General Programming Concepts:
Writing and Debugging Programs

Inter-Process Communication (IPC) Limits

This document describes system limits for 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. For more information on extending shmat, see Understanding Memory Mapping.

The following tables summarize the semaphore limits on IPC mechanisms.

Semaphores	4.3.0	4.3.1	4.3.2	5.1	5.2
Maximum number of semaphore IDs:	4096	4096	131072	131072	131072
Maximum semaphores per semaphore ID	65535	65535	65535	65535	65535
Maximum operations per semop call	1024	1024	1024	1024	1024
Maximum undo entries per process	1024	1024	1024	1024	1024
Size in bytes of undo structure	8208	8208	8208	8208	8208
Semaphore maximum value	32767	32767	32767	32767	32767
Adjust on exit maximum value	16384	16384	16384	16384	16384

The following tables summarize the message queue limits on IPC mechanisms.

Message Queue	4.3.0	4.3.1	4.3.2	5.1	5.2
Maximum message size	4 MB	4 MB	4 MB	4 MB	4 MB
Maximum bytes on queue	4 MB	4 MB	4 MB	4 MB	4 MB
Maximum number of message queue IDs	4096	4096	131072	131072	131072
Maximum messages per queue ID	524288	524288	524288	524288	524288

The following tables summarize the shared memory limits on IPC mechanisms.

Shared Memory	4.3.0	4.3.1	4.3.2	5.1	5.2
Maximum segment size (32-bit)	256 MB	2 GB	2 GB	2 GB	2 GB
Maximum segment size (64-bit)	256 MB	2 GB	2 GB	64 GB	1 TB
Minimum segment size	1	1	1	1	1
Maximum number of shared memory IDs	4096	4096	131072	131072	131072
Maximum number of segments per process (32-bit)	11	11	11	11	11
Maximum number of segments per process (64-bit)	268435456	268435456	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 4.3

For semaphores and message queues, the table shows the system limits
For shared memory, the maximum shared memory segment size is 256GB.
For shared memory without the extended shmat capability:
- A process can attach a maximum of 11 shared memory segments.
For shared memory with the extended shmat capability:
- When a shared memory segments is attached, its size is rounded to a multiple of 4096 bytes
- A process can attach as many shared memory segments as will fit in the available address space. The maximum available address space size is 11 segments, or 11 times 256 MB.
The extended shmat capability is used if the environment variable EXTSHM has the value ON when the process starts executing.
The available address space for attaching shared memory segments is reduced if the large or very large address-space model is used. For more information, see Large Program Support.

IPC Limits on AIX 4.3.1

The maximum size of a shared memory segment increases from 256 MB to 2 GB. When a shared memory segment larger than 256 MB is attached, its size is rounded to a multiple of 256 MB, even if the extended shmat capability is being used.

IPC Limits on AIX 4.3.2

The maximum number of message queues, semaphore IDs, and shared memory segments is 131072.
The maximum number of messages per queue is 524288.

IPC Limits on AIX 5.1

The maximum size of a shared memory segment for 64-bit processes is 64 GB. A 32-bit process cannot attach a shared memory segment larger than 2 GB.

IPC Limits on AIX 5.2

The maximum size of a shared memory segment for 64-bit processes is 1 TB. A 32-bit process cannot attach a shared memory segment larger than 2 GB.
32-bit applications can use the shmat capability to obtain more than 11 segments when using the very large address space model without having to use extended shmat. For more information on the very large address space model, see Understanding the Very Large Address-Space Model.
Applications can query the IPC limits on the system using the vmgetinfo system call.