[pgpool-hackers: 113] Re: Explanation of max_pool parameter

Gurjeet Singh singh.gurjeet at gmail.com
Thu Aug 23 09:40:08 JST 2012


Because it is quite elaborate explanation, it might not be a good fit for
docs, which are supposed to be concise. Probably it should reside somewhere
in wiki and the docs for max_pool should link there. But don't mind me if
you think these can fit right in the docs.

On Wed, Aug 22, 2012 at 7:06 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:

> Great explanation. I would like to add this to somewhere in the document.
> Thanks!
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese: http://www.sraoss.co.jp
>
> > Hi Hackers,
> >
> >     I have answered max_pool related question a few times, and more than
> > once the following explanation cleared up the other party's confusion
> just
> > y reading it, without any more explanation from me. Do you think this
> > deserves some place on pgpool docs?
> >
> >     max_pool parameter configures how many connections to cache _per
> > child_. So if num_init_children is configured to 100, and max_pool is
> > configured to 3, then pgpool can potentially open 300 (=3*100)
> connections
> > to the backend database.
> >
> >     A child process opens a new backend connection only if the requested
> > [user,database] pair is not already in the cache. So if the application
> > uses only one user to connect to only one database, say [pguser1,pgdb1],
> > then each child will continue to reuse the first connection and will
> never
> > open a second connection, so in effect pgpool will open no more than 100
> > backend connections even though max_pool is set to 3.
> >
> >     But if the application uses more than one pair of [user,database]
> > connection parameters, then each child will cache the connection it might
> > already have for another pair, and open a new backend connection for the
> > requested pair.
> >
> >     For eg., if the application now uses these 4 pairs: [user1,db1]
> > [user1,db2] [user2,db1] [user2,db2] to connect to pgpool, then each child
> > process can cache up to 3 connections for the first 3 different pairs it
> > receives connection requests for. But as soon as it receives a request
> for
> > the 4th pair that it does not yet have a connection for, then it will
> > disconnect the oldest connection in the cache and open a new connection
> for
> > the 4th pair.
> >
> >     As we already know that there's no guarantee as to which child
> process
> > will handle an incoming connection request, max_pool tries to improve the
> > performance a little bit by caching connections of different pairs, in
> the
> > hopes that an incoming connection request might match one of the
> > connections cached by the child process. But this also causes an
> explosion
> > in the number of connections that pgpool would request from the database.
> >
> >     So, in order to guarantee that the application connection requests
> are
> > never rejected, and that the connection requests wait until a database
> > connection is available, the following condition should be met:
> >
> > max_pool*num_init_children <= (max_connections -
> > superuser_reserved_connections)
> >
> >     If the application uses superuser connections (which is not
> > recommended), then the condition is reduced to:
> >
> > max_pool*num_init_children <= max_connections
> >
> >     Setting max_pool to 1 will guarantee that the number of database
> > connections opened by pgpool child processes never exceeds the
> > num_init_children value. If for performance reasons, as explained above,
> > you do wish to set max_pool to more than 1, then max_connections will
> also
> > have to be increased accordingly so that application connection requests
> do
> > not get denied.
> >
> > Best regards,
> > --
> > Gurjeet Singh
>



-- 
Gurjeet Singh
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