Where is the number of shared memory segments set?

[ggitlin]

Hello!
Does anyone know where the maximum number of shared memoty segments and the size of the segment is set?
Our customer's monitoring program reports that 12 is required, but "OS imposed" limit is 11. According to a web site I found, the EXTSHM=ON allows a process to allocate more than 11 shared memory segments (providing the size of the segment is less than 256 MB). When this variable was set in the startup script - it still didn't work.
Application - SAP.
AIX 4.3.3 ML9.

 

[Jonsie123]

In AIX memory management is dynamic and you do not need to specifically set this parameter from the OS. This parameter can be set from the applications like Oracle when the SGA is allocated basically in the form of semaphores

Cheers

 

[bi]

ggitlin, if increasing the size of the SGA doesn't help you, I'm wondering if bringing your AIX system up to ML 10 or 11 would help? What level does SAP recommend?

 

[AIXSPadmin]

There are Segments 0-15:

segment 0 - kernel
segment 1 - text
segment 2 - data
segments 3-12 shared/memory mapped
segment 13 - shared library text
segment 14 - additional shmat/mmap
segment 15 - shared library data

Each segment is 256MB. So there are 16,777,216 possible 256MB segments in 4PB of virtual memory. The VMM allocates 16 segments per process, thus the VM image is 4GB (16 * 256MB).

SAP tends to use shared memory pools, so it doesn't violate the 16 SHM.

mmap can be used to generate anonymous shared memory segments.

 

[ggitlin]

Thank to everyone for the responces.
What about "rss", "stack" and "data" attributes in the /etc/security/limits file? Do they play any role in memory allocation?

 

[ggitlin]

To AIXSPadmin:
Thanks for the detailed response. What about the size of the memory segment? Is it 256 MB by default or is it set somewhere?

 

[AIXSPadmin]

Default.

 

[ggitlin]

Thank You.