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PostgreSQL Reference Manual - Volume 3 - Server Administration Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 204 pages
ISBN 0954612043
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3.4.1 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 Windows port, 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 3.3.1 Server Start-up Failures.) The relevant kernel parameters are named consistently across different systems; Table 3-1 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 3-1: System V IPC parameters
Name Description Reasonable values
SHMMAX Maximum size of shared memory segment (bytes) at least several megabytes (see text)
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 depending on several PostgreSQL configuration parameters, as shown in Table 3-2. You can, as a temporary solution, lower some of those settings to avoid the failure. As a rough approximation, you can estimate the required segment size as 500 kB plus the variable amounts shown in the table. (Any error message you might get will include the exact size of the failed allocation request.) While it is possible to get PostgreSQL to run with SHMMAX as small as 1 MB, you need at least 4 MB for acceptable performance, and desirable settings are in the tens of megabytes.

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 500 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 ( max_connections), 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 3-1). 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 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 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 default settings are only suitable for small installations (for example, default SHMMAX is 32 MB). Changes can be made via the sysctl or loader interfaces. The following parameters can be set using sysctl:
$ sysctl -w kern.ipc.shmall=32768
$ sysctl -w kern.ipc.shmmax=134217728
$ sysctl -w kern.ipc.semmap=256
To have these settings persist over reboots, modify ‘/etc/sysctl.conf’. The remaining semaphore settings are read-only as far as sysctl is concerned, but can be changed before boot using the loader prompt:
(loader) set kern.ipc.semmni=256
(loader) set kern.ipc.semmns=512
(loader) set kern.ipc.semmnu=256
Similarly these can be saved between reboots in ‘/boot/loader.conf’. 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 sysctl setting kern.ipc.shm_use_phys. If running in FreeBSD jails by enabling sysctl's security.jail.sysvipc_allowed, postmasters 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.) FreeBSD versions before 4.0 work like NetBSD and OpenBSD (see below).
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 (OpenBSD uses option instead):
options        SYSVSHM
options        SHMMAXPGS=4096
options        SHMSEG=256

options        SYSVSEM
options        SEMMNI=256
options        SEMMNS=512
options        SEMMNU=256
options        SEMMAP=256
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 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 settings are only suitable for small installations (the default max segment size is 32 MB). However the remaining defaults are quite generously sized, and usually do not require changes. The max segment size can be changed via the sysctl interface. For example, to allow 128 MB, and explicitly set the maximum total shared memory size to 2097152 pages (the default):
$ sysctl -w kernel.shmmax=134217728
$ sysctl -w kernel.shmall=2097152
In addition these settings can be saved between reboots in ‘/etc/sysctl.conf’. Older distributions may not have the sysctl program, but equivalent changes can be made by manipulating the ‘/proc’ file system:
$ echo 134217728 >/proc/sys/kernel/shmmax
$ echo 2097152 >/proc/sys/kernel/shmall
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 and later, these commands have been moved to ‘/etc/rc’ and must be edited there. 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. In OS X 10.3.9 and later, instead of editing ‘/etc/rc’ you may create a file named ‘/etc/sysctl.conf’, containing variable assignments such as
This method is better than editing ‘/etc/rc’ because your changes will be preserved across system updates. Note that all five shared-memory parameters must be set in ‘/etc/sysctl.conf’, else the values will be ignored. Beware that recent releases of OS X ignore attempts to set SHMMAX to a value that isn't an exact multiple of 4096. SHMALL is measured in 4 kB pages on this platform. In all OS X versions, you'll need to reboot to make changes in the shared memory parameters take effect.
SCO OpenServer
In the default configuration, only 512 kB of shared memory per segment is allowed. 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.
On UnixWare 7, the maximum size for shared memory segments is only 512 kB in the default configuration. 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.
Table 3-2: Configuration parameters affecting
PostgreSQL's shared memory usage
Name Approximate multiplier (bytes per increment)
max_connections 400 + 270 * max_locks_per_transaction
max_prepared_transactions 600 + 270 * max_locks_per_transaction
shared_buffers 8300 (assuming 8K BLCKSZ)
wal_buffers 8200 (assuming 8K XLOG_BLCKSZ)
max_fsm_relations 70
max_fsm_pages 6
ISBN 0954612043PostgreSQL Reference Manual - Volume 3 - Server Administration GuideSee the print edition