AIX Version 4.3 Differences Guide

4.7 Shared Memory Enhancements
The following enhancements reflect changes to the shared memory function in
AIX.
4.7.1 Larger Shared Memory Regions (4.3.1)
The maximum size of a shared memory region created by the shmget() routine
and attached to a process’ address space by the shmat() routine has been
increased from 256 MB to 2 GB. Prior to AIX 4.3.1, it was possible to mmap() a
memory mapped file of up to 2 GB, but an anonymous memory region was limited
to 256 MB. This meant that a large memory region had to be created in several
256 MB portions and each portion attached individually. AIX 4.3.1 has removed
this restriction, so it is now possible to attach a 2 GB memory region with one call
to the shmat() routine.
If EXTSHM=ON is set and an application performs a shmget() with a size greater
than SEGSIZE-PAGESIZE, the system will use the traditional shmat() and not
mmap() as would be the case when EXTSHM=ON.
4.7.2 128 KB Shared Memory IDs (4.3.2)
AIX 4.3.2 now supports 128 KB mem, sem, and shm IDs, up from 4 KB in the
previous releases.
4.7.3 Shared Memory Debugging Enhancements (4.3.2)
AIX 4.3.2 has added the facility for additional information to be included in the
core file that may be produced by an application program when it encounters
certain types of error.
There are two methods of enabling the extra information to be included in the
core dump.
• The application can use the sigaction routine to set the SA_FULLDUMP flag
for the signal that will cause the core file to be generated.
• Enable full core information as the default, either from the SMIT Change /
Show Characteristics of Operating System panel, the WebSM Devices panel
by selecting sys0, or by using the command:
chdev -l sys0 -a fullcore=’true’
When an application faults, and a full core is generated, the core will include all
the shared memory regions of the faulting process that are currently attached.
The dbx debugger has been changed to understand the extra information in the
core file and allow the developer to interrogate the user defined variables
contained in the shared memory regions of the process at the time of termination.
4.8 DMA Pre-Translation (4.3.2)
DMA pretranslation of memory buffers reduces the cost of setting up DMA
operations. Its objective is to reduce DMA setup pathlength for selected and
predetermined I/O operations to improve performance. The enhances the
Application Development and Pthreads 79