[pgpool-hackers: 140] Re: Explanation of max_pool parameter

[Gurjeet Singh]

Hi Tatsuo,

    How can one create a login-account so that they can contribute to the
wiki. As of now, my las name on that post is wrong, (should be Singh,not
Sing). I don't see a way to create an account, and the pgpool-bugzilla
account does not work there.

Thanks in advance,

On Fri, Aug 24, 2012 at 9:35 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:

> Done.
>
> http://www.pgpool.net/mediawiki/index.php/Relationship_between_max_pool,_num_init_children,_and_max_connections
>
> Linked from document section.
> http://www.pgpool.net/mediawiki/index.php/Documentation
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese: http://www.sraoss.co.jp
>
> > "Relationship between max_pool, num_init_children, and max_connections" ?
> >
> > On Thu, Aug 23, 2012 at 5:43 PM, Tatsuo Ishii <ishii at postgresql.org>
> wrote:
> >
> >> Ok. Can you please provide the document title?
> >> --
> >> Tatsuo Ishii
> >> SRA OSS, Inc. Japan
> >> English: http://www.sraoss.co.jp/index_en.php
> >> Japanese: http://www.sraoss.co.jp
> >>
> >> > It doesn't sound like an FAQ item. Either a page in Wiki or in User
> >> > contributed docs would be fine with me.
> >> >
> >> > On Wed, Aug 22, 2012 at 9:34 PM, Tatsuo Ishii <ishii at postgresql.org>
> >> wrote:
> >> >
> >> >> Putting into Wiki and add link in docs sounds nice idea.
> >> >> What about adding to FAQ?
> >> >>
> >> >> Or do you want to place "User contributed documentation" section?
> >> >> http://www.pgpool.net/mediawiki/index.php/Documentation
> >> >> --
> >> >> Tatsuo Ishii
> >> >> SRA OSS, Inc. Japan
> >> >> English: http://www.sraoss.co.jp/index_en.php
> >> >> Japanese: http://www.sraoss.co.jp
> >> >>
> >> >> > 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
> >> >>
> >> >
> >> >
> >> >
> >> > --
> >> > Gurjeet Singh
> >>
> >
> >
> >
> > --
> > Gurjeet Singh
>



-- 
Gurjeet Singh

http://gurjeet.singh.im/
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