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Re: extending relations more efficiently

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Christopher Browne <cbbrowne(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: extending relations more efficiently
Date: 2012-05-01 19:35:04
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Lists: pgsql-hackers
On Tue, May 1, 2012 at 3:01 PM, Alvaro Herrera
<alvherre(at)commandprompt(dot)com> wrote:
>> The system is supposed to be designed to avoid that; we try to hand out
>> pages with free space to different backends.  One of the advantages of
>> the current page-at-a-time setup is that when there is *no* free space
>> according to the FSM, each such backend will create and fill its own
>> page independently.  They do serialize on the extension lock, but the
>> intervening tuple additions are parallelized.  We have to be careful to
>> not make that case worse in a multi-page-extension scheme.
> This didn't work all that well for toast tables that have lots of very
> large tuples inserted concurrently, at least with older releases.  Each
> backend would acquire the extension lock many times per tuple inserted.
> The result was really high contention on the extension lock.

Hmm, this sounds like a good test case - a small number of SQL
statements triggering a large amount of relation extension work.
Using an unlogged table would probably make it easier to see the
relation-extension contention, too, since you'd get WALInsertLock
mostly out of the way.

Robert Haas
The Enterprise PostgreSQL Company

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