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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.
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 asked 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 possible
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 by multiplying the number of buffers and
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 approximately 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 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 the following to your kernel configuration file. A SHMALL value of 1024 represents 4 MB 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 4.3 or later, you will probably 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 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"
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.)
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/shmpara m.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.
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:
./link_unix
and reboot.
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 http://sunsite.uakom.sk/sunworldonline/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. 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
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:
default:\
...
:datasize-cur=256M:\
:maxproc-cur=256:\
:openfiles-cur=256:\
...
(-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.
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. 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.