PostgreSQL can sometimes exhaust various operating system resource limits, especially when multiple copies of the server are running on the same system, or in very large installations. This section explains the kernel resources used by PostgreSQL and the steps you can take to resolve problems related to kernel resource consumption.
PostgreSQL requires the operating system to provide inter-process communication (IPC) features, specifically shared memory and semaphores. Unix-derived systems typically provide “System V” IPC, “POSIX” IPC, or both. Windows has its own implementation of these features and is not discussed here.
By default, PostgreSQL allocates
a very small amount of System V shared memory, as well as a much larger
amount of anonymous mmap
shared memory.
Alternatively, a single large System V shared memory region can be used
(see shared_memory_type).
In addition a significant number of semaphores, which can be either
System V or POSIX style, are created at server startup. Currently,
POSIX semaphores are used on Linux and FreeBSD systems while other
platforms use System V semaphores.
System V IPC features are typically constrained by system-wide allocation limits. When PostgreSQL exceeds one of these limits, the server will refuse to start and should leave an instructive error message describing the problem and what to do about it. (See also Section 19.3.1.) The relevant kernel parameters are named consistently across different systems; Table 19.1 gives an overview. The methods to set them, however, vary. Suggestions for some platforms are given below.
Table 19.1. System V IPC Parameters
Name | Description | Values needed to run one PostgreSQL instance |
---|---|---|
SHMMAX | Maximum size of shared memory segment (bytes) | at least 1kB, but the default is usually much higher |
SHMMIN | Minimum size of shared memory segment (bytes) | 1 |
SHMALL | Total amount of shared memory available (bytes or pages) | same as SHMMAX if bytes,
or ceil(SHMMAX/PAGE_SIZE) if pages,
plus room for other applications |
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 + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16) plus room for other applications |
SEMMNS | Maximum number of semaphores system-wide | ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 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; do not change unless necessary) |
PostgreSQL requires a few bytes of System V shared memory
(typically 48 bytes, on 64-bit platforms) for each copy of the server.
On most modern operating systems, this amount can easily be allocated.
However, if you are running many copies of the server or you explicitly
configure the server to use large amounts of System V shared memory (see
shared_memory_type and dynamic_shared_memory_type), it may be necessary to
increase SHMALL
, which is the total amount of System V shared
memory system-wide. Note that 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 32 bytes 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.
When using System V semaphores,
PostgreSQL uses one semaphore per allowed connection
(max_connections), allowed autovacuum worker process
(autovacuum_max_workers) and allowed background
process (max_worker_processes), 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
autovacuum_max_workers
plus max_wal_senders
,
plus max_worker_processes
, plus one extra for each 16
allowed connections plus workers (see the formula in Table 19.1). 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 + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 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
. If the system has this parameter
(many do not), it 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.
Various other settings related to “semaphore undo”, such as
SEMMNU
and SEMUME
, do not affect
PostgreSQL.
When using POSIX semaphores, the number of semaphores needed is the same as for System V, that is one semaphore per allowed connection (max_connections), allowed autovacuum worker process (autovacuum_max_workers) and allowed background process (max_worker_processes). On the platforms where this option is preferred, there is no specific kernel limit on the number of POSIX semaphores.
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 might, 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
) might be too low.
The default shared memory settings are usually good enough, unless
you have set shared_memory_type
to sysv
.
System V semaphores are not used on this platform.
The default IPC settings can be changed using
the sysctl
or
loader
interfaces. The following
parameters can be set using sysctl
:
#
sysctl kern.ipc.shmall=32768
#
sysctl kern.ipc.shmmax=134217728
To make these settings persist over reboots, modify
/etc/sysctl.conf
.
If you have set shared_memory_type
to
sysv
, you might also want to configure your kernel
to lock System V 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 a FreeBSD jail, you should set its
sysvshm
parameter to new
, so that
it has its own separate System V shared memory namespace.
(Before FreeBSD 11.0, it was necessary to enable shared access to
the host's IPC namespace from jails, and take measures to avoid
collisions.)
The default shared memory settings are usually good enough, unless
you have set shared_memory_type
to sysv
.
You will usually want to increase kern.ipc.semmni
and kern.ipc.semmns
,
as NetBSD's default settings
for these are uncomfortably small.
IPC parameters can be adjusted using sysctl
,
for example:
#
sysctl -w kern.ipc.semmni=100
To make these settings persist over reboots, modify
/etc/sysctl.conf
.
If you have set shared_memory_type
to
sysv
, you might also want to configure your kernel
to lock System V 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 shared memory settings are usually good enough, unless
you have set shared_memory_type
to sysv
.
You will usually want to
increase kern.seminfo.semmni
and kern.seminfo.semmns
,
as OpenBSD's default settings
for these are uncomfortably small.
IPC parameters can be adjusted using sysctl
,
for example:
#
sysctl kern.seminfo.semmni=100
To make these settings persist over reboots, modify
/etc/sysctl.conf
.
The default settings tend to suffice for normal installations.
IPC parameters can be set in the System Administration Manager (SAM) under → . Choose when you're done.
The default shared memory settings are usually good enough, unless
you have set shared_memory_type
to sysv
,
and even then only on older kernel versions that shipped with low defaults.
System V semaphores are not used on this platform.
The shared memory size settings can be changed via the
sysctl
interface. For example, to allow 16 GB:
$
sysctl -w kernel.shmmax=17179869184
$
sysctl -w kernel.shmall=4194304
To make these settings persist over reboots, see
/etc/sysctl.conf
.
The default shared memory and semaphore settings are usually good enough, unless
you have set shared_memory_type
to sysv
.
The recommended method for configuring shared memory in macOS
is to create a file named /etc/sysctl.conf
,
containing variable assignments such as:
kern.sysv.shmmax=4194304 kern.sysv.shmmin=1 kern.sysv.shmmni=32 kern.sysv.shmseg=8 kern.sysv.shmall=1024
Note that in some macOS versions,
all five shared-memory parameters must be set in
/etc/sysctl.conf
, else the values will be ignored.
SHMMAX
can only be set to a multiple of 4096.
SHMALL
is measured in 4 kB pages on this platform.
It is possible to change all but SHMMNI
on the fly, using
sysctl. But it's still best to set up your preferred
values via /etc/sysctl.conf
, so that the values will be
kept across reboots.
The default shared memory and semaphore settings are usually good enough for most
PostgreSQL applications. Solaris defaults
to a SHMMAX
of one-quarter of system RAM.
To further adjust this setting, use a project setting associated
with the postgres
user. For example, run the
following as root
:
projadd -c "PostgreSQL DB User" -K "project.max-shm-memory=(privileged,8GB,deny)" -U postgres -G postgres user.postgres
This command adds the user.postgres
project and
sets the shared memory maximum for the postgres
user to 8GB, and takes effect the next time that user logs
in, or when you restart PostgreSQL (not reload).
The above assumes that PostgreSQL is run by
the postgres
user in the postgres
group. No server reboot is required.
Other recommended kernel setting changes for database servers which will have a large number of connections are:
project.max-shm-ids=(priv,32768,deny) project.max-sem-ids=(priv,4096,deny) project.max-msg-ids=(priv,4096,deny)
Additionally, if you are running PostgreSQL
inside a zone, you may need to raise the zone resource usage
limits as well. See "Chapter2: Projects and Tasks" in the
System Administrator's Guide for more
information on projects
and prctl
.
If systemd is in use, some care must be taken
that IPC resources (including shared memory) are not prematurely
removed by the operating system. This is especially of concern when
installing PostgreSQL from source. Users of distribution packages of
PostgreSQL are less likely to be affected, as
the postgres
user is then normally created as a system
user.
The setting RemoveIPC
in logind.conf
controls whether IPC objects are
removed when a user fully logs out. System users are exempt. This
setting defaults to on in stock systemd, but
some operating system distributions default it to off.
A typical observed effect when this setting is on is that shared memory objects used for parallel query execution are removed at apparently random times, leading to errors and warnings while attempting to open and remove them, like
WARNING: could not remove shared memory segment "/PostgreSQL.1450751626": No such file or directory
Different types of IPC objects (shared memory vs. semaphores, System V vs. POSIX) are treated slightly differently by systemd, so one might observe that some IPC resources are not removed in the same way as others. But it is not advisable to rely on these subtle differences.
A “user logging out” might happen as part of a maintenance
job or manually when an administrator logs in as
the postgres
user or something similar, so it is hard
to prevent in general.
What is a “system user” is determined
at systemd compile time from
the SYS_UID_MAX
setting
in /etc/login.defs
.
Packaging and deployment scripts should be careful to create
the postgres
user as a system user by
using useradd -r
, adduser --system
,
or equivalent.
Alternatively, if the user account was created incorrectly or cannot be changed, it is recommended to set
RemoveIPC=no
in /etc/systemd/logind.conf
or another appropriate
configuration file.
At least one of these two things has to be ensured, or the PostgreSQL server will be very unreliable.
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 might 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.
The default virtual memory behavior on Linux is not optimal for PostgreSQL. Because of the way that the kernel implements memory overcommit, the kernel might terminate the PostgreSQL postmaster (the supervisor server process) if the memory demands of either PostgreSQL or 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 (postgres).
This indicates that the postgres
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. If memory is tight, increasing the swap space of the operating system can help avoid the problem, because the out-of-memory (OOM) killer is invoked only when physical memory and swap space are exhausted.
If PostgreSQL itself is the cause of the
system running out of memory, you can avoid the problem by changing
your configuration. In some cases, it may help to lower memory-related
configuration parameters, particularly
shared_buffers
,
work_mem
, and
hash_mem_multiplier
.
In other cases, the problem may be caused by allowing too many
connections to the database server itself. In many cases, it may
be better to reduce
max_connections
and instead make use of external connection-pooling software.
It is possible to modify the
kernel's behavior so that it will not “overcommit” memory.
Although this setting will not prevent the OOM killer from being invoked
altogether, it will lower the chances significantly and will therefore
lead to more robust system behavior. 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 might also wish to modify the related setting
vm.overcommit_ratio
. For details see the kernel documentation
file https://www.kernel.org/doc/Documentation/vm/overcommit-accounting.
Another approach, which can be used with or without altering
vm.overcommit_memory
, is to set the process-specific
OOM score adjustment value for the postmaster process to
-1000
, thereby guaranteeing it will not be targeted by the OOM
killer. The simplest way to do this is to execute
echo -1000 > /proc/self/oom_score_adj
in the postmaster's startup script just before invoking the postmaster. Note that this action must be done as root, or it will have no effect; so a root-owned startup script is the easiest place to do it. If you do this, you should also set these environment variables in the startup script before invoking the postmaster:
export PG_OOM_ADJUST_FILE=/proc/self/oom_score_adj export PG_OOM_ADJUST_VALUE=0
These settings will cause postmaster child processes to run with the
normal OOM score adjustment of zero, so that the OOM killer can still
target them at need. You could use some other value for
PG_OOM_ADJUST_VALUE
if you want the child processes to run
with some other OOM score adjustment. (PG_OOM_ADJUST_VALUE
can also be omitted, in which case it defaults to zero.) If you do not
set PG_OOM_ADJUST_FILE
, the child processes will run with the
same OOM score adjustment as the postmaster, which is unwise since the
whole point is to ensure that the postmaster has a preferential setting.
Using huge pages reduces overhead when using large contiguous chunks of
memory, as PostgreSQL does, particularly when
using large values of shared_buffers. To use this
feature in PostgreSQL you need a kernel
with CONFIG_HUGETLBFS=y
and
CONFIG_HUGETLB_PAGE=y
. You will also have to configure
the operating system to provide enough huge pages of the desired size.
To estimate the number of huge pages needed, start
PostgreSQL without huge pages enabled and check
the postmaster's anonymous shared memory segment size, as well as the
system's default and supported huge page sizes, using the
/proc
and /sys
file systems.
This might look like:
$head -1 $PGDATA/postmaster.pid
4170 $pmap 4170 | awk '/rw-s/ && /zero/ {print $2}'
6490428K $grep ^Hugepagesize /proc/meminfo
Hugepagesize: 2048 kB $ls /sys/kernel/mm/hugepages
hugepages-1048576kB hugepages-2048kB
In this example the default is 2MB, but you can also explicitly request
either 2MB or 1GB with huge_page_size.
Assuming 2MB
huge pages,
6490428
/ 2048
gives approximately
3169.154
, so in this example we need at
least 3170
huge pages. A larger setting would be
appropriate if other programs on the machine also need huge pages.
We can set this with:
# sysctl -w vm.nr_hugepages=3170
Don't forget to add this setting to /etc/sysctl.conf
so that it is reapplied after reboots. For non-default huge page sizes,
we can instead use:
# echo 3170 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
It is also possible to provide these settings at boot time using
kernel parameters such as hugepagesz=2M hugepages=3170
.
Sometimes the kernel is not able to allocate the desired number of huge pages immediately due to fragmentation, so it might be necessary to repeat the command or to reboot. (Immediately after a reboot, most of the machine's memory should be available to convert into huge pages.) To verify the huge page allocation situation for a given size, use:
$ cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
It may also be necessary to give the database server's operating system
user permission to use huge pages by setting
vm.hugetlb_shm_group
via sysctl, and/or
give permission to lock memory with ulimit -l
.
The default behavior for huge pages in
PostgreSQL is to use them when possible, with
the system's default huge page size, and
to fall back to normal pages on failure. To enforce the use of huge
pages, you can set huge_pages
to on
in postgresql.conf
.
Note that with this setting PostgreSQL will fail to
start if not enough huge pages are available.
For a detailed description of the Linux huge pages feature have a look at https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt.