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Update discussion of shared memory sizing for 8.3: adjust multipliers,

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mention autovacuum_max_workers.  Minor copy-editing too.
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Tom Lane 2007-11-01 19:06:01 +00:00
parent 5c996884a0
commit 388c7e28c5
1 changed files with 40 additions and 26 deletions
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doc/src/sgml/runtime.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.381 2007/03/30 03:19:02 momjian Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.382 2007/11/01 19:06:01 tgl Exp $ -->
<chapter Id="runtime"> <chapter Id="runtime">
<title>Operating System Environment</title> <title>Operating System Environment</title>
@ -478,9 +478,10 @@ psql: could not connect to server: No such file or directory
relevant for <productname>PostgreSQL</>). Almost all modern relevant for <productname>PostgreSQL</>). Almost all modern
operating systems provide these features, but not all of them have operating systems provide these features, but not all of them have
them turned on or sufficiently sized by default, especially systems them turned on or sufficiently sized by default, especially systems
with BSD heritage. (For the <systemitem class="osname">Windows</> with BSD heritage. (On <systemitem class="osname">Windows</>,
port, <productname>PostgreSQL</> provides its own replacement <productname>PostgreSQL</> provides its own replacement
implementation of these facilities.) implementation of these facilities, and so most of this section
can be disregarded.)
</para> </para>
<para> <para>
@ -550,13 +551,13 @@ psql: could not connect to server: No such file or directory
<row> <row>
<entry><varname>SEMMNI</></> <entry><varname>SEMMNI</></>
<entry>Maximum number of semaphore identifiers (i.e., sets)</> <entry>Maximum number of semaphore identifiers (i.e., sets)</>
<entry>at least <literal>ceil(max_connections / 16)</literal></> <entry>at least <literal>ceil((max_connections + autovacuum_max_workers) / 16)</literal></>
</row> </row>
<row> <row>
<entry><varname>SEMMNS</></> <entry><varname>SEMMNS</></>
<entry>Maximum number of semaphores system-wide</> <entry>Maximum number of semaphores system-wide</>
<entry><literal>ceil(max_connections / 16) * 17</literal> plus room for other applications</> <entry><literal>ceil((max_connections + autovacuum_max_workers) / 16) * 17</literal> plus room for other applications</>
</row> </row>
<row> <row>
@ -590,15 +591,13 @@ psql: could not connect to server: No such file or directory
likely that this limit has been exceeded. The size of the required likely that this limit has been exceeded. The size of the required
shared memory segment varies depending on several shared memory segment varies depending on several
<productname>PostgreSQL</> configuration parameters, as shown in <productname>PostgreSQL</> configuration parameters, as shown in
<xref linkend="shared-memory-parameters">. <xref linkend="shared-memory-parameters">. (Any error message you might
get will include the exact size of the failed allocation request.)
You can, as a temporary solution, lower some of those settings to You can, as a temporary solution, lower some of those settings to
avoid the failure. As a rough approximation, you can estimate the avoid the failure. While it is possible to get
required segment size as 500 kB plus the variable amounts shown in
the table. (Any error message you might get will include the exact
size of the failed allocation request.) While it is possible to get
<productname>PostgreSQL</> to run with <varname>SHMMAX</> as small as <productname>PostgreSQL</> to run with <varname>SHMMAX</> as small as
1 MB, you need at least 4 MB for acceptable performance, and desirable 2 MB, you need considerably more for acceptable performance. Desirable
settings are in the tens of megabytes. settings are in the tens to hundreds of megabytes.
</para> </para>
<para> <para>
@ -620,17 +619,21 @@ psql: could not connect to server: No such file or directory
<para> <para>
<productname>PostgreSQL</> uses one semaphore per allowed connection <productname>PostgreSQL</> uses one semaphore per allowed connection
(<xref linkend="guc-max-connections">), in sets of 16. Each such set will (<xref linkend="guc-max-connections">) and allowed autovacuum worker
process (<xref linkend="guc-autovacuum-max-workers">), in sets of 16.
Each such set will
also contain a 17th semaphore which contains a <quote>magic also contain a 17th semaphore which contains a <quote>magic
number</quote>, to detect collision with semaphore sets used by number</quote>, to detect collision with semaphore sets used by
other applications. The maximum number of semaphores in the system other applications. The maximum number of semaphores in the system
is set by <varname>SEMMNS</>, which consequently must be at least is set by <varname>SEMMNS</>, which consequently must be at least
as high as <varname>max_connections</> plus one extra for each 16 as high as <varname>max_connections</> plus
allowed connections (see the formula in <xref <varname>autovacuum_max_workers</>, plus one extra for each 16
allowed connections plus workers (see the formula in <xref
linkend="sysvipc-parameters">). The parameter <varname>SEMMNI</> linkend="sysvipc-parameters">). The parameter <varname>SEMMNI</>
determines the limit on the number of semaphore sets that can determines the limit on the number of semaphore sets that can
exist on the system at one time. Hence this parameter must be at exist on the system at one time. Hence this parameter must be at
least <literal>ceil(max_connections / 16)</>. Lowering the number least <literal>ceil((max_connections + autovacuum_max_workers) / 16)</>.
Lowering the number
of allowed connections is a temporary workaround for failures, of allowed connections is a temporary workaround for failures,
which are usually confusingly worded <errorname>No space which are usually confusingly worded <errorname>No space
left on device</>, from the function <function>semget</>. left on device</>, from the function <function>semget</>.
@ -675,7 +678,7 @@ psql: could not connect to server: No such file or directory
sort of configuration commonly used for other databases such sort of configuration commonly used for other databases such
as <application>DB/2</application>.</para> as <application>DB/2</application>.</para>
<para> It might , however, be necessary to modify the global <para> It might, however, be necessary to modify the global
<command>ulimit</command> information in <command>ulimit</command> information in
<filename>/etc/security/limits</filename>, as the default hard <filename>/etc/security/limits</filename>, as the default hard
limits for file sizes (<varname>fsize</varname>) and numbers of limits for file sizes (<varname>fsize</varname>) and numbers of
@ -865,8 +868,9 @@ options SEMMAP=256
<indexterm><primary>Linux</><secondary>IPC configuration</></> <indexterm><primary>Linux</><secondary>IPC configuration</></>
<listitem> <listitem>
<para> <para>
The default settings are only suitable for small installations The default maximum segment size is 32 MB, which is only adequate
(the default maximum segment size is 32 MB). However, the remaining for small <productname>PostgreSQL</productname> installations.
However, the remaining
defaults are quite generously sized, and usually do not require defaults are quite generously sized, and usually do not require
changes. The maximum shared memory segment size can be changed via the changes. The maximum shared memory segment size can be changed via the
<command>sysctl</command> interface. For example, to allow 128 MB, <command>sysctl</command> interface. For example, to allow 128 MB,
@ -985,7 +989,7 @@ kern.sysv.shmall=1024
<listitem> <listitem>
<para> <para>
At least in version 2.6, the default maximum size of a shared At least in version 2.6, the default maximum size of a shared
memory segments is too low for <productname>PostgreSQL</>. The memory segment is too low for <productname>PostgreSQL</>. The
relevant settings can be changed in <filename>/etc/system</>, relevant settings can be changed in <filename>/etc/system</>,
for example: for example:
<programlisting> <programlisting>
@ -1051,24 +1055,29 @@ set semsys:seminfo_semmsl=32
<thead> <thead>
<row> <row>
<entry>Name</> <entry>Name</>
<entry>Approximate multiplier (bytes per increment)</> <entry>Approximate multiplier (bytes per increment) as of 8.3</>
</row> </row>
</thead> </thead>
<tbody> <tbody>
<row> <row>
<entry><xref linkend="guc-max-connections"></> <entry><xref linkend="guc-max-connections"></>
<entry>400 + 270 * <xref linkend="guc-max-locks-per-transaction"></entry> <entry>1800 + 270 * <xref linkend="guc-max-locks-per-transaction"></entry>
</row>
<row>
<entry><xref linkend="guc-autovacuum-max-workers"></>
<entry>1800 + 270 * <xref linkend="guc-max-locks-per-transaction"></entry>
</row> </row>
<row> <row>
<entry><xref linkend="guc-max-prepared-transactions"></> <entry><xref linkend="guc-max-prepared-transactions"></>
<entry>600 + 270 * <xref linkend="guc-max-locks-per-transaction"></entry> <entry>770 + 270 * <xref linkend="guc-max-locks-per-transaction"></entry>
</row> </row>
<row> <row>
<entry><xref linkend="guc-shared-buffers"></> <entry><xref linkend="guc-shared-buffers"></>
<entry>8300 (assuming 8 kB <symbol>BLCKSZ</>)</entry> <entry>8400 (assuming 8 kB <symbol>BLCKSZ</>)</entry>
</row> </row>
<row> <row>
@ -1085,6 +1094,11 @@ set semsys:seminfo_semmsl=32
<entry><xref linkend="guc-max-fsm-pages"></> <entry><xref linkend="guc-max-fsm-pages"></>
<entry>6</> <entry>6</>
</row> </row>
<row>
<entry>Fixed space requirements</>
<entry>770 kB</entry>
</row>
</tbody> </tbody>
</tgroup> </tgroup>
</table> </table>