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From: Jim Vanns <jvanns@ilm.com>
Date: Wed, 31 Jul 2024 16:41:51 +0100
Message-ID: <CAH7vdhMB1Tej0XQC8oObDF_iraiknmjOmBkEEf4=m9abbtBLNg@mail.gmail.com>
Subject: Re: Suggestions to overcome 'multixact "members" limit exceeded' in
 temporary tables
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I've been able to observe that the performance degradation with TRUNCATE
appears to happen when other ancillary processes are running that are also
heavy users of temporary tables. If I used an exclusive tablespace, would
that improve things?

Cheers

Jim


On Wed, 31 Jul 2024 at 15:16, Jim Vanns <jvanns@ilm.com> wrote:

> I've reached the limit of my understanding and attempts at correcting my
> code/use of temporary tables in the face of multixact members and have come
> to ask for your help! Here's a brief description of my software;
>
> Pool of N connection sessions, persistent for the duration of the program
> lifetime.
> Upon each session initialisation, a set of CREATE TEMPORARY TABLE ON
> COMMIT DELETE ROWS statements are made for bulk ingest.
> Each session is acquired by a thread for use when ingesting data and
> therefore each temporary table remains until the session is terminated
> The thread performs a COPY <temp table> FROM STDIN in binary format
> Then an INSERT INTO <main table> SELECT FROM <temp table> WHERE...
>
> This has been working great for a while and with excellent throughput.
> However, upon scaling up I eventually hit this error;
>
> ERROR:  multixact "members" limit exceeded
> DETAIL:  This command would create a multixact with 2 members, but the
> remaining space is only enough for 0 members.
> HINT:  Execute a database-wide VACUUM in database with OID 16467 with
> reduced vacuum_multixact_freeze_min_age and
> vacuum_multixact_freeze_table_age settings.
>
> And it took me quite a while to identify that it appears to be coming from
> the temporary table (the other 'main' tables were being autovacuumed OK) -
> which makes sense because they have a long lifetime, aren't auto vacuumed
> and shared by transactions (in turn).
>
> I first attempted to overcome this by introducing an initial step of
> always creating the temporary table before the copy (and using on commit
> drop) but this lead to a terrible performance degradation.
> Next, I reverted the above and instead I introduced a VACUUM step every
> 1000000 (configurable) ingest operations
> Finally, I introduced a TRUNCATE step in addition to the occasional VACUUM
> since the TRUNCATE allowed the COPY option of FREEZE.
>
> The new overhead appears minimal until after several hours and again I've
> hit a performance degradation seemingly dominated by the TRUNCATE.
>
> My questions are;
>
> 1) Is the VACUUM necessary if I use TRUNCATE + COPY FREEZE (on the
> temporary table)?
> 2) Is there really any benefit to using FREEZE here or is it best to just
> VACUUM the temporary tables occasionally?
> 3) Is there a better way of managing all this!? Perhaps re-CREATING the TT
> every day or something?
>
> I understand that I can create a Linux tmpfs partition for a tablespace
> for the temporary tables and that may speed up the TRUNCATE but that seems
> like a hack and I'd rather not do it at all if it's avoidable.
>
> Thanks for your help,
>
> Jim
>
> PS. PG version in use is 15.4 if that matters here
>
> --
> Jim Vanns
> Principal Production Engineer
> Industrial Light & Magic, London
>


-- 
Jim Vanns
Principal Production Engineer
Industrial Light & Magic, London

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<div dir=3D"ltr">I&#39;ve been able to observe that the performance degrada=
tion=C2=A0with TRUNCATE appears to happen when other ancillary=C2=A0process=
es are running that are also heavy users of temporary tables. If I used an =
exclusive tablespace, would that improve things?<div><br></div><div>Cheers<=
/div><div><br></div><div>Jim</div><div><br></div></div><br><div class=3D"gm=
ail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Wed, 31 Jul 2024 at 15:=
16, Jim Vanns &lt;<a href=3D"mailto:jvanns@ilm.com" target=3D"_blank">jvann=
s@ilm.com</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=
=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding=
-left:1ex"><div dir=3D"ltr">I&#39;ve reached the limit of my understanding =
and attempts at correcting my code/use of temporary tables in the face of=
=C2=A0multixact members and have come to ask for your help! Here&#39;s a br=
ief description of my software;<div><br></div><div>Pool of N connection ses=
sions, persistent for the duration of the program lifetime.</div><div>Upon =
each session initialisation, a set of CREATE TEMPORARY TABLE ON COMMIT DELE=
TE ROWS statements are made for bulk ingest.</div><div>Each session is acqu=
ired by a thread for use when ingesting=C2=A0data and therefore each tempor=
ary table remains until the session is terminated</div><div>The thread perf=
orms a COPY &lt;temp table&gt; FROM STDIN in binary format</div><div>Then a=
n INSERT INTO &lt;main table&gt; SELECT FROM &lt;temp table&gt; WHERE...</d=
iv><div><br></div><div>This has been working great for a while and with exc=
ellent throughput. However, upon scaling up I eventually hit this error;</d=
iv><div><br></div><div>ERROR: =C2=A0multixact &quot;members&quot; limit exc=
eeded<br>DETAIL: =C2=A0This command would create a multixact with 2 members=
, but the remaining space is only enough for 0 members.<br>HINT: =C2=A0Exec=
ute a database-wide VACUUM in database with OID 16467 with reduced vacuum_m=
ultixact_freeze_min_age and vacuum_multixact_freeze_table_age settings.<br>=
</div><div><div><br></div><div>And it took me quite a while to identify tha=
t it appears to be coming from the temporary table (the other &#39;main&#39=
; tables were being autovacuumed=C2=A0OK) - which makes sense because they =
have a long lifetime, aren&#39;t auto vacuumed and shared by transactions (=
in turn).</div><div><br></div><div>I first attempted to overcome this by in=
troducing an initial step of always creating the temporary table before the=
 copy (and using on commit drop) but this lead to a terrible performance de=
gradation.</div><div>Next, I reverted the above and instead I introduced a =
VACUUM step every 1000000 (configurable) ingest operations</div><div>Finall=
y, I introduced a TRUNCATE step in addition to the occasional VACUUM since =
the TRUNCATE allowed the COPY option of FREEZE.</div><div><br></div><div>Th=
e new overhead appears minimal until after several hours and again I&#39;ve=
 hit a performance degradation=C2=A0seemingly dominated=C2=A0by the TRUNCAT=
E.</div><div><br></div><div>My questions are;</div><div><br></div><div>1) I=
s the VACUUM necessary if I use TRUNCATE=C2=A0+ COPY FREEZE (on the tempora=
ry table)?</div><div>2) Is there really any benefit to using FREEZE here or=
 is it best to just VACUUM the temporary tables occasionally?</div><div>3) =
Is there a better way of managing all this!? Perhaps re-CREATING the TT eve=
ry day or something?</div><div><br></div><div>I understand that I can creat=
e a Linux tmpfs partition for a tablespace for the temporary tables and tha=
t may speed up the TRUNCATE but that seems like a hack and I&#39;d rather n=
ot do it at all if it&#39;s avoidable.</div><div><br></div><div>Thanks for =
your help,</div><div><br></div><div>Jim</div><div><br></div><div>PS. PG ver=
sion in use is 15.4 if that matters here</div><div><br></div><span class=3D=
"gmail_signature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_sign=
ature"><div dir=3D"ltr">Jim Vanns<br>Principal Production Engineer<br>Indus=
trial Light &amp; Magic, London</div></div></div></div>
</blockquote></div><br clear=3D"all"><div><br></div><span class=3D"gmail_si=
gnature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signature"><d=
iv dir=3D"ltr">Jim Vanns<br>Principal Production Engineer<br>Industrial Lig=
ht &amp; Magic, London</div></div>

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