[Pgpool-general] connection limits

[Tatsuo Ishii]

> I am curious what information I can get regarding connection limits per
> pgpool server.   I am sure there are several variables involved but my
> primary purpose for asking is to decided whether or not to have many load
> balanced web servers talk to individual pgpool servers running locally that
> each talk to various db servers in a replication pool or if I should only
> have one pgpool server per db replicant.  There are advantages to both
> sides, however some understanding about the limitations (or lack of) that
> pgpool has in the number of connections it can accept (app to pgpool, not
> pgpool to pg), and what kind/measure of system resources are consumed for
> those connections.
> 
> I suspect it's probably not a deal breaker either way.

The connection limits for app to pgpool is deinfed by
"num_init_children". For example, if num_init_children = 32, there are
32 pgpool child processes and each process issues accept(2). Thus up
to 32 apps can connect to pgpool child processes. If 33th app tries to
connect to pgpool, the request will be blocked and added to the
kernel's listen queue. This is very different from
PostgreSQL(postmaster)'s behavior, in that postmaster will not accept
more than mx_connections request and just throws an error.

Note that there is a limit for the length of listen queue. pgpool
tries to set the length as long as possible, but the actual length
will vary depending the kernel you are using (I'm not sure the exact
number). If your SQL pattern is heavy enough to run out the listen
queue, you might want to set larger value for num_init_children, or
start multiple pgpool instances as you suggest.
--
Tatsuo Ishii
SRA OSS, Inc. Japan