the reports in RDS, so we had not identified the increase in queries.

Hirsute