A large PostgreSQL installation can quickly hit various operating system resource limits. (On some systems, the factory defaults are so low that you don't even need a really "large" installation.) If you have encountered this kind of problem then keep reading.
Shared memory and semaphores are collectively referred to as "System V IPC" (together with message queues, which are not relevant for PostgreSQL). 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. (For the QNX and BeOS ports, PostgreSQL provides its own replacement implementation of these facilities.)
The complete lack of these facilities is usually manifested by an Illegal system call error upon postmaster start. In that case there's nothing left to do but to reconfigure your kernel -- PostgreSQL won't work without them.
When PostgreSQL exceeds one of the various hard 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. (See also Section 3.3.1.) The relevant kernel parameters are named consistently across different systems; Table 3-2 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 at least, possibly even recompile the kernel, to change these settings.
Table 3-2. System V IPC parameters
| Name | Description | Reasonable values | 
|---|---|---|
| SHMMAX | Maximum size of shared memory segment (bytes) | 250kB + 8.2kB * shared_buffers+ 14.2kB *max_connectionsor infinity | 
| SHMMIN | Minimum size of shared memory segment (bytes) | 1 | 
| SHMALL | Total amount of shared memory available (bytes or pages) | if bytes, same as SHMMAX; if pages, ceil(SHMMAX/PAGE_SIZE) | 
| SHMSEG | Maximum number of shared memory segments per process | only 1 segment is needed, but the default is much higher | 
| SHMMNI | Maximum number of shared memory segments system-wide | like SHMSEGplus room for other applications | 
| SEMMNI | Maximum number of semaphore identifiers (i.e., sets) | >= ceil(max_connections / 16) | 
| SEMMNS | Maximum number of semaphores system-wide | ceil(max_connections / 16) * 17 + room for other applications | 
| SEMMSL | Maximum number of semaphores per set | >= 17 | 
| SEMMAP | Number of entries in semaphore map | see text | 
| SEMVMX | Maximum value of semaphore | >= 255 (The default is often 32767, don't change unless asked to.) | 
    
    The most important shared memory parameter is SHMMAX,
    the maximum size, in bytes, that a shared memory segment can have.
    If you get an error message from shmget along the
    lines of Invalid argument then it is possible that
    this limit has been exceeded. The size of the required shared
    memory segments varies both with the number of requested buffers
    (-B option) and the number of allowed connections
    (-N option), although the former is the dominant item.
    (You can therefore, as a temporary solution, lower these settings
    to get rid of the failures.) As a rough approximation you can
    estimate the required segment size as the number of buffers times
    the block size (8 kB by default) plus ample overhead (at least
    half a megabyte). Any error message you might get will contain the
    size of the failed allocation request.
   
    Less likely to cause problems is the minimum size for shared
    memory segments (SHMMIN), which should be at most
    somewhere around 256 kB for PostgreSQL (it is
    usually just 1). The maximum number of segments system-wide
    (SHMMNI) or per-process (SHMSEG) should
    not cause a problem unless your system has them set to zero. Some
    systems also have a limit on the total amount of shared memory in
    the system; see the platform-specific instructions below.
   
    PostgreSQL uses one semaphore per allowed connection
    (-N option), in sets of 16.  Each such set will also
    contain a 17th semaphore which contains a "magic
    number", to detect collision with semaphore sets used by
    other applications. The maximum number of semaphores in the system
    is set by SEMMNS, which consequently must be at least
    as high as the connection setting plus one extra for each 16
    allowed connections (see the formula in Table 3-2).  The parameter 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 ceil(max_connections / 16). Lowering the number
    of allowed connections is a temporary workaround for failures,
    which are usually confusingly worded "No space
    left on device", from the function semget().
   
    In some cases it might also turn out to be necessary to increase
    SEMMAP to be at least on the order of
    SEMMNS. 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 therefore over time lead to less
    available semaphores than there should be.
   
    The SEMMSL parameter, which determines how many
    semaphores can be in a set, must be at least 17 for
    PostgreSQL.
   
    Various other settings related to "semaphore undo", such as
    SEMMNU and SEMUME, are not of concern
    for PostgreSQL.
   
Shared Memory.          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 number of shared buffers supported by the
	 postmaster, add the following to your kernel configuration file. A
	 SHMALL value of 1024 represents 4MB of shared
	 memory. The following increases the maximum shared memory area
	 to 32 MB:
options "SHMALL=8192" options "SHMMAX=\(SHMALL*PAGE_SIZE\)"
        For those running 4.1 or later, just make the above changes,
        recompile the kernel, and reboot. For those running earlier
        releases, use bpatch to find the
        sysptsize value in the current kernel. This is
        computed dynamically at boot time.
$ bpatch -r sysptsize 0x9 = 9
        Next, add SYSPTSIZE as a hard-coded value in the
        kernel configuration file. Increase the value you found using
        bpatch. Add 1 for every additional 4 MB of
        shared memory you desire.
options "SYSPTSIZE=16"
        sysptsize cannot be changed by sysctl.
       
Semaphores. You may need to increase the number of semaphores. By default, PostgreSQL allocates 34 semaphores, which is over half the default system total of 60.
Set the values you want in your kernel configuration file, e.g.:
options "SEMMNI=40" options "SEMMNS=240" options "SEMUME=40" options "SEMMNU=120"
        The options SYSVSHM and SYSVSEM need
        to be enabled when the kernel is compiled. (They are by
        default.) The maximum size of shared memory is determined by
        the option SHMMAXPGS (in pages). The following
        shows an example of how to set the various parameters:
options SYSVSHM options SHMMAXPGS=4096 options SHMSEG=256 options SYSVSEM options SEMMNI=256 options SEMMNS=512 options SEMMNU=256 options SEMMAP=256
(On NetBSD and OpenBSD the key word is actually option singular.)
        The default settings tend to suffice for normal installations.
        On HP-UX 10, the factory default for
        SEMMNS is 128, which might be too low for larger
        database sites.
       
IPC parameters can be set in the System Administration Manager (SAM) under Kernel Configuration->Configurable Parameters. Hit Create A New Kernel when you're done.
        The default shared memory limit (both
        SHMMAX and SHMALL) is 32
        MB in 2.2 kernels, but it can be changed in the
        proc file system (without reboot).  For
        example, to allow 128 MB:
$ echo 134217728 >/proc/sys/kernel/shmall $ echo 134217728 >/proc/sys/kernel/shmmax
You could put these commands into a script run at boot-time.
Alternatively, you can use sysctl, if available, to control these parameters. Look for a file called /etc/sysctl.conf and add lines like the following to it:
kernel.shmall = 134217728 kernel.shmmax = 134217728
This file is usually processed at boot time, but sysctl can also be called explicitly later.
Other parameters are sufficiently sized for any application. If you want to see for yourself look into /usr/src/linux/include/asm-xxx/shmparam.h and /usr/src/linux/include/linux/sem.h.
        In the default configuration, only 512 kB of shared memory per
        segment is allowed, which is about enough for -B 24 -N
        12. To increase the setting, first change the directory to
        /etc/conf/cf.d. To display the current value of
        SHMMAX, in bytes, run
./configure -y SHMMAX
        To set a new value for SHMMAX, run:
./configure SHMMAX=value
        where value is the new value you want to use
        (in bytes). After setting SHMMAX, rebuild the kernel
./link_unix
and reboot.
At least in version 2.6, the maximum size of a shared memory segment is set too low for PostgreSQL. The relevant settings can be changed in /etc/system, for example:
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
You need to reboot to make the changes effective.
See also http://www.sunworld.com/swol-09-1997/swol-09-insidesolaris.html for information on shared memory under Solaris.
        On UnixWare 7, the maximum size for shared
        memory segments is 512 kB in the default configuration. This
        is enough for about -B 24 -N 12. To display the
        current value of SHMMAX, run
/etc/conf/bin/idtune -g SHMMAX
        which displays the current, default, minimum, and maximum
        values, in bytes. To set a new value for SHMMAX,
        run:
/etc/conf/bin/idtune SHMMAX value
        where value is the new value you want to use
        (in bytes). After setting SHMMAX, rebuild the
        kernel
/etc/conf/bin/idbuild -B
and reboot.
    Unix-like operating systems enforce various kinds of resource
    limits that might interfere with the operation of your
    PostgreSQL server.  Of importance are
    especially the limits on the number of processes per user, the
    number of open files per process, and the amount of memory
    available to a process.  Each of these have a "hard"
    and a "soft" 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 setrlimit is responsible for setting
    these parameters.  The shell's built-in command
    ulimit (Bourne shells) or
    limit (csh) is used to control the resource
    limits from the command line.  On BSD-derived systems the file
    /etc/login.conf controls what values the
    various resource limits are set to upon login.  See
    login.conf for details.  The relevant
    parameters are maxproc,
    openfiles, and datasize.
    For example:
default:\
...
        :datasize-cur=256M:\
        :maxproc-cur=256:\
        :openfiles-cur=256:\
...(-cur is the soft limit. Append -max to set the hard limit.)
Kernels generally also have an implementation-dependent system-wide limit on some resources.
On Linux /proc/sys/fs/file-max 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 /etc/sysctl.conf. The maximum limit of files per process is fixed at the time the kernel is compiled; see /usr/src/linux/Documentation/proc.txt for more information.
The PostgreSQL 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.
The factory default limit on open files is often set to "socially friendly" 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 may want to raise this limit.
    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 don't want to alter the system-wide limit, you
    can set PostgreSQL's
    max_files_per_process configuration parameter
    to limit its consumption of open files.