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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="KERNEL-RESOURCES"
>17.4. Managing Kernel Resources</A
></H1
><P
> A large <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> installation can quickly exhaust
various operating system resource limits. (On some systems, the
factory defaults are so low that you don't even need a really
<SPAN
CLASS="QUOTE"
>"large"</SPAN
> installation.) If you have encountered this kind of
problem, keep reading.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SYSVIPC"
>17.4.1. Shared Memory and Semaphores</A
></H2
><A
NAME="AEN23755"
></A
><A
NAME="AEN23757"
></A
><P
> Shared memory and semaphores are collectively referred to as
<SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
>
<ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
>"</SPAN
> (together with message queues, which are not
relevant for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>). Almost all modern
operating systems provide these features, but not all of them have
them turned on or sufficiently sized by default, especially systems
with BSD heritage. (On <SPAN
CLASS="SYSTEMITEM"
>Windows</SPAN
>,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides its own replacement
implementation of these facilities, and so most of this section
can be disregarded.)
</P
><P
> The complete lack of these facilities is usually manifested by an
<SPAN
CLASS="ERRORNAME"
>Illegal system call</SPAN
> error upon server start. In
that case there's nothing left to do but to reconfigure your
kernel. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> won't work without them.
</P
><P
> When <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> exceeds one of the various hard
<ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
> limits, the server will refuse to start and
should leave an instructive error message describing the problem
encountered and what to do about it. (See also <A
HREF="server-start.html#SERVER-START-FAILURES"
>Section 17.3.1</A
>.) The relevant kernel
parameters are named consistently across different systems; <A
HREF="kernel-resources.html#SYSVIPC-PARAMETERS"
>Table 17-1</A
> gives an overview. The methods to set
them, however, vary. Suggestions for some platforms are given below.
Be warned that it is often necessary to reboot your machine, and
possibly even recompile the kernel, to change these settings.
</P
><DIV
CLASS="TABLE"
><A
NAME="SYSVIPC-PARAMETERS"
></A
><P
><B
>Table 17-1. <SPAN
CLASS="SYSTEMITEM"
>System V</SPAN
> <ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
> parameters</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Description</TH
><TH
>Reasonable values</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="VARNAME"
>SHMMAX</TT
></TD
><TD
>Maximum size of shared memory segment (bytes)</TD
><TD
>at least several megabytes (see text)</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMMIN</TT
></TD
><TD
>Minimum size of shared memory segment (bytes)</TD
><TD
>1</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMALL</TT
></TD
><TD
>Total amount of shared memory available (bytes or pages)</TD
><TD
>if bytes, same as <TT
CLASS="VARNAME"
>SHMMAX</TT
>; if pages, <TT
CLASS="LITERAL"
>ceil(SHMMAX/PAGE_SIZE)</TT
></TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMSEG</TT
></TD
><TD
>Maximum number of shared memory segments per process</TD
><TD
>only 1 segment is needed, but the default is much higher</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SHMMNI</TT
></TD
><TD
>Maximum number of shared memory segments system-wide</TD
><TD
>like <TT
CLASS="VARNAME"
>SHMSEG</TT
> plus room for other applications</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMNI</TT
></TD
><TD
>Maximum number of semaphore identifiers (i.e., sets)</TD
><TD
>at least <TT
CLASS="LITERAL"
>ceil((max_connections + autovacuum_max_workers) / 16)</TT
></TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMNS</TT
></TD
><TD
>Maximum number of semaphores system-wide</TD
><TD
><TT
CLASS="LITERAL"
>ceil((max_connections + autovacuum_max_workers) / 16) * 17</TT
> plus room for other applications</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMSL</TT
></TD
><TD
>Maximum number of semaphores per set</TD
><TD
>at least 17</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMMAP</TT
></TD
><TD
>Number of entries in semaphore map</TD
><TD
>see text</TD
></TR
><TR
><TD
><TT
CLASS="VARNAME"
>SEMVMX</TT
></TD
><TD
>Maximum value of semaphore</TD
><TD
>at least 1000 (The default is often 32767, don't change unless forced to)</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <A
NAME="AEN23841"
></A
> The most important
shared memory parameter is <TT
CLASS="VARNAME"
>SHMMAX</TT
>, the maximum size, in
bytes, of a shared memory segment. If you get an error message from
<CODE
CLASS="FUNCTION"
>shmget</CODE
> like <SPAN
CLASS="ERRORNAME"
>Invalid argument</SPAN
>, it is
likely that this limit has been exceeded. The size of the required
shared memory segment varies depending on several
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> configuration parameters, as shown in
<A
HREF="kernel-resources.html#SHARED-MEMORY-PARAMETERS"
>Table 17-2</A
>. (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
avoid the failure. While it is possible to get
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> to run with <TT
CLASS="VARNAME"
>SHMMAX</TT
> as small as
2 MB, you need considerably more for acceptable performance. Desirable
settings are in the tens to hundreds of megabytes.
</P
><P
> Some systems also have a limit on the total amount of shared memory in
the system (<TT
CLASS="VARNAME"
>SHMALL</TT
>). Make sure this is large enough
for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> plus any other applications that
are using shared memory segments. (Caution: <TT
CLASS="VARNAME"
>SHMALL</TT
>
is measured in pages rather than bytes on many systems.)
</P
><P
> Less likely to cause problems is the minimum size for shared
memory segments (<TT
CLASS="VARNAME"
>SHMMIN</TT
>), which should be at most
approximately 500 kB for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> (it is
usually just 1). The maximum number of segments system-wide
(<TT
CLASS="VARNAME"
>SHMMNI</TT
>) or per-process (<TT
CLASS="VARNAME"
>SHMSEG</TT
>) are unlikely
to cause a problem unless your system has them set to zero.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> uses one semaphore per allowed connection
(<A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
>) and allowed autovacuum worker
process (<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
>), in sets of 16.
Each such set will
also contain a 17th semaphore which contains a <SPAN
CLASS="QUOTE"
>"magic
number"</SPAN
>, to detect collision with semaphore sets used by
other applications. The maximum number of semaphores in the system
is set by <TT
CLASS="VARNAME"
>SEMMNS</TT
>, which consequently must be at least
as high as <TT
CLASS="VARNAME"
>max_connections</TT
> plus
<TT
CLASS="VARNAME"
>autovacuum_max_workers</TT
>, plus one extra for each 16
allowed connections plus workers (see the formula in <A
HREF="kernel-resources.html#SYSVIPC-PARAMETERS"
>Table 17-1</A
>). The parameter <TT
CLASS="VARNAME"
>SEMMNI</TT
>
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 <TT
CLASS="LITERAL"
>ceil((max_connections + autovacuum_max_workers) / 16)</TT
>.
Lowering the number
of allowed connections is a temporary workaround for failures,
which are usually confusingly worded <SPAN
CLASS="ERRORNAME"
>No space
left on device</SPAN
>, from the function <CODE
CLASS="FUNCTION"
>semget</CODE
>.
</P
><P
> In some cases it might also be necessary to increase
<TT
CLASS="VARNAME"
>SEMMAP</TT
> to be at least on the order of
<TT
CLASS="VARNAME"
>SEMMNS</TT
>. This parameter defines the size of the semaphore
resource map, in which each contiguous block of available 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
semaphores get lost (until reboot). Fragmentation of the semaphore
space could over time lead to fewer available semaphores than there
should be.
</P
><P
> The <TT
CLASS="VARNAME"
>SEMMSL</TT
> parameter, which determines how many
semaphores can be in a set, must be at least 17 for
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</P
><P
> Various other settings related to <SPAN
CLASS="QUOTE"
>"semaphore undo"</SPAN
>, such as
<TT
CLASS="VARNAME"
>SEMMNU</TT
> and <TT
CLASS="VARNAME"
>SEMUME</TT
>, are not of concern
for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><SPAN
CLASS="SYSTEMITEM"
>AIX</SPAN
></DT
><DD
><P
> At least as of version 5.1, it should not be necessary to do
any special configuration for such parameters as
<TT
CLASS="VARNAME"
>SHMMAX</TT
>, as it appears this is configured to
allow all memory to be used as shared memory. That is the
sort of configuration commonly used for other databases such
as <SPAN
CLASS="APPLICATION"
>DB/2</SPAN
>.</P
><P
> It might, however, be necessary to modify the global
<TT
CLASS="COMMAND"
>ulimit</TT
> information in
<TT
CLASS="FILENAME"
>/etc/security/limits</TT
>, as the default hard
limits for file sizes (<TT
CLASS="VARNAME"
>fsize</TT
>) and numbers of
files (<TT
CLASS="VARNAME"
>nofiles</TT
>) might be too low.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>BSD/OS</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Shared Memory. </B
> By default, only 4 MB of shared memory is supported. Keep in
mind that shared memory is not pageable; it is locked in RAM.
To increase the amount of shared memory supported by your
system, add something like the following to your kernel configuration
file:
</P><PRE
CLASS="PROGRAMLISTING"
>options "SHMALL=8192"
options "SHMMAX=\(SHMALL*PAGE_SIZE\)"</PRE
><P>
<TT
CLASS="VARNAME"
>SHMALL</TT
> is measured in 4 kB pages, so a value of
1024 represents 4 MB of shared memory. Therefore the above increases
the maximum shared memory area to 32 MB.
For those running 4.3 or later, you will probably also need to increase
<TT
CLASS="VARNAME"
>KERNEL_VIRTUAL_MB</TT
> above the default <TT
CLASS="LITERAL"
>248</TT
>.
Once all changes have been made, recompile the kernel, and reboot.
</P
></DIV
><P
> For those running 4.0 and earlier releases, use <TT
CLASS="COMMAND"
>bpatch</TT
>
to find the <TT
CLASS="VARNAME"
>sysptsize</TT
> value in the current
kernel. This is computed dynamically at boot time.
</P><PRE
CLASS="SCREEN"
>$ <KBD
CLASS="USERINPUT"
>bpatch -r sysptsize</KBD
>
<SAMP
CLASS="COMPUTEROUTPUT"
>0x9 = 9</SAMP
></PRE
><P>
Next, add <TT
CLASS="VARNAME"
>SYSPTSIZE</TT
> as a hard-coded value in the
kernel configuration file. Increase the value you found using
<TT
CLASS="COMMAND"
>bpatch</TT
>. Add 1 for every additional 4 MB of
shared memory you desire.
</P><PRE
CLASS="PROGRAMLISTING"
>options "SYSPTSIZE=16"</PRE
><P>
<TT
CLASS="VARNAME"
>sysptsize</TT
> cannot be changed by <TT
CLASS="COMMAND"
>sysctl</TT
>.
</P
><DIV
CLASS="FORMALPARA"
><P
><B
>Semaphores. </B
> You will probably want to increase the number of semaphores
as well; the default system total of 60 will only allow about
50 <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> connections. Set the
values you want in your kernel configuration file, e.g.:
</P><PRE
CLASS="PROGRAMLISTING"
>options "SEMMNI=40"
options "SEMMNS=240"</PRE
><P>
</P
></DIV
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>FreeBSD</SPAN
></DT
><DD
><P
> The default settings are only suitable for small installations
(for example, default <TT
CLASS="VARNAME"
>SHMMAX</TT
> is 32
MB). Changes can be made via the <TT
CLASS="COMMAND"
>sysctl</TT
> or
<TT
CLASS="COMMAND"
>loader</TT
> interfaces. The following
parameters can be set using <TT
CLASS="COMMAND"
>sysctl</TT
>:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.shmall=32768</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.shmmax=134217728</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.semmap=256</KBD
></PRE
><P>
To have these settings persist over reboots, modify
<TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
</P
><P
> The remaining semaphore settings are read-only as far as
<TT
CLASS="COMMAND"
>sysctl</TT
> is concerned, but can be changed
before boot using the <TT
CLASS="COMMAND"
>loader</TT
> prompt:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>(loader)</SAMP
> <KBD
CLASS="USERINPUT"
>set kern.ipc.semmni=256</KBD
>
<SAMP
CLASS="PROMPT"
>(loader)</SAMP
> <KBD
CLASS="USERINPUT"
>set kern.ipc.semmns=512</KBD
>
<SAMP
CLASS="PROMPT"
>(loader)</SAMP
> <KBD
CLASS="USERINPUT"
>set kern.ipc.semmnu=256</KBD
></PRE
><P>
Similarly these can be saved between reboots in
<TT
CLASS="FILENAME"
>/boot/loader.conf</TT
>.
</P
><P
> You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
This can be accomplished using the <TT
CLASS="COMMAND"
>sysctl</TT
>
setting <TT
CLASS="LITERAL"
>kern.ipc.shm_use_phys</TT
>.
</P
><P
> If running in FreeBSD jails by enabling <SPAN
CLASS="APPLICATION"
>sysctl</SPAN
>'s
<TT
CLASS="LITERAL"
>security.jail.sysvipc_allowed</TT
>, <SPAN
CLASS="APPLICATION"
>postmaster</SPAN
>s
running in different jails should be run by different operating system
users. This improves security because it prevents non-root users
from interfering with shared memory or semaphores in a different jail,
and it allows the PostgreSQL IPC cleanup code to function properly.
(In FreeBSD 6.0 and later the IPC cleanup code doesn't properly detect
processes in other jails, preventing the running of postmasters on the
same port in different jails.)
</P
><P
> <SPAN
CLASS="SYSTEMITEM"
>FreeBSD</SPAN
> versions before 4.0 work like
<SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
> (see below).
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
></DT
><DD
><P
> In <SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
> 5.0 and later,
IPC parameters can be adjusted using <TT
CLASS="COMMAND"
>sysctl</TT
>,
for example:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kern.ipc.shmmax=16777216</KBD
></PRE
><P>
To have these settings persist over reboots, modify
<TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
</P
><P
> You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
This can be accomplished using the <TT
CLASS="COMMAND"
>sysctl</TT
>
setting <TT
CLASS="LITERAL"
>kern.ipc.shm_use_phys</TT
>.
</P
><P
> <SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
> versions before 5.0 work like
<SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
> (see below), except that
parameters should be set with the keyword <TT
CLASS="LITERAL"
>options</TT
> not
<TT
CLASS="LITERAL"
>option</TT
>.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
></DT
><DD
><P
> The options <TT
CLASS="VARNAME"
>SYSVSHM</TT
> and <TT
CLASS="VARNAME"
>SYSVSEM</TT
> need
to be enabled when the kernel is compiled. (They are by
default.) The maximum size of shared memory is determined by
the option <TT
CLASS="VARNAME"
>SHMMAXPGS</TT
> (in pages). The following
shows an example of how to set the various parameters:
</P><PRE
CLASS="PROGRAMLISTING"
>option SYSVSHM
option SHMMAXPGS=4096
option SHMSEG=256
option SYSVSEM
option SEMMNI=256
option SEMMNS=512
option SEMMNU=256
option SEMMAP=256</PRE
><P>
</P
><P
> You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
This can be accomplished using the <TT
CLASS="COMMAND"
>sysctl</TT
>
setting <TT
CLASS="LITERAL"
>kern.ipc.shm_use_phys</TT
>.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>HP-UX</SPAN
></DT
><DD
><P
> The default settings tend to suffice for normal installations.
On <SPAN
CLASS="PRODUCTNAME"
>HP-UX</SPAN
> 10, the factory default for
<TT
CLASS="VARNAME"
>SEMMNS</TT
> is 128, which might be too low for larger
database sites.
</P
><P
> <ACRONYM
CLASS="ACRONYM"
>IPC</ACRONYM
> parameters can be set in the <SPAN
CLASS="APPLICATION"
>System
Administration Manager</SPAN
> (<ACRONYM
CLASS="ACRONYM"
>SAM</ACRONYM
>) under
<SPAN
CLASS="GUIMENU"
>Kernel
Configuration</SPAN
>-><SPAN
CLASS="GUIMENUITEM"
>Configurable Parameters</SPAN
>. Hit
<SPAN
CLASS="GUIBUTTON"
>Create A New Kernel</SPAN
> when you're done.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Linux</SPAN
></DT
><DD
><P
> The default maximum segment size is 32 MB, which is only adequate
for small <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> installations.
However, the remaining
defaults are quite generously sized, and usually do not require
changes. The maximum shared memory segment size can be changed via the
<TT
CLASS="COMMAND"
>sysctl</TT
> interface. For example, to allow 128 MB,
and explicitly set the maximum total shared memory size to 2097152
pages (the default):
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kernel.shmmax=134217728</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kernel.shmall=2097152</KBD
></PRE
><P>
In addition these settings can be saved between reboots in
<TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
</P
><P
> Older distributions might not have the <TT
CLASS="COMMAND"
>sysctl</TT
> program,
but equivalent changes can be made by manipulating the
<TT
CLASS="FILENAME"
>/proc</TT
> file system:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>echo 134217728 >/proc/sys/kernel/shmmax</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>echo 2097152 >/proc/sys/kernel/shmall</KBD
></PRE
><P>
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>MacOS X</SPAN
></DT
><DD
><P
> In OS X 10.2 and earlier, edit the file
<TT
CLASS="FILENAME"
>/System/Library/StartupItems/SystemTuning/SystemTuning</TT
>
and change the values in the following commands:
</P><PRE
CLASS="PROGRAMLISTING"
>sysctl -w kern.sysv.shmmax
sysctl -w kern.sysv.shmmin
sysctl -w kern.sysv.shmmni
sysctl -w kern.sysv.shmseg
sysctl -w kern.sysv.shmall</PRE
><P>
</P
><P
> In OS X 10.3 and later, these commands have been moved to
<TT
CLASS="FILENAME"
>/etc/rc</TT
> and must be edited there. Note that
<TT
CLASS="FILENAME"
>/etc/rc</TT
> is usually overwritten by OS X updates (such as
10.3.6 to 10.3.7) so you should expect to have to redo your editing
after each update.
</P
><P
> In OS X 10.3.9 and later, instead of editing <TT
CLASS="FILENAME"
>/etc/rc</TT
>
you can create a file named <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>,
containing variable assignments such as:
</P><PRE
CLASS="PROGRAMLISTING"
>kern.sysv.shmmax=4194304
kern.sysv.shmmin=1
kern.sysv.shmmni=32
kern.sysv.shmseg=8
kern.sysv.shmall=1024</PRE
><P>
This method is better than editing <TT
CLASS="FILENAME"
>/etc/rc</TT
> because
your changes will be preserved across system updates. Note that
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>all five</I
></SPAN
> shared-memory parameters must be set in
<TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>, else the values will be ignored.
</P
><P
> Beware that recent releases of OS X ignore attempts to set
<TT
CLASS="VARNAME"
>SHMMAX</TT
> to a value that isn't an exact multiple of 4096.
</P
><P
> <TT
CLASS="VARNAME"
>SHMALL</TT
> is measured in 4 kB pages on this platform.
</P
><P
> In all OS X versions, you'll need to reboot to make changes in the
shared memory parameters take effect.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>SCO OpenServer</SPAN
></DT
><DD
><P
> In the default configuration, only 512 kB of shared memory per
segment is allowed. To increase the setting, first change to the
directory <TT
CLASS="FILENAME"
>/etc/conf/cf.d</TT
>. To display the current value of
<TT
CLASS="VARNAME"
>SHMMAX</TT
>, run:
</P><PRE
CLASS="PROGRAMLISTING"
>./configure -y SHMMAX</PRE
><P>
To set a new value for <TT
CLASS="VARNAME"
>SHMMAX</TT
>, run:
</P><PRE
CLASS="PROGRAMLISTING"
>./configure SHMMAX=<TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></PRE
><P>
where <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> is the new value you want to use
(in bytes). After setting <TT
CLASS="VARNAME"
>SHMMAX</TT
>, rebuild the kernel:
</P><PRE
CLASS="PROGRAMLISTING"
>./link_unix</PRE
><P>
and reboot.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Solaris</SPAN
></DT
><DD
><P
> At least in version 2.6, the default maximum size of a shared
memory segment is too low for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. The
relevant settings can be changed in <TT
CLASS="FILENAME"
>/etc/system</TT
>,
for example:
</P><PRE
CLASS="PROGRAMLISTING"
>set shmsys:shminfo_shmmax=0x2000000
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=256
set shmsys:shminfo_shmseg=256
set semsys:seminfo_semmap=256
set semsys:seminfo_semmni=512
set semsys:seminfo_semmns=512
set semsys:seminfo_semmsl=32</PRE
><P>
You need to reboot for the changes to take effect.
</P
><P
> See also <A
HREF="http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html"
TARGET="_top"
>http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html</A
>
for information on shared memory under
<SPAN
CLASS="PRODUCTNAME"
>Solaris</SPAN
>.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>UnixWare</SPAN
></DT
><DD
><P
> On <SPAN
CLASS="PRODUCTNAME"
>UnixWare</SPAN
> 7, the maximum size for shared
memory segments is only 512 kB in the default configuration.
To display the current value of <TT
CLASS="VARNAME"
>SHMMAX</TT
>, run:
</P><PRE
CLASS="PROGRAMLISTING"
>/etc/conf/bin/idtune -g SHMMAX</PRE
><P>
which displays the current, default, minimum, and maximum
values. To set a new value for <TT
CLASS="VARNAME"
>SHMMAX</TT
>,
run:
</P><PRE
CLASS="PROGRAMLISTING"
>/etc/conf/bin/idtune SHMMAX <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
></PRE
><P>
where <TT
CLASS="REPLACEABLE"
><I
>value</I
></TT
> is the new value you want to use
(in bytes). After setting <TT
CLASS="VARNAME"
>SHMMAX</TT
>, rebuild the
kernel:
</P><PRE
CLASS="PROGRAMLISTING"
>/etc/conf/bin/idbuild -B</PRE
><P>
and reboot.
</P
></DD
></DL
></DIV
><DIV
CLASS="TABLE"
><A
NAME="SHARED-MEMORY-PARAMETERS"
></A
><P
><B
>Table 17-2. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> shared memory usage</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Usage</TH
><TH
>Approximate shared memory bytes required (as of 8.3)</TH
></TR
></THEAD
><TBODY
><TR
><TD
>Connections</TD
><TD
>(1800 + 270 * <A
HREF="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION"
>max_locks_per_transaction</A
>) * <A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
></TD
></TR
><TR
><TD
>Autovacuum workers</TD
><TD
>(1800 + 270 * <A
HREF="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION"
>max_locks_per_transaction</A
>) * <A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
></TD
></TR
><TR
><TD
>Prepared transactions</TD
><TD
>(770 + 270 * <A
HREF="runtime-config-locks.html#GUC-MAX-LOCKS-PER-TRANSACTION"
>max_locks_per_transaction</A
>) * <A
HREF="runtime-config-resource.html#GUC-MAX-PREPARED-TRANSACTIONS"
>max_prepared_transactions</A
></TD
></TR
><TR
><TD
>Shared disk buffers</TD
><TD
>(<A
HREF="runtime-config-preset.html#GUC-BLOCK-SIZE"
>block_size</A
> + 208) * <A
HREF="runtime-config-resource.html#GUC-SHARED-BUFFERS"
>shared_buffers</A
></TD
></TR
><TR
><TD
>WAL buffers</TD
><TD
>(<A
HREF="runtime-config-preset.html#GUC-WAL-BLOCK-SIZE"
>wal_block_size</A
> + 8) * <A
HREF="runtime-config-wal.html#GUC-WAL-BUFFERS"
>wal_buffers</A
></TD
></TR
><TR
><TD
>Fixed space requirements</TD
><TD
>770 kB</TD
></TR
></TBODY
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN24159"
>17.4.2. Resource Limits</A
></H2
><P
> Unix-like operating systems enforce various kinds of resource limits
that might interfere with the operation of your
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server. Of particular
importance are limits on the number of processes per user, the
number of open files per process, and the amount of memory available
to each process. Each of these have a <SPAN
CLASS="QUOTE"
>"hard"</SPAN
> and a
<SPAN
CLASS="QUOTE"
>"soft"</SPAN
> limit. The soft limit is what actually counts
but it can be changed by the user up to the hard limit. The hard
limit can only be changed by the root user. The system call
<CODE
CLASS="FUNCTION"
>setrlimit</CODE
> is responsible for setting these
parameters. The shell's built-in command <TT
CLASS="COMMAND"
>ulimit</TT
>
(Bourne shells) or <TT
CLASS="COMMAND"
>limit</TT
> (<SPAN
CLASS="APPLICATION"
>csh</SPAN
>) is
used to control the resource limits from the command line. On
BSD-derived systems the file <TT
CLASS="FILENAME"
>/etc/login.conf</TT
>
controls the various resource limits set during login. See the
operating system documentation for details. The relevant
parameters are <TT
CLASS="VARNAME"
>maxproc</TT
>,
<TT
CLASS="VARNAME"
>openfiles</TT
>, and <TT
CLASS="VARNAME"
>datasize</TT
>. For
example:
</P><PRE
CLASS="PROGRAMLISTING"
>default:\
...
:datasize-cur=256M:\
:maxproc-cur=256:\
:openfiles-cur=256:\
...</PRE
><P>
(<TT
CLASS="LITERAL"
>-cur</TT
> is the soft limit. Append
<TT
CLASS="LITERAL"
>-max</TT
> to set the hard limit.)
</P
><P
> Kernels can also have system-wide limits on some resources.
<P
></P
></P><UL
><LI
><P
> On <SPAN
CLASS="PRODUCTNAME"
>Linux</SPAN
>
<TT
CLASS="FILENAME"
>/proc/sys/fs/file-max</TT
> determines the
maximum number of open files that the kernel will support. It can
be changed by writing a different number into the file or by
adding an assignment in <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
The maximum limit of files per process is fixed at the time the
kernel is compiled; see
<TT
CLASS="FILENAME"
>/usr/src/linux/Documentation/proc.txt</TT
> for
more information.
</P
></LI
></UL
><P>
</P
><P
> The <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server uses one process
per connection so you should provide for at least as many processes
as allowed connections, in addition to what you need for the rest
of your system. This is usually not a problem but if you run
several servers on one machine things might get tight.
</P
><P
> The factory default limit on open files is often set to
<SPAN
CLASS="QUOTE"
>"socially friendly"</SPAN
> values that allow many users to
coexist on a machine without using an inappropriate fraction of
the system resources. If you run many servers on a machine this
is perhaps what you want, but on dedicated servers you might want to
raise this limit.
</P
><P
> On the other side of the coin, some systems allow individual
processes to open large numbers of files; if more than a few
processes do so then the system-wide limit can easily be exceeded.
If you find this happening, and you do not want to alter the
system-wide limit, you can set <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s <A
HREF="runtime-config-resource.html#GUC-MAX-FILES-PER-PROCESS"
>max_files_per_process</A
> configuration parameter to
limit the consumption of open files.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN24191"
>17.4.3. Linux Memory Overcommit</A
></H2
><P
> In Linux 2.4 and later, the default virtual memory behavior is not
optimal for <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>. Because of the
way that the kernel implements memory overcommit, the kernel might
terminate the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server (the
master server process) if the memory demands of
another process cause the system to run out of virtual memory.
</P
><P
> If this happens, you will see a kernel message that looks like
this (consult your system documentation and configuration on where
to look for such a message):
</P><PRE
CLASS="PROGRAMLISTING"
>Out of Memory: Killed process 12345 (postgres). </PRE
><P>
This indicates that the <TT
CLASS="FILENAME"
>postgres</TT
> process
has been terminated due to memory pressure.
Although existing database connections will continue to function
normally, no new connections will be accepted. To recover,
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will need to be restarted.
</P
><P
> One way to avoid this problem is to run
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> on a machine where you can
be sure that other processes will not run the machine out of
memory. If memory is tight, increasing the swap space of the
operating system can help avoiding the problem, because the
out-of-memory (OOM) killer is invoked whenever physical memory and
swap space are exhausted.
</P
><P
> On Linux 2.6 and later, an additional measure is to modify the
kernel's behavior so that it will not <SPAN
CLASS="QUOTE"
>"overcommit"</SPAN
> memory.
Although this setting will not prevent the <A
HREF="http://lwn.net/Articles/104179/"
TARGET="_top"
>OOM killer</A
> from being invoked
altogether, it will lower the chances significantly and will therefore
lead to more robust system behavior. This is done by selecting strict
overcommit mode via <TT
CLASS="COMMAND"
>sysctl</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>sysctl -w vm.overcommit_memory=2</PRE
><P>
or placing an equivalent entry in <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
You might also wish to modify the related setting
<TT
CLASS="LITERAL"
>vm.overcommit_ratio</TT
>. For details see the kernel documentation
file <TT
CLASS="FILENAME"
>Documentation/vm/overcommit-accounting</TT
>.
</P
><P
> Some vendors' Linux 2.4 kernels are reported to have early versions
of the 2.6 overcommit <TT
CLASS="COMMAND"
>sysctl</TT
> parameter. However, setting
<TT
CLASS="LITERAL"
>vm.overcommit_memory</TT
> to 2
on a kernel that does not have the relevant code will make
things worse not better. It is recommended that you inspect
the actual kernel source code (see the function
<CODE
CLASS="FUNCTION"
>vm_enough_memory</CODE
> in the file <TT
CLASS="FILENAME"
>mm/mmap.c</TT
>)
to verify what is supported in your copy before you try this in a 2.4
installation. The presence of the <TT
CLASS="FILENAME"
>overcommit-accounting</TT
>
documentation file should <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> be taken as evidence that the
feature is there. If in any doubt, consult a kernel expert or your
kernel vendor.
</P
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