is it correct parameters for pgbouncer.ini and postgresql.conf?

Question

I have pgbouncer.ini file with the below configuration

[databases]
test_db = host=localhost port=5432 dbname=test_db 

[pgbouncer]
logfile = /var/log/postgresql/pgbouncer.log
pidfile = /var/run/postgresql/pgbouncer.pid
listen_addr = 0.0.0.0
listen_port = 5433
unix_socket_dir = /var/run/postgresql 
auth_type = md5
auth_file = /etc/pgbouncer/userlist.txt
admin_users = postgres

#pool_mode = transaction
pool_mode = session
server_reset_query = RESET ALL; 
ignore_startup_parameters = extra_float_digits
max_client_conn = 25000
autodb_idle_timeout = 3600
default_pool_size = 250
max_db_connections = 250
max_user_connections = 250

and I have in my postgresql.conf file

max_connections = 2000

does it effect badly on the performance ? because of max_connections in my postgresql.conf ? or it doesn't mean anything and already the connection handled by the pgbouncer ?

one more question. in pgpouncer configuration, does it right listen_addr = 0.0.0.0 ? or should to be listen_addr = * ?

Is it better to set default_pool_size on PgBouncer equal to the number of CPU cores available on this server?

Shall all of default_pool_size, max_db_connections and max_user_connections to be set with the same value ?

Answer 1

So the idea of using pgbouncer is to pool connections when you can't afford to have a higher number of max_connections in PG itself. NOTE: Please DO NOT set max_connections to a number like 2000 just like that.

Let's start with an example, if you have a connection limit of 20 and then your app or organization wants to have a 1000 connections at a given time, that is where pooler comes into picture and in this specific case you want the 20 connections to pool that 1000 coming in from the application.

To understand how it actually works let's take a step back and understand what happens when you do not have a connection pooler and only rely on PG config setting for the max connections which in our case is 20.

So when a connection comes in from a client\application etc. the main process of postgresql(PID, i.e. parent ID) spawns a child for that. So each new connection spawns a child process under the main postgres process, like so:

PID USER      PR NI VIRT RES  SHR S %CPU %MEM TIME+  COMMAND             
24379 postgres  20 0 346m 148m 122m R 61.7  7.4 0:46.36 postgres: sysbench sysbench ::1(40120) 
24381 postgres  20 0 346m 143m 119m R 62.7  7.1 0:46.14 postgres: sysbench sysbench ::1(40124) 
24380 postgres  20 0 338m 137m 121m R 57.7  6.8 0:46.04 postgres: sysbench sysbench ::1(40122) 
24382 postgres  20 0 338m 129m 115m R 57.4  6.5 0:46.09 postgres: sysbench sysbench ::1(40126)

So now once a connection request is sent, it is received by the POSTMASTER process and creates a child process at OS level under the main parent process. This connection then has a life span of "unlimited" unless close by the application or you have a time out set for idle connections in postgresql.

Now here comes the situation where it can be a very costly affair to manage the connections with a given compute, if they exceed a certain limit. Meaning n number of connections when served have a given compute cost and after some time the OS won't be able to handle a situation with HUGE connections and will in turn cause contentions at different compute level(i.e. Memory, CPU, I/O).

What if you can use the presently spawned child processes(backends) if they are not doing any work. You will save time on getting the child process(backends) and the additional cost as well(this can be different at times). This is where the pool of connections that are always open help to serve different client requests comes in and is also called pooling.

So basically now you have only n connections available but the pooler can manage n+i number of connections to serve the client requests.

This where pg-bouncer helps to reuse the connections. It can be configured with 3 types of pooling i.e Session pooling, Statement pooling and Transaction pooling. Basically bouncer returns the connection back to the pool once it has done, statement level work or transaction level work etc. Only during session pooling it keeps the connections unless it disconnects.

So basically lower down the number of connections at PG conf file level and tune all settings in the bouncer.ini.

To answer the second part:

one more question. in pgpouncer configuration, does it right listen_addr = 0.0.0.0 ? or should to be listen_addr = * ?

It depends if you have a standalone deployment, server etc. basically if its on the server itself and you want it to allow connections from everywhere(incoming) use "*" if you want only the local network to be allowed use "127.0.0.0".

For the rest of your questions check this link: pgbouncer docs

I have tried to share a little of what I know, feel free to ask away if anything was unclear or or correct if it was incorrectly mentioned.