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Forums & tsearch2 - best practices reg. concurrency

From: "Markus Wollny" <Markus(dot)Wollny(at)computec(dot)de>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: Forums & tsearch2 - best practices reg. concurrency
Date: 2005-06-22 09:14:56
Message-ID: 2266D0630E43BB4290742247C891057507A0D796@dozer.computec.de (view raw or flat)
Thread:
Lists: pgsql-performance
Hello!
 
We're using PostgreSQL 8.0.1 as general backend for all of our websites,
including our online forums (aka bulletin boards or whatever you wish to
call that). As for full text search capabilities, we've chosen to
implement this via tsearch2. However, the tables themselves are quite
large, and as there's lots of weird user input in them (just no way of
limiting our users to "proper" orthography), so are the indices; we have
already split up the main posting-table in two, one containing the more
recent messages (<6 months) and one for everything else.

Search capabilities have been limited to accessing only one of those,
either recent or archive. Still, the tsearch2-GiST-index for a table is
around 325MB in size; the "recent messages" table itself without any
indices weighs in at about 1.8GB containing over one million rows, the
archive-table is a little over 3GB and contains about 1.3 million rows.
A full text search in the table with the recent postings can take up to
five minutes.

This wouldn't be much of a problem, as we're providing other, quicker
search options (like searching for an author or a full text search just
on the topics); the problem with the full text search lies in the
locking mechanisms: As long as there's a search going on, all the
subsequent INSERTs or UPDATEs on that table fail due to timeout. This
means that currently, whenever we allow full text searching, there may
be a timeframe of more than one hour, during which users cannot write
any new postings in our forum or edit (i.e. update) anything. This is
hardly acceptable...

This is what I did to actually diagnose that simple tsearch2-related
SELECTs where causing the write-locks:

First I started a full text search query which I knew would run over
four minutes. Then I waited for other users to try and post some
messages; soon enough a 'ps ax|grep wait' showed several "INSERT/UPDATE
waiting"-backends. So I took a look at the locks:

select s.current_query as statement,
        l.mode as lock_mode,
        l.granted as lock_granted,
        c.relname as locked_relation,
        c.relnamespace as locked_relnamespace,
        c.reltype as locked_reltype
from pg_stat_activity s,
        pg_locks l,
        pg_class c
where
        l.pid = s.procpid
and
        l.relation = c.oid
order by age(s.query_start) desc;

I found four locks for the search query at the very beginning of the
resultset - all of them of the AccessShareLock persuasion and granted
alright: one on the message-table, one on the thread-table, one on the
tsearch2-index and another one on the primary key index of the
thread-table.

The hanging inserts/updates were waiting for an AccessExclusiveLock on
the tsearch2-index - all the other locks of these queries were marked as
granted.

As far as I understand from some of the previous messages on the mailing
list regarding concurrency issues with GiST-type indices, any SELECT
that's using a tsearch2-index would completely lock write-access to that
index for the runtime of the query - is that correct so far?

Now I'd like to find out about possible solutions or workarounds for
this issue. Surely some of you must have encountered quite similar
situations, so what did you do about it? I already pondered the idea of
a separate insert/update-queue-table which would then be processed by a
cron-job, thus separating the information-entry from the actual insert
into the table that's blocked due to the lock on the index. Another
possibility (which I find a little bit more compelling) would involve
replicating the message-table via Slony-I to another database which
could then be used as only target for any search-queries which require
use of the GiST-index. Would this provide the needed "asynchronicity" to
avoid this race condition between the AccessShareLock from the
search-SELECT and the AccessExclusiveLock from the write access queries?

I'd be very glad to know your opinions on this matter.

Kind regards

   Markus

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Next:From: Tobias BroxDate: 2005-06-22 09:36:44
Subject: Re: Querying 19million records very slowly
Previous:From: Kjell Tore FossbakkDate: 2005-06-22 09:10:42
Subject: Re: Querying 19million records very slowly

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