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16.5. Managing Kernel Resources

A large PostgreSQL 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 "large" installation.) If you have encountered this kind of problem, keep reading.

16.5.1. Shared Memory and Semaphores

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 server 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 IPC 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 Section 16.3.1.) The relevant kernel parameters are named consistently across different systems; Table 16-2 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, and possibly even recompile the kernel, to change these settings.

Table 16-2. System V IPC parameters

Name Description Reasonable values
SHMMAX Maximum size of shared memory segment (bytes) 250 kB + 8.2 kB * shared_buffers + 14.2 kB * max_connections up to 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 SHMSEG plus room for other applications
SEMMNI Maximum number of semaphore identifiers (i.e., sets) at least ceil(max_connections / 16)
SEMMNS Maximum number of semaphores system-wide ceil(max_connections / 16) * 17 plus room for other applications
SEMMSL Maximum number of semaphores per set at least 17
SEMMAP Number of entries in semaphore map see text
SEMVMX Maximum value of semaphore at least 1000 (The default is often 32767, don't change unless forced to)

The most important shared memory parameter is SHMMAX, the maximum size, in bytes, of a shared memory segment. If you get an error message from shmget like Invalid argument, it is likely that this limit has been exceeded. The size of the required shared memory segment varies both with the number of requested buffers (-B option) and the number of allowed connections (-N option), although the former is the most significant. (You can, as a temporary solution, lower these settings to eliminate the failure.) As a rough approximation, you can estimate the required segment size as suggested in Table 16-2. Any error message you might get will contain the size of the failed allocation request.

Some systems also have a limit on the total amount of shared memory in the system (SHMALL). Make sure this is large enough for PostgreSQL plus any other applications that are using shared memory segments. (Caution: SHMALL is measured in pages rather than bytes on many systems.)

Less likely to cause problems is the minimum size for shared memory segments (SHMMIN), which should be at most approximately 256 kB for PostgreSQL (it is usually just 1). The maximum number of segments system-wide (SHMMNI) or per-process (SHMSEG) are unlikely to cause a problem unless your system has them set to zero.

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 max_connections plus one extra for each 16 allowed connections (see the formula in Table 16-2). 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 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 over time lead to fewer 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 amount of shared memory supported by your system, add something like the following to your kernel configuration file:

options "SHMALL=8192"

SHMALL is measured in 4KB 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 KERNEL_VIRTUAL_MB above the default 248. Once all changes have been made, recompile the kernel, and reboot.

For those running 4.0 and 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 will probably want to increase the number of semaphores as well; the default system total of 60 will only allow about 50 PostgreSQL connections. Set the values you want in your kernel configuration file, e.g.:

options "SEMMNI=40"
options "SEMMNS=240"

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 OpenBSD the key word is actually option singular.)

You might also want to configure your kernel to lock shared memory into RAM and prevent it from being paged out to swap. Use the sysctl setting kern.ipc.shm_use_phys.


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 in /usr/src/linux/include/asm-xxx/shmparam.h and /usr/src/linux/include/linux/sem.h.


In OS X 10.2 and earlier, edit the file /System/Library/StartupItems/SystemTuning/SystemTuning and change the values in the following commands:

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

In OS X 10.3, these commands have been moved to /etc/rc and must be edited there. You'll need to reboot to make changes take effect. Note that /etc/rc 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.

SHMALL is measured in 4KB pages on this platform.

SCO OpenServer

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 to the directory /etc/conf/cf.d. To display the current value of SHMMAX, 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:


and reboot.


At least as of version 5.1, it should not be necessary to do any special configuration for such parameters as SHMMAX, 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 DB/2.

It may, however, be necessary to modify the global ulimit information in /etc/security/limits, as the default hard limits for file sizes (fsize) and numbers of files (nofiles) may be too low.


At least in version 2.6, the default maximum size of a shared memory segments is 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 for the changes to take effect.

See also 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. 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.

16.5.2. Resource Limits

Unix-like operating systems enforce various kinds of resource limits that might interfere with the operation of your PostgreSQL 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 "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 the various resource limits set during login. See the operating system documentation for details. The relevant parameters are maxproc, openfiles, and datasize. For example:


(-cur is the soft limit. Append -max to set the hard limit.)

Kernels can also have system-wide limits 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 you do not want to alter the system-wide limit, you can set PostgreSQL's max_files_per_process configuration parameter to limit the consumption of open files.

16.5.3. Linux Memory Overcommit

In Linux 2.4 and later, the default virtual memory behavior is not optimal for PostgreSQL. Because of the way that the kernel implements memory overcommit, the kernel may terminate the PostgreSQL server (the postmaster process) if the memory demands of another process cause the system to run out of virtual memory.

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):

Out of Memory: Killed process 12345 (postmaster). 

This indicates that the postmaster 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, PostgreSQL will need to be restarted.

One way to avoid this problem is to run PostgreSQL on a machine where you can be sure that other processes will not run the machine out of memory.

On Linux 2.6 and later, a better solution is to modify the kernel's behavior so that it will not "overcommit" memory. This is done by selecting strict overcommit mode via sysctl:

sysctl -w vm.overcommit_memory=2

or placing an equivalent entry in /etc/sysctl.conf. You may also wish to modify the related setting vm.overcommit_ratio. For details see the kernel documentation file Documentation/vm/overcommit-accounting.

Some vendors' Linux 2.4 kernels are reported to have early versions of the 2.6 overcommit sysctl parameter. However, setting vm.overcommit_memory 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 vm_enough_memory in the file mm/mmap.c) to verify what is supported in your copy before you try this in a 2.4 installation. The presence of the overcommit-accounting documentation file should not be taken as evidence that the feature is there. If in any doubt, consult a kernel expert or your kernel vendor.