From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
cc: Amit Kapila <amit.kapila16@gmail.com>, Noah Misch <noah@leadboat.com>,
        Amit Khandekar <amitdkhan.pg@gmail.com>,
        Alvaro Herrera from 2ndQuadrant <alvherre@alvh.no-ip.org>,
        Andres Freund <andres@anarazel.de>,
        Juan =?utf-8?B?Sm9zw6kgU2FudGFtYXLDrWE=?= Flecha
 <juanjo.santamaria@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
        Robert Haas <robertmhaas@gmail.com>,
        Thomas Munro <thomas.munro@gmail.com>
Subject: Re: logical decoding : exceeded maxAllocatedDescs for .spill files
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Comments: In-reply-to Tomas Vondra <tomas.vondra@2ndquadrant.com>
	message dated "Sun, 12 Jan 2020 05:10:08 +0100"
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Date: Sat, 11 Jan 2020 23:20:56 -0500
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Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> On Sat, Jan 11, 2020 at 10:53:57PM -0500, Tom Lane wrote:
>> remind me where the win came from, exactly?

> Well, the problem is that in 10 we allocate tuple data in the main
> memory ReorderBuffer context, and when the transaction gets decoded we
> pfree() it. But in AllocSet that only moves the data to the freelists,
> it does not release it entirely. So with the right allocation pattern
> (sufficiently diverse chunk sizes) this can easily result in allocation
> of large amount of memory that is never released.

> I don't know if this is what's happening in this particular test, but I
> wouldn't be surprised by it.

Nah, don't think I believe that: the test inserts a bunch of tuples,
but they look like they will all be *exactly* the same size.

CREATE TABLE decoding_test(x integer, y text);
...

    FOR i IN 1..10 LOOP
        BEGIN
            INSERT INTO decoding_test(x) SELECT generate_series(1,5000);
        EXCEPTION
            when division_by_zero then perform 'dummy';
        END;

			regards, tom lane