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<Chapter Id="runtime">

limits of the IPC resources then the postmaster will refuse to

start up and should leave a marginally instructive error message

about which problem was encountered and what needs to be done

about it. The relevant kernel parametershave a relatively

consistent nomenclatureacross systems; <xref

about it. The relevant kernel parametersare named

consistentlyacross different systems; <xref

linkend="sysvipc-parameters"> gives an overview. The methods to

set them, however, vary; suggestions for some platforms are given

below. Be aware, however, that you will have to reboot your

machine atbest, orpossibly recompile the kernel, to change these

below. Be aware that you will have to reboot your

machine atleast,possibly even recompile the kernel, to change these

settings.

</para>

<row>

<entry><varname>SHMSEG</></>

<entry>Maximum number of shared memory segments per process</>

<entry>Must be at least 3, but the default is much higher.</>

<entry>must be at least 3, but the default is much higher</>

</row>

<row>

<row>

<entry><varname>SEMMNI</></>

<entry>Maximum number of semaphore identifiers (i.e., sets)</>

<entry>&gt;= ceil(max_connections% 16)</>

<entry>&gt;= ceil(max_connections/ 16)</>

</row>

<row>

<entry><varname>SEMMNS</></>

<entry>Maximum number of semaphores system-wide</>

<entry>number of allowed connections,rounded up to multiple of 16</>

<entry>max_connectionsrounded up to multiple of 16, + room for other applications</>

</row>

<row>

The parameter <varname>SEMMNI</> determines the limit on the

number of semaphore sets that can exist on the system at one time.

Hence this parameter must be at least

<literal>ceil(max_connections% 16)</>. Lowering the number of

<literal>ceil(max_connections/ 16)</>. Lowering the number of

allowed connections is a temporary workaround for failures, which

are usually confusingly worded <quote><errorname>No space left on

device</></>, from the function <function>semget()</>.

semaphores needs an entry. When a semaphore set is freed it is

either added to an existing entry that is adjacent to the freed

block or it is registered under a new map entry. If the map is

full, the freed semaphoresgets lost (until reboot). Fragmentation

full, the freed semaphoresget lost (until reboot). Fragmentation

of the semaphore space could therefore over time lead to less

available semaphores than there should be.

</para>

</para>

<para>

Various settings related to <quote>semaphore undo</>, such as

<varname>SEMMNU</> and <varname>SEMUME</>, are not ofrelevance

with <productname>Postgres</>.

Variousothersettings related to <quote>semaphore undo</>, such as

<varname>SEMMNU</> and <varname>SEMUME</>, are not ofconcern

for <productname>Postgres</>.

</para>

<programlisting>

/sys/sys/shm.h:69:#define SHMMAXPGS 1024 /* max hardware pages... */

</programlisting>

The default setting of 1024is for a maximum of 4 MB of shared

The default setting of 1024provides a maximum of 4 MB of shared

memory.

</para>

</formalpara>

to be enabled when the kernel is compiled. (They are by

default.) The maximum size of shared memory is determined by

the option <varname>SHMMAXPGS</> (in pages). The following

shows an exampleon how to set the various parameters:

shows an exampleof how to set the various parameters:

<programlisting>

options SYSVSHM

options SHMMAXPGS=4096