Should I decrease max_connections in PostgreSQL when using PgBouncer?

Question

Why shall I decrease max_connections in PostgreSQL when I use PgBouncer? Will there be a difference if I set max_connections in PostgreSQL's config equal 100 or 1000 when I use PgBouncer to limit connections below either?

Answer 1

Each possible connection reserves some resources in shared memory, and some backend private memory is also scaled to it. Reserving this memory when it will never be used is a waste of resources. This was more of an issue in the past, when shared memory resources were much more fiddly than they are on modern OS.

Also, there is some code which needs to iterate over all of those resources, possibly while holding locks, so it takes more time to do that if there is more data to iterate over. The exact nature of the iteration and locks have changed from version to version, as code was optimized to make it more scalable to large number of CPUs.

Neither of these effects is likely to be huge when the most of the possible connections are not actually used. Maybe the most important reason to lower max_connections is to get instant diagnosis in case pgbouncer has been misconfigured and is not doing its job correctly.