Update SysV IPC information.
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Peter Eisentraut
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@ -1,5 +1,5 @@
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.41 2000/12/03 14:36:45 petere Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.42 2000/12/17 11:22:00 petere Exp $
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-->
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<Chapter Id="runtime">
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@ -1300,11 +1300,12 @@ env PGOPTIONS='-c geqo=off' psql
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limits of the IPC resources then the postmaster will refuse to
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start up and should leave a marginally instructive error message
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about which problem was encountered and what needs to be done
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about it. The relevant kernel parameters are named
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about it. (See also <xref linkend="postmaster-start-failures">.)
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The relevant kernel parameters are named
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consistently across different systems; <xref
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linkend="sysvipc-parameters"> gives an overview. The methods to
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set them, however, vary; suggestions for some platforms are given
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below. Be aware that you will have to reboot your
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below. Be aware that you will probably have to reboot your
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machine at least, possibly even recompile the kernel, to change these
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settings.
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</para>
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@ -1332,13 +1333,13 @@ env PGOPTIONS='-c geqo=off' psql
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<row>
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<entry><varname>SHMMIN</></>
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<entry>Minimum size of shared memory segment (bytes)</>
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<entry>1 (at most 144)</>
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<entry>1 (at most about 256 kB)</>
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</row>
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<row>
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<entry><varname>SHMSEG</></>
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<entry>Maximum number of shared memory segments per process</>
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<entry>must be at least 3, but the default is much higher</>
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<entry>only 1 segment is needed, but the default is much higher</>
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</row>
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<row>
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@ -1356,13 +1357,13 @@ env PGOPTIONS='-c geqo=off' psql
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<row>
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<entry><varname>SEMMNS</></>
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<entry>Maximum number of semaphores system-wide</>
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<entry>max_connections rounded up to multiple of 16, + room for other applications</>
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<entry>ceil(max_connections / 16) * 17 + room for other applications</>
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</row>
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<row>
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<entry><varname>SEMMSL</></>
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<entry>Maximum number of semaphores per set</>
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<entry>>= 16</>
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<entry>>= 17</>
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</row>
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<row>
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@ -1396,34 +1397,36 @@ env PGOPTIONS='-c geqo=off' psql
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estimate the required segment size as the number of buffers times
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the block size (8192 kB by default) plus ample overhead (at least
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half a megabyte). Any error message you might get will contain the
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size of the failed allocation. (<productname>Postgres</> will
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actually use three shared memory segments, but the size of the
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other two is negligible for this consideration.)
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size of the failed allocation.
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</para>
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<para>
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Less likely to cause problems is the minimum size for shared
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memory segments (<varname>SHMMIN</>), which must be at least 144
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for <productname>Postgres</> (it's usually just 1), and the
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maximum number of segments system-wide (<varname>SHMMNI</>, as
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mentioned, 3 are needed) or per-process (<varname>SHMSEG</>,
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ditto). Some systems also have a limit on the total amount of
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shared memory in the system; see the platform-specific
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instructions below.
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memory segments (<varname>SHMMIN</>), which should be at most
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somewhere around 256 kB for <productname>Postgres</> (it is
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usually just 1). The maximum number of segments system-wide
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(<varname>SHMMNI</>) or per-process (<varname>SHMSEG</>) should
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not cause a problem unless your system has them set to zero. Some
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systems also have a limit on the total amount of shared memory in
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the system; see the platform-specific instructions below.
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</para>
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<para>
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<productname>Postgres</> uses one semaphore per allowed connection
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(<option>-N</> option), in sets of 16. The maximum number of
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semaphores in the system is set by <varname>SEMMNS</>, which
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consequently must be at least as high as the connection setting.
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The parameter <varname>SEMMNI</> determines the limit on the
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number of semaphore sets that can exist on the system at one time.
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Hence this parameter must be at least
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<literal>ceil(max_connections / 16)</>. Lowering the number of
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allowed connections is a temporary workaround for failures, which
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are usually confusingly worded <quote><errorname>No space left on
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device</></>, from the function <function>semget()</>.
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(<option>-N</> option), in sets of 16. Each such set will also
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contain a 17th semaphore which contains a <quote>magic
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number</quote>, to avoid collision with semaphore sets used by
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other applications. The maximum number of semaphores in the system
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is set by <varname>SEMMNS</>, which consequently must be at least
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as high as the connection setting plus one extra for each 16
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allowed connections (see the formula in <xref
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linkend="sysvipc-parameters">. The parameter <varname>SEMMNI</>
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determines the limit on the number of semaphore sets that can
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exist on the system at one time. Hence this parameter must be at
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least <literal>ceil(max_connections / 16)</>. Lowering the number
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of allowed connections is a temporary workaround for failures,
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which are usually confusingly worded <quote><errorname>No space
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left on device</></>, from the function <function>semget()</>.
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</para>
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<para>
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@ -1441,7 +1444,7 @@ env PGOPTIONS='-c geqo=off' psql
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<para>
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The <varname>SEMMSL</> parameter, which determines how many
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semaphores can be in a set, must be at least 16 for
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semaphores can be in a set, must be at least 17 for
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<productname>Postgres</>.
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</para>
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@ -1558,11 +1561,11 @@ options SEMMAP=256
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<varlistentry>
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<term>HPUX</>
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<term>HP-UX</>
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<listitem>
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<para>
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The default settings tend to suffice for normal installations.
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On <productname>HPUX</> 10, the factory default for
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On <productname>HP-UX</> 10, the factory default for
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<varname>SEMMNS</> is 128, which might be too low for larger
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database sites.
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</para>
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@ -1581,11 +1584,23 @@ options SEMMAP=256
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<term>Linux</>
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<listitem>
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<para>
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System V IPC is enabled by default and sufficiently sized for
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most uses. The relevant parameters are in
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The default shared memory limit (both
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<varname>SHMMAX</varname> and <varname>SHMALL</varname>) is 32
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MB in 2.2 kernels, but it can be changed in the
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<filename>proc</filename> file system (without reboot). For
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example, to allow 128 MB:
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<screen>
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<prompt>$</prompt> <userinput>echo 134217728 >/proc/sys/kernel/shmall</userinput>
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<prompt>$</prompt> <userinput>echo 134217728 >/proc/sys/kernel/shmmax</userinput>
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</screen>
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You could put these commands into a script run at boot-time.
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</para>
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<para>
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Other parameters are sufficiently sized for any application.
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If you want to see for yourself look into
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<filename>/usr/src/linux/include/asm-<replaceable>xxx</>/shmparam.h</>
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and <filename>/usr/src/linux/include/linux/sem.h</>. Be sure
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to do <command>make dep</> before rebuilding the kernel.
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and <filename>/usr/src/linux/include/linux/sem.h</>.
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</para>
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</listitem>
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</varlistentry>
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