18.4. Managing Kernel Resources
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## 18.4. Managing Kernel Resources

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.

### 18.4.1. Shared Memory and Semaphores

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.

The complete lack of these facilities is usually manifested by an “Illegal system call” error upon server start. In that case there is no alternative but to reconfigure your kernel. PostgreSQL won't work without them. This situation is rare, however, among modern operating systems.

Upon starting the server, PostgreSQL normally allocates a very small amount of System V shared memory, as well as a much larger amount of POSIX (`mmap`) shared memory. 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.

### Note

Prior to PostgreSQL 9.3, only System V shared memory was used, so the amount of System V shared memory required to start the server was much larger. If you are running an older version of the server, please consult the documentation for your server version.

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 18.3.1.) The relevant kernel parameters are named consistently across different systems; Table 18.1 gives an overview. The methods to set them, however, vary. Suggestions for some platforms are given below.

Table 18.1. 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_worker_processes + 5) / 16)` plus room for other applications
`SEMMNS` | Maximum number of semaphores system-wide | `ceil((max_connections + autovacuum_max_workers + 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 if other applications are also using System V shared memory, 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_worker_processes`, plus one extra for each 16 allowed connections plus workers (see the formula in Table 18.1). 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_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.

AIX

  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 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.

FreeBSD

  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`.

  These semaphore-related settings are read-only as far as `sysctl` is concerned, but can be set in `/boot/loader.conf`:

      kern.ipc.semmni=256
      kern.ipc.semmns=512

  After modifying that file, a reboot is required for the new settings to take effect.

  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 different jails, and it allows the PostgreSQL IPC cleanup code to function properly. (In FreeBSD 6.0 and later the IPC cleanup code does not 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 old OpenBSD (see below).

NetBSD

  In NetBSD 5.0 and later, 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`.

  You will usually want to increase `kern.ipc.semmni` and `kern.ipc.semmns`, as NetBSD's default settings for these are uncomfortably small.

  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`.

  NetBSD versions before 5.0 work like old OpenBSD (see below), except that kernel parameters should be set with the keyword `options` not `option`.

OpenBSD

  In OpenBSD 3.3 and later, 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`.

  You will usually want to increase `kern.seminfo.semmni` and `kern.seminfo.semmns`, as OpenBSD's default settings for these are uncomfortably small.

  In older OpenBSD versions, you will need to build a custom kernel to change the IPC parameters. Make sure that the options `SYSVSHM` and `SYSVSEM` are enabled, too. (They are by default.) The following shows an example of how to set the various parameters in the kernel configuration file:

      option        SYSVSHM
      option        SHMMAXPGS=4096
      option        SHMSEG=256

      option        SYSVSEM
      option        SEMMNI=256
      option        SEMMNS=512
      option        SEMMNU=256

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. Choose Create A New Kernel when you're done.

Linux

  The default maximum segment size is 32 MB, and the default maximum total size is 2097152 pages. A page is almost always 4096 bytes except in unusual kernel configurations with “huge pages” (use `getconf PAGE_SIZE` to verify).

  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

  In addition these settings can be preserved between reboots in the file `/etc/sysctl.conf`. Doing that is highly recommended.

  Ancient distributions might not have the `sysctl` program, but equivalent changes can be made by manipulating the `/proc` file system:

      $ echo 17179869184 >/proc/sys/kernel/shmmax
      $ echo 4194304 >/proc/sys/kernel/shmall

  The remaining defaults are quite generously sized, and usually do not require changes.

macOS

  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.

  Beware that recent releases of macOS 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 older macOS versions, you will need to reboot to have changes in the shared memory parameters take effect. As of 10.5 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 file `/etc/sysctl.conf` is only honored in macOS 10.3.9 and later. If you are running a previous 10.3.x release, you must edit the file `/etc/rc` 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

  Note that `/etc/rc` is usually overwritten by macOS system updates, so you should expect to have to redo these edits after each update.

  In macOS 10.2 and earlier, instead edit these commands in the file `/System/Library/StartupItems/SystemTuning/SystemTuning`.

Solaris 2.6 to 2.9 (Solaris 6 to Solaris 9)

  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 older versions of Solaris.

Solaris 2.10 (Solaris 10) and later
OpenSolaris

  In Solaris 10 and later, and OpenSolaris, the default shared memory and semaphore settings are good enough for most PostgreSQL applications. Solaris now 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`.

### 18.4.2. systemd RemoveIPC

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.

### Caution

At least one of these two things has to be ensured, or the PostgreSQL server will be very unreliable.

### 18.4.3. 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:

    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.

### 18.4.4. 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 might terminate the PostgreSQL postmaster (the master 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` and `work_mem`. 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.

On Linux 2.6 and later, 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.

Older Linux kernels do not offer `/proc/self/oom_score_adj`, but may have a previous version of the same functionality called `/proc/self/oom_adj`. This works the same except the disable value is `-17` not `-1000`.

### Note

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 2.4 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 kernel 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.

### 18.4.5. Linux Huge Pages

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 adjust the kernel setting `vm.nr_hugepages`. 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 huge page size, using the `/proc` file system. 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

`6490428` / `2048` gives approximately `3169.154`, so in this example we need at least `3170` huge pages, which we can set with:

    $ sysctl -w vm.nr_hugepages=3170

A larger setting would be appropriate if other programs on the machine also need huge pages. Don't forget to add this setting to `/etc/sysctl.conf` so that it will be reapplied after reboots.

Sometimes the kernel is not able to allocate the desired number of huge pages immediately, 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, use:

    $ grep Huge /proc/meminfo

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 and to fall back to normal pages when failing. 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.

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