From: Tom Lane <tgl@sss.pgh.pa.us>
To: Kevin Grittner <kgrittn@ymail.com>
cc: Dave Owens <dave@teamunify.com>, Merlin Moncure <mmoncure@gmail.com>,
 Matheus de Oliveira <matioli.matheus@gmail.com>,
 postgres performance list <pgsql-performance@postgresql.org>
Subject: Re: query against pg_locks leads to large memory alloc
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Comments: In-reply-to Kevin Grittner <kgrittn@ymail.com>
 message dated "Mon, 18 Aug 2014 15:01:47 -0700"
Date: Mon, 18 Aug 2014 19:24:33 -0400
Message-ID: <12486.1408404273@sss.pgh.pa.us>
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Kevin Grittner <kgrittn@ymail.com> writes:
> Dave Owens <dave@teamunify.com> wrote:
>> max_connections = 450 ...we have found that we run out of shared
>> memory when max_pred_locks_per_transaction is less than 30k.

> It gathers the information in memory to return for all those locks
> (I think both the normal heavyweight locks and the predicate locks
> do that).  450 * 30000 is 13.5 million predicate locks you could
> have, so they don't need a very big structure per lock to start
> adding up.  I guess we should refactor that to use a tuplestore, so
> it can spill to disk when it gets to be more than work_mem.

Seems to me the bigger issue is why does he need such a huge
max_pred_locks_per_transaction setting?  It's hard to believe that
performance wouldn't tank with 10 million predicate locks active.
Whether you can do "select * from pg_locks" seems pretty far down
the list of concerns about this setting.

			regards, tom lane


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