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 VIPC" (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
IPC limits, the postmaster 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.) The relevant kernel
parameters are named consistently across different systems; Table 3-3 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.
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)
>= 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, 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 the connection setting plus one extra for each 16
allowed connections (see the formula in Table 3-3). 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.
BSD/OS
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 4 MB of
shared memory. The following increases the maximum shared
memory area to 32 MB:
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 sysptsize0x9 = 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.:
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:
(On NetBSD and OpenBSD the key word is actually
option singular.)
You may also want to use the sysctl setting to
lock shared memory into RAM and prevent it from being paged out
to swap.
HP-UX
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.
Linux
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:
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:
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.
OS/X
Edit the file
/System/Library/StartupItems/SystemTuning/SystemTuning
and edit the following values:
These values have the same meanings on OS/X as those listed for
previous operating systems.
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 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.
Solaris
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 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
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
login.conf 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 don't 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.
