| PostgreSQL 9.4.26 Documentation | |||
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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.
Shared memory and semaphores are collectively referred to as "System V IPC" (together with message queues, which are not relevant for PostgreSQL). Except on Windows, where PostgreSQL provides its own replacement implementation of these facilities, these facilities are required in order to run PostgreSQL.
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.
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 and what to do about it. (See also Section 17.3.1.) The relevant kernel parameters are named consistently across different systems; Table 17-1 gives an overview. The methods to set them, however, vary. Suggestions for some platforms are given below.
Note: Prior to PostgreSQL 9.3, 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.
Table 17-1. System V IPC Parameters
| Name | Description | Reasonable values |
|---|---|---|
| SHMMAX | Maximum size of shared memory segment (bytes) | at least 1kB (more if running many copies of the server) |
| 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 + autovacuum_max_workers + max_worker_processes + 5) / 16) |
| 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 SHMMAX, the maximum size in bytes of a shared memory segment, or SHMALL, 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.
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 17-1). 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.
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, do not affect PostgreSQL.
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.
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).
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.