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Operating Systems AIX Permanently set maxdata to maximum Post 302954937 by MichaelFelt on Sunday 13th of September 2015 12:54:13 PM
Old 09-13-2015
After many hours of travel, and some sleep I have for you - this link with the basis information: https://www-01.ibm.com/support/knowl...rg_support.htm

Basically, what happens is that system and code remain in segments 0 and 1.
When maxdata is not equal to 0 (zero) - which is default, segment 2 is used for the stack and application .data (pre-initialized data) and .bss (aka heap for malloc calls) start in segment 3 and continue for the number of segments specified.

As you mentioned in your initial question the "official" large memory support model has a maximum MAXDATA value of 0x80000000 - so that is 2.25 G total (256MB (roughly) for the stack, and 2 GByte (8x 256 MByte) for application .data and .bss

From the discussion, I do not think you will want to use the DSA - dynamic segment allocation - option (so-called or the VERY large memory option). However, if 2G Byte for .data and .bss is not going to be enough you may need to use that.

Back to "shared" memory - the default memory model setup uses only segments 0, 1, and 2 for the memory model - the segments there are 12 segments (numbers 3-12 and 14 are used by shmap or mmap routines). Historically, segments 13 and 15 were used by global shared libraries text and data. When you use the DSA option global shared libraries are not used - instead the shared libraries are loaded "privately" into segment 15 - both text and data. This free up segment 13 giving an application at least one additional segment.

If you are not using shmap or mmap routines then the "very large" model should work fairly easy. However, if you do use either of these routines they may impact the maximum memory available via malloc(). - keyword here: potential impact!

I hope this answers your question well enough.

Michael
 

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IPCS(1) 						    BSD General Commands Manual 						   IPCS(1)

NAME
ipcs -- report System V interprocess communication facilities status SYNOPSIS
ipcs [-abcMmopQqSsTt] DESCRIPTION
The ipcs utility provides information on System V interprocess communication (IPC) facilities on the system. The options are as follows: -a Show the maximum amount of information possible when displaying active semaphores, message queues, and shared memory segments. (This is shorthand for specifying the -b, -c, -o, -p, and -t options.) -b Show the maximum allowed sizes for active semaphores, message queues, and shared memory segments. The ``maximum allowed size'' is the maximum number of bytes in a message on a message queue, the size of a shared memory segment, or the number of semaphores in a set of semaphores. -c Show the creator's name and group for active semaphores, message queues, and shared memory segments. -M Display system information about shared memory. -m Display information about active shared memory segments. -o Show outstanding usage for active message queues, and shared memory segments. The ``outstanding usage'' is the number of messages in a message queue, or the number of processes attached to a shared memory segment. -p Show the process ID information for active semaphores, message queues, and shared memory segments. The ``process ID information'' is the last process to send a message to or receive a message from a message queue, the process that created a semaphore, or the last process to attach or detach a shared memory segment. -Q Display system information about messages queues. -q Display information about active message queues. -S Display system information about semaphores. -s Display information about active semaphores. -T Display system information about shared memory, message queues and semaphores. -t Show access times for active semaphores, message queues, and shared memory segments. The access times is the time of the last con- trol operation on an IPC object, the last send or receive of a message, the last attach or detach of a shared memory segment, or the last operation on a semaphore. If none of the -M, -m, -Q, -q, -S, or -s options are specified, information about all active IPC facilities is listed. RESTRICTIONS
System data structures may change while ipcs is running; the output of ipcs is not guaranteed to be consistent. BUGS
This manual page is woefully incomplete, because it does not at all attempt to explain the information printed by ipcs. SEE ALSO
ipcrm(1) AUTHORS
Thorsten Lockert <tholo@sigmasoft.com> BSD
June 18, 1994 BSD
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