AWS RDS Postgresql and max_worker_processes

Question

AWS RDS Postgresql 12.10

According to https://www.enterprisedb.com/postgres-tutorials/comprehensive-guide-how-tune-database-parameters-and-configuration-postgresql,

max_worker_processes Set this to the number of CPUs you want to share for PostgreSQL exclusively. This is the number of background processes the database engine can use. Setting this parameter will require a server restart. The default is 8.

The default is also 8 in AWS RDS Postgresql, no matter the Instance type (and thus number of vCPUs). Am I cheating myself by paying for a db.r5.12xlarge (48 CPUs) while using the default max_worker_processes value?

• jjanes: On v13, the RDS default is GREATEST(${DBInstanceVCPU*2},8)

Answer 1

If you depend on large scale parallel query from just a few concurrent sessions to get your work done, then surely you have been cheating yourself. But if you don't, then probably not. For example, if you have a large number of simultaneous connections all submitting CPU intensive queries at the same time, you will probably be able to keep all 24 CPUs busy without any parallelization. ("virtual" CPUs doesn't mean much, for r5.12xlarge more than 24 CPU-intensive jobs running at the same time will start competing with each other for processing time, regardless of the number of "vCPU" that are claimed)

• RonJohn: Your answer seems to say that max_worker_processes is per-connection, but the EDB link says "This is the number of background processes the database engine can use." ISTM that is a system-wide number, not per-connection. What am I missing?
• jjanes: @RonJohn, sorry, I don't understand what part of my answer you read that way. Connections always get at least one process, regardless of max_worker_processes. It is only additional workers (parallel workers) that are restricted.
• RonJohn: It's not your answer, it's the EDB document.