[pgpool-general: 3260] Re: Backend selection algorithm

Tatsuo Ishii ishii at postgresql.org
Thu Nov 6 07:35:04 JST 2014

> Hi everyone,
> In playing with some timeout settings with some real-world live traffic, I
> noticed that pgpool keeps a lot of server-side idle connections open.  I
> started with a child_life_time of 30 seconds, and it quickly consumed up
> all of my num_init_children setting (32) on the postgres server.  I'd like
> to minimize the number of connections left open (and idle) on the postgres
> server.

Yes, that's the effect of connection pooling of pgpool-II. If you
don't like this, you can turn it off by setting connection_cache off.

> When I dropped the child_life_time down to 5 seconds, it reduced the idle
> connections significantly.

Yes, becuase each pgpool-II child is responsible for keeping the
connection pooling in their process space.

> While that effectively solves my problem, I'm
> wondering what algorithm is used to select a server pool slot when a new
> client connects.  As I was watching it, it seems like an LRU algorithm,
> picking the server side slot with the largest idle time.

No. pgpool-II employes pre-fork model, which means multiple child
process are forked and are waiting for connect(2) request comes from
client. If the request arrives to the port pgpool-II is listening on,
the kernel selects one of the child process and let it process. I
don't know what algorithm is used within the kernel process. Maybe
it's URL, maybe not.

Best regards,
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php

> If that's the
> case, I think I would prefer it to pick the server side slot with the
> *smallest* idle time.  The pool should then shrink to the minimal number of
> server-side connection slots required to handle the current client-side
> load.  Then to account for burst activity, the idle time could be upped to
> keep a small amount of extra slots for burst capacity.
> The way it seems now, I have to shorten the idle timeout to something very
> short in order to keep from overrunning my connection limits on the
> postgres server.
> Does any of this seem correct, or am I way off base here?
> Thank you!
> --Cal

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