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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="KERNEL-RESOURCES"
>18.4. Managing Kernel Resources</A
></H1
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> can sometimes exhaust various operating system
resource limits, especially when multiple copies of the server are running
on the same system, or in very large installations. This section explains
the kernel resources used by <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> and the steps you
can take to resolve problems related to kernel resource consumption.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SYSVIPC"
>18.4.1. Shared Memory and Semaphores</A
></H2
><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
>). Except on
<SPAN
CLASS="SYSTEMITEM"
>Windows</SPAN
>, where <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
provides its own replacement implementation of these facilities, these
facilities are required in order to run
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</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 is no alternative but to reconfigure your
kernel. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> won't work without them.
This situation is rare, however, among modern operating systems.
</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
and what to do about it. (See also <A
HREF="server-start.html#SERVER-START-FAILURES"
>Section 18.3.1</A
>.) The relevant kernel
parameters are named consistently across different systems; <A
HREF="kernel-resources.html#SYSVIPC-PARAMETERS"
>Table 18-1</A
> gives an overview. The methods to set
them, however, vary. Suggestions for some platforms are given below.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Prior to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.3, the amount of System V shared
memory required to start the server was much larger. If you are running
an older version of the server, please consult the documentation for
your server version.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="TABLE"
><A
NAME="SYSVIPC-PARAMETERS"
></A
><P
><B
>Table 18-1. <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 1kB (more if running many copies of the server)</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 + max_worker_processes + 5) / 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 + max_worker_processes + 5) / 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; do not change unless necessary)</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> requires a few bytes of System V shared memory
(typically 48 bytes, on 64-bit platforms) for each copy of the server.
On most modern operating systems, this amount can easily be allocated.
However, if you are running many copies of the server, or if other
applications are also using System V shared memory, it may be necessary
to increase <TT
CLASS="VARNAME"
>SHMMAX</TT
>, the maximum size in bytes of a shared
memory segment, or <TT
CLASS="VARNAME"
>SHMALL</TT
>, the total amount of System V shared
memory system-wide. Note that <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 32 bytes 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
>), allowed autovacuum worker process
(<A
HREF="runtime-config-autovacuum.html#GUC-AUTOVACUUM-MAX-WORKERS"
>autovacuum_max_workers</A
>) and allowed background
process (<A
HREF="runtime-config-resource.html#GUC-MAX-WORKER-PROCESSES"
>max_worker_processes</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 <TT
CLASS="VARNAME"
>max_worker_processes</TT
>,
plus one extra for each 16
allowed connections plus workers (see the formula in <A
HREF="kernel-resources.html#SYSVIPC-PARAMETERS"
>Table 18-1</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 + max_worker_processes + 5) / 16)</TT
>.
Lowering the number
of allowed connections is a temporary workaround for failures,
which are usually confusingly worded <SPAN
CLASS="QUOTE"
>"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
>. If the system has this parameter
(many do not), it 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
>, do not affect
<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"
>FreeBSD</SPAN
>
</DT
><DD
><P
> The default IPC settings can be changed using
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 kern.ipc.shmall=32768</KBD
>
<SAMP
CLASS="PROMPT"
>#</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl kern.ipc.shmmax=134217728</KBD
></PRE
><P>
To make these settings persist over reboots, modify
<TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
</P
><P
> These semaphore-related settings are read-only as far as
<TT
CLASS="COMMAND"
>sysctl</TT
> is concerned, but can be set in
<TT
CLASS="FILENAME"
>/boot/loader.conf</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>kern.ipc.semmni=256
kern.ipc.semmns=512</PRE
><P>
After modifying that file, a reboot is required for the new
settings to take effect.
</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 different jails,
and it allows the PostgreSQL IPC cleanup code to function properly.
(In FreeBSD 6.0 and later the IPC cleanup code does not 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
old <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.semmni=100</KBD
></PRE
><P>
To make these settings persist over reboots, modify
<TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
</P
><P
> You will usually want to increase <TT
CLASS="LITERAL"
>kern.ipc.semmni</TT
>
and <TT
CLASS="LITERAL"
>kern.ipc.semmns</TT
>,
as <SPAN
CLASS="SYSTEMITEM"
>NetBSD</SPAN
>'s default settings
for these are uncomfortably small.
</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 old <SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
>
(see below), except that kernel 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
> In <SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
> 3.3 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 kern.seminfo.semmni=100</KBD
></PRE
><P>
To make these settings persist over reboots, modify
<TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>.
</P
><P
> You will usually want to
increase <TT
CLASS="LITERAL"
>kern.seminfo.semmni</TT
>
and <TT
CLASS="LITERAL"
>kern.seminfo.semmns</TT
>,
as <SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
>'s default settings
for these are uncomfortably small.
</P
><P
> In older <SPAN
CLASS="SYSTEMITEM"
>OpenBSD</SPAN
> versions,
you will need to build a custom kernel to change the IPC parameters.
Make sure that the options <TT
CLASS="VARNAME"
>SYSVSHM</TT
>
and <TT
CLASS="VARNAME"
>SYSVSEM</TT
> are enabled, too. (They are by
default.) The following shows an example of how to set the various
parameters in the kernel configuration file:
</P><PRE
CLASS="PROGRAMLISTING"
>option SYSVSHM
option SHMMAXPGS=4096
option SHMSEG=256
option SYSVSEM
option SEMMNI=256
option SEMMNS=512
option SEMMNU=256</PRE
><P>
</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
>. Choose
<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, and the
default maximum total size is 2097152
pages. A page is almost always 4096 bytes except in unusual
kernel configurations with <SPAN
CLASS="QUOTE"
>"huge pages"</SPAN
>
(use <TT
CLASS="LITERAL"
>getconf PAGE_SIZE</TT
> to verify).
</P
><P
> The shared memory size settings can be changed via the
<TT
CLASS="COMMAND"
>sysctl</TT
> interface. For example, to allow 16 GB:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kernel.shmmax=17179869184</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>sysctl -w kernel.shmall=4194304</KBD
></PRE
><P>
In addition these settings can be preserved between reboots in
the file <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>. Doing that is
highly recommended.
</P
><P
> Ancient 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"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>echo 17179869184 >/proc/sys/kernel/shmmax</KBD
>
<SAMP
CLASS="PROMPT"
>$</SAMP
> <KBD
CLASS="USERINPUT"
>echo 4194304 >/proc/sys/kernel/shmall</KBD
></PRE
><P>
</P
><P
> The remaining defaults are quite generously sized, and usually
do not require changes.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>OS X</SPAN
>
</DT
><DD
><P
> The recommended method for configuring shared memory in OS X
is to 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>
Note that in some OS X versions,
<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 older OS X versions, you will need to reboot to have changes in the
shared memory parameters take effect. As of 10.5 it is possible to
change all but <TT
CLASS="VARNAME"
>SHMMNI</TT
> on the fly, using
<SPAN
CLASS="APPLICATION"
>sysctl</SPAN
>. But it's still best to set up your preferred
values via <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
>, so that the values will be
kept across reboots.
</P
><P
> The file <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
> is only honored in OS X
10.3.9 and later. If you are running a previous 10.3.x release,
you must edit the file <TT
CLASS="FILENAME"
>/etc/rc</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>
Note that
<TT
CLASS="FILENAME"
>/etc/rc</TT
> is usually overwritten by OS X system updates,
so you should expect to have to redo these edits after each update.
</P
><P
> In OS X 10.2 and earlier, instead edit these commands in the file
<TT
CLASS="FILENAME"
>/System/Library/StartupItems/SystemTuning/SystemTuning</TT
>.
</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
> 2.6 to 2.9 (Solaris
6 to Solaris 9)
</DT
><DD
><P
> 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. 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 older versions of Solaris.
</P
></DD
><DT
><SPAN
CLASS="SYSTEMITEM"
>Solaris</SPAN
> 2.10 (Solaris
10) and later<BR><SPAN
CLASS="SYSTEMITEM"
>OpenSolaris</SPAN
></DT
><DD
><P
> In Solaris 10 and later, and OpenSolaris, the default shared memory and
semaphore settings are good enough for most
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> applications. Solaris now defaults
to a <TT
CLASS="VARNAME"
>SHMMAX</TT
> of one-quarter of system <ACRONYM
CLASS="ACRONYM"
>RAM</ACRONYM
>.
To further adjust this setting, use a project setting associated
with the <TT
CLASS="LITERAL"
>postgres</TT
> user. For example, run the
following as <TT
CLASS="LITERAL"
>root</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>projadd -c "PostgreSQL DB User" -K "project.max-shm-memory=(privileged,8GB,deny)" -U postgres -G postgres user.postgres</PRE
><P>
</P
><P
> This command adds the <TT
CLASS="LITERAL"
>user.postgres</TT
> project and
sets the shared memory maximum for the <TT
CLASS="LITERAL"
>postgres</TT
>
user to 8GB, and takes effect the next time that user logs
in, or when you restart <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> (not reload).
The above assumes that <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is run by
the <TT
CLASS="LITERAL"
>postgres</TT
> user in the <TT
CLASS="LITERAL"
>postgres</TT
>
group. No server reboot is required.
</P
><P
> Other recommended kernel setting changes for database servers which will
have a large number of connections are:
</P><PRE
CLASS="PROGRAMLISTING"
>project.max-shm-ids=(priv,32768,deny)
project.max-sem-ids=(priv,4096,deny)
project.max-msg-ids=(priv,4096,deny)</PRE
><P>
</P
><P
> Additionally, if you are running <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
inside a zone, you may need to raise the zone resource usage
limits as well. See "Chapter2: Projects and Tasks" in the
<I
CLASS="CITETITLE"
>System Administrator's Guide</I
> for more
information on <TT
CLASS="LITERAL"
>projects</TT
> and <TT
CLASS="COMMAND"
>prctl</TT
>.
</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 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
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SYSTEMD-REMOVEIPC"
>18.4.2. systemd RemoveIPC</A
></H2
><P
> If <SPAN
CLASS="PRODUCTNAME"
>systemd</SPAN
> is in use, some care must be taken
that IPC resources (shared memory and semaphores) are not prematurely
removed by the operating system. This is especially of concern when
installing PostgreSQL from source. Users of distribution packages of
PostgreSQL are less likely to be affected, as
the <TT
CLASS="LITERAL"
>postgres</TT
> user is then normally created as a system
user.
</P
><P
> The setting <TT
CLASS="LITERAL"
>RemoveIPC</TT
>
in <TT
CLASS="FILENAME"
>logind.conf</TT
> controls whether IPC objects are
removed when a user fully logs out. System users are exempt. This
setting defaults to on in stock <SPAN
CLASS="PRODUCTNAME"
>systemd</SPAN
>, but
some operating system distributions default it to off.
</P
><P
> A typical observed effect when this setting is on is that the semaphore
objects used by a PostgreSQL server are removed at apparently random
times, leading to the server crashing with log messages like
</P><PRE
CLASS="SCREEN"
>LOG: semctl(1234567890, 0, IPC_RMID, ...) failed: Invalid argument</PRE
><P>
Different types of IPC objects (shared memory vs. semaphores, System V
vs. POSIX) are treated slightly differently
by <SPAN
CLASS="PRODUCTNAME"
>systemd</SPAN
>, so one might observe that some IPC
resources are not removed in the same way as others. But it is not
advisable to rely on these subtle differences.
</P
><P
> A <SPAN
CLASS="QUOTE"
>"user logging out"</SPAN
> might happen as part of a maintenance
job or manually when an administrator logs in as
the <TT
CLASS="LITERAL"
>postgres</TT
> user or something similar, so it is hard
to prevent in general.
</P
><P
> What is a <SPAN
CLASS="QUOTE"
>"system user"</SPAN
> is determined
at <SPAN
CLASS="PRODUCTNAME"
>systemd</SPAN
> compile time from
the <TT
CLASS="SYMBOL"
>SYS_UID_MAX</TT
> setting
in <TT
CLASS="FILENAME"
>/etc/login.defs</TT
>.
</P
><P
> Packaging and deployment scripts should be careful to create
the <TT
CLASS="LITERAL"
>postgres</TT
> user as a system user by
using <TT
CLASS="LITERAL"
>useradd -r</TT
>, <TT
CLASS="LITERAL"
>adduser --system</TT
>,
or equivalent.
</P
><P
> Alternatively, if the user account was created incorrectly or cannot be
changed, it is recommended to set
</P><PRE
CLASS="PROGRAMLISTING"
>RemoveIPC=no</PRE
><P>
in <TT
CLASS="FILENAME"
>/etc/systemd/logind.conf</TT
> or another appropriate
configuration file.
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> At least one of these two things has to be ensured, or the PostgreSQL
server will be very unreliable.
</P
></TD
></TR
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN32077"
>18.4.3. 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="LINUX-MEMORY-OVERCOMMIT"
>18.4.4. 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
> postmaster (the
master server process) if the memory demands of either
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> or 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 avoid the problem, because the
out-of-memory (OOM) killer is invoked only when physical memory and
swap space are exhausted.
</P
><P
> If <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> itself is the cause of the
system running out of memory, you can avoid the problem by changing
your configuration. In some cases, it may help to lower memory-related
configuration parameters, particularly
<A
HREF="runtime-config-resource.html#GUC-SHARED-BUFFERS"
><TT
CLASS="VARNAME"
>shared_buffers</TT
></A
>
and <A
HREF="runtime-config-resource.html#GUC-WORK-MEM"
><TT
CLASS="VARNAME"
>work_mem</TT
></A
>. In
other cases, the problem may be caused by allowing too many connections
to the database server itself. In many cases, it may be better to reduce
<A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
><TT
CLASS="VARNAME"
>max_connections</TT
></A
>
and instead make use of external connection-pooling software.
</P
><P
> On Linux 2.6 and later, it is possible 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="VARNAME"
>vm.overcommit_ratio</TT
>. For details see the kernel documentation
file <A
HREF="https://www.kernel.org/doc/Documentation/vm/overcommit-accounting"
TARGET="_top"
>https://www.kernel.org/doc/Documentation/vm/overcommit-accounting</A
>.
</P
><P
> Another approach, which can be used with or without altering
<TT
CLASS="VARNAME"
>vm.overcommit_memory</TT
>, is to set the process-specific
<I
CLASS="FIRSTTERM"
>OOM score adjustment</I
> value for the postmaster process to
<TT
CLASS="LITERAL"
>-1000</TT
>, thereby guaranteeing it will not be targeted by the OOM
killer. The simplest way to do this is to execute
</P><PRE
CLASS="PROGRAMLISTING"
>echo -1000 > /proc/self/oom_score_adj</PRE
><P>
in the postmaster's startup script just before invoking the postmaster.
Note that this action must be done as root, or it will have no effect;
so a root-owned startup script is the easiest place to do it. If you
do this, you should also set these environment variables in the startup
script before invoking the postmaster:
</P><PRE
CLASS="PROGRAMLISTING"
>export PG_OOM_ADJUST_FILE=/proc/self/oom_score_adj
export PG_OOM_ADJUST_VALUE=0</PRE
><P>
These settings will cause postmaster child processes to run with the
normal OOM score adjustment of zero, so that the OOM killer can still
target them at need. You could use some other value for
<TT
CLASS="ENVAR"
>PG_OOM_ADJUST_VALUE</TT
> if you want the child processes to run
with some other OOM score adjustment. (<TT
CLASS="ENVAR"
>PG_OOM_ADJUST_VALUE</TT
>
can also be omitted, in which case it defaults to zero.) If you do not
set <TT
CLASS="ENVAR"
>PG_OOM_ADJUST_FILE</TT
>, the child processes will run with the
same OOM score adjustment as the postmaster, which is unwise since the
whole point is to ensure that the postmaster has a preferential setting.
</P
><P
> Older Linux kernels do not offer <TT
CLASS="FILENAME"
>/proc/self/oom_score_adj</TT
>,
but may have a previous version of the same functionality called
<TT
CLASS="FILENAME"
>/proc/self/oom_adj</TT
>. This works the same except the disable
value is <TT
CLASS="LITERAL"
>-17</TT
> not <TT
CLASS="LITERAL"
>-1000</TT
>.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> 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 2.4 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 kernel 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
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LINUX-HUGE-PAGES"
>18.4.5. Linux Huge Pages</A
></H2
><P
> Using huge pages reduces overhead when using large contiguous chunks of
memory, as <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does, particularly when
using large values of <A
HREF="runtime-config-resource.html#GUC-SHARED-BUFFERS"
>shared_buffers</A
>. To use this
feature in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> you need a kernel
with <TT
CLASS="VARNAME"
>CONFIG_HUGETLBFS=y</TT
> and
<TT
CLASS="VARNAME"
>CONFIG_HUGETLB_PAGE=y</TT
>. You will also have to adjust
the kernel setting <TT
CLASS="VARNAME"
>vm.nr_hugepages</TT
>. To estimate the
number of huge pages needed, start <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
without huge pages enabled and check the
postmaster's <TT
CLASS="VARNAME"
>VmPeak</TT
> value, as well as the system's
huge page size, using the <TT
CLASS="FILENAME"
>/proc</TT
> file system. This might
look like:
</P><PRE
CLASS="PROGRAMLISTING"
>$ <KBD
CLASS="USERINPUT"
>head -1 $PGDATA/postmaster.pid</KBD
>
4170
$ <KBD
CLASS="USERINPUT"
>grep ^VmPeak /proc/4170/status</KBD
>
VmPeak: 6490428 kB
$ <KBD
CLASS="USERINPUT"
>grep ^Hugepagesize /proc/meminfo</KBD
>
Hugepagesize: 2048 kB</PRE
><P>
<TT
CLASS="LITERAL"
>6490428</TT
> / <TT
CLASS="LITERAL"
>2048</TT
> gives approximately
<TT
CLASS="LITERAL"
>3169.154</TT
>, so in this example we need at
least <TT
CLASS="LITERAL"
>3170</TT
> huge pages, which we can set with:
</P><PRE
CLASS="PROGRAMLISTING"
>$ <KBD
CLASS="USERINPUT"
>sysctl -w vm.nr_hugepages=3170</KBD
></PRE
><P>
A larger setting would be appropriate if other programs on the machine
also need huge pages. Don't forget to add this setting
to <TT
CLASS="FILENAME"
>/etc/sysctl.conf</TT
> so that it will be reapplied
after reboots.
</P
><P
> Sometimes the kernel is not able to allocate the desired number of huge
pages immediately, so it might be necessary to repeat the command or to
reboot. (Immediately after a reboot, most of the machine's memory
should be available to convert into huge pages.) To verify the huge
page allocation situation, use:
</P><PRE
CLASS="PROGRAMLISTING"
>$ <KBD
CLASS="USERINPUT"
>grep Huge /proc/meminfo</KBD
></PRE
><P>
</P
><P
> It may also be necessary to give the database server's operating system
user permission to use huge pages by setting
<TT
CLASS="VARNAME"
>vm.hugetlb_shm_group</TT
> via <SPAN
CLASS="APPLICATION"
>sysctl</SPAN
>, and/or
give permission to lock memory with <TT
CLASS="COMMAND"
>ulimit -l</TT
>.
</P
><P
> The default behavior for huge pages in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is to use them when possible and
to fall back to normal pages when failing. To enforce the use of huge
pages, you can set <A
HREF="runtime-config-resource.html#GUC-HUGE-PAGES"
>huge_pages</A
>
to <TT
CLASS="LITERAL"
>on</TT
> in <TT
CLASS="FILENAME"
>postgresql.conf</TT
>.
Note that with this setting <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> will fail to
start if not enough huge pages are available.
</P
><P
> For a detailed description of the <SPAN
CLASS="PRODUCTNAME"
>Linux</SPAN
> huge
pages feature have a look
at <A
HREF="https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt"
TARGET="_top"
>https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt</A
>.
</P
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