Interprocess communication limits

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This topic describes system limits for Interprocess communication (IPC) mechanisms.

On some UNIX® systems, the system administrator can edit the /etc/master file and define limits for IPC mechanisms (semaphores, shared memory segments, and message queues). The problem with this method is that the higher the limits, the more memory the operating system uses, and performance can be adversely affected.

AIX® uses a different method. In AIX, upper limits are defined for the IPC mechanisms, which are not configurable. The individual IPC data structures are allocated and deallocated as needed, so memory requirements depend on the current system usage of IPC mechanisms.

This difference in methods sometimes confuses users who are installing or using databases. The limit that is the most confusing is the maximum number of shared memory segments that can be attached simultaneously per process. For 64-bit processes, the maximum number of shared memory segments is 268435456. For 32-bit processes, the maximum number of shared memory segments is 11, unless the extended shmat capability is used.

The following tables summarize the semaphore limits on IPC mechanisms.
Semaphores 7.1 7.2 7.3
Maximum number of semaphore IDs for 64-bit kernel 1048576 1048576 1048576
Maximum semaphores per semaphore ID 65535 65535 65535
Maximum operations per semop call 1024 1024 1024
Maximum undo entries per process 1024 1024 1024
Size in bytes of undo structure 8208 8208 8208
Semaphore maximum value 32767 32767 32767
Adjust on exit maximum value 16384 16384 16384
The following tables summarize the message queue limits on IPC mechanisms.
Message queue 7.1 7.2 7.3
Maximum message size 4 MB 4 MB 4 MB
Maximum bytes on queue 4 MB 4 MB 4 MB - 2 GB
Maximum number of message queue IDs for 32-bit kernel 131072 131072 131072
Maximum number of message queue IDs for 64-bit kernel 1048576 1048576 1048576
Maximum messages per queue ID 524288 524288 524288
The following tables summarize the shared memory limits on IPC mechanisms.
Shared memory 7.1 7.2 7.3
Maximum segment size (32-bit process) 2 GB 2 GB 2 GB
Maximum segment size (64-bit process) for 32-bit kernel N/A N/A N/A
Maximum segment size (64-bit process) for 64-bit kernel 32 TB 32 TB 32 TB
Minimum segment size 1 1 1
Maximum number of shared memory IDs (32-bit kernel) 131072 131072 131072
Maximum number of shared memory IDs (64-bit kernel) 1048576 1048576 1048576
Maximum number of segments per process (32-bit process) 11 11 11
Maximum number of segments per process (64-bit process) 268435456 268435456 268435456
Note: For 32-bit processes, the maximum number of segments per process is limited only by the size of the address space when the extended shmat capability is used.

IPC limits on AIX 7.3

The maximum number of bytes on a message queue is controlled by the ipc_msgmnb tunable parameter of the vmo command.