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Preallocate some DSM space at startup.

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Create an optional region in the main shared memory segment that can be
used to acquire and release "fast" DSM segments, and can benefit from
huge pages allocated at cluster startup time, if configured.  Fall back
to the existing mechanisms when that space is full.  The size is
controlled by a new GUC min_dynamic_shared_memory, defaulting to 0.

Main region DSM segments initially contain whatever garbage the memory
held last time they were used, rather than zeroes.  That change revealed
that DSA areas failed to initialize themselves correctly in memory that
wasn't zeroed first, so fix that problem.

Discussion: https://postgr.es/m/CA%2BhUKGLAE2QBv-WgGp%2BD9P_J-%3Dyne3zof9nfMaqq1h3EGHFXYQ%40mail.gmail.com
This commit is contained in:
Thomas Munro
2020-07-31 17:27:09 +12:00
parent 7b1110d2fd
commit 84b1c63ad4
9 changed files with 216 additions and 26 deletions
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24
doc/src/sgml/config.sgml
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@@ -1906,6 +1906,30 @@ include_dir 'conf.d'
</listitem>
</varlistentry>
<varlistentry id="guc-min-dynamic-shared-memory" xreflabel="min_dynamic_shared_memory">
<term><varname>min_dynamic_shared_memory</varname> (<type>integer</type>)
<indexterm>
<primary><varname>min_dynamic_shared_memory</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the amount of memory that should be allocated at server
startup time for use by parallel queries. When this memory region is
insufficient or exhausted by concurrent queries, new parallel queries
try to allocate extra shared memory temporarily from the operating
system using the method configured with
<varname>dynamic_shared_memory_type</varname>, which may be slower due
to memory management overheads. Memory that is allocated at startup
time with <varname>min_dynamic_shared_memory</varname> is affected by
the <varname>huge_pages</varname> setting on operating systems where
that is supported, and may be more likely to benefit from larger pages
on operating systems where that is managed automatically.
The default value is <literal>0</literal> (none).
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>