AIX working with the LDR_CNTRL environment variable

The LDR_CNTRL environment variable controls the way AIX® handles the memory space available to a 32-bit program and the page sizes used in each segment. The POWER4™ and later PowerPC® processors support the use of 16 MB large pages in addition to the default 4 KB pages. The POWER5+™ and later PowerPC processors add support for two new page sizes, 64 KB and 16 GB.
Controlling the available memory space

The SDK typically sets appropriate LDR_CNTRL=MAXDATA settings automatically for the size of the Java™ heap. However, if you explicitly set the LDR_CNTRL=MAXDATA environment variable the SDK will not override the value you have specified. Do not override the automatic settings unless you have a complete understanding of the AIX memory models. For more information about the AIX memory models and the automatic settings used by the SDK, see SDK use of AIX large program support.
Page sizes

AIX v5.2 and later operating systems support 16 MB pages. AIX v5.3 maintenance package 5300-04 on POWER5+ processors adds support for the 64 KB and 16 GB page sizes. The 16 MB and 16 GB pages require AIX system configuration changes. For information on using 16 MB pages with AIX, see http://www.ibm.com/servers/aix/whitepapers/large_page.html. For information about using either 64 KB or 16 GB pages with AIX, see http://www.ibm.com/servers/aix/whitepapers/multiple_page.pdf. The default AIX page size is 4 KB.

The LDR_CNTRL=LARGE_PAGE_DATA environment variable can be used under the AIX v5.2 and later operating systems to control the use of 16 MB pages for the native data area and native heap of a program. You can use 16 MB large pages, if they are available, by setting LDR_CNTRL=LARGE_PAGE_DATA=Y. Using large pages might improve the performance of Java applications that require a large amount of native heap space. In particular, the native heap is used for machine code generated by the just-in-time compiler and Java applications with a large number of compiled methods might benefit from using 16 MB pages for the native heap.

AIX v5.3 maintenance package 5300-04 added LDR_CNTRL variants that independently control the use of different page sizes for the text (TEXTPSIZE), stack (STACKPSIZE) and native data or heap (DATAPSIZE) areas. See the Guide to Multiple Page Size Support on AIX 5L Version 5.3 for general information about these variants: http://www.ibm.com/servers/aix/whitepapers/multiple_page.pdf.
An example of the use of TEXTPSIZE, STACKPSIZE, and DATAPSIZE variants is:

LDR_CNTRL=TEXTPSIZE=4K@STACKPSIZE=64K@DATAPSIZE=64K

This example uses 4 KB pages for text, 64 KB pages for stack and 64 KB pages for the native data and native heap areas. A DATAPSIZE setting will override any LARGE_PAGE_DATA setting.

In the default AIX 5L™ 32-bit process address space model, the initial thread stack size and data of a process are located in the same PowerPC 256 MB segment. Only one page size can be used in a segment. If different page sizes are specified for the stack and data of a standard 32-bit process, the smaller page size will be used for both. See the Guide to Multiple Page Size Support on AIX 5L Version 5.3 for information about how to use different segments for the stack and data of a process: http://www.ibm.com/servers/aix/whitepapers/multiple_page.pdf.
When page sizes are set using LDR_CNTRL, the SDK does not automatically set MAXDATA and it is set to MAXDATA=0. To use a Java heap larger than 2.5 GB, set MAXDATA to a different value. See SDK use of AIX large program support for more information about possible settings. For example:

LDR_CNTR=MAXDATA=0XB0000000@DSA@TEXTPSIZE=4K@STACKPSIZE=64K@DATAPSIZE=64K

The new 64 KB pages are general-purpose and most workloads will see a benefit by using 64 KB pages for text, stack, native data, and the Java heap. The 16 GB pages are intended only for use in very high performance environments.
Note: Use the -Xlp option variants to request that the JVM allocates the Java heap with a specific size of pages.

For more information about using the LDR_CNTRL environment variables and on configuring AIX support for large pages, see http://www.ibm.com/servers/aix/whitepapers/large_page.html for AIX v5.2 information on 16 MB pages, and http://www.ibm.com/servers/aix/whitepapers/multiple_page.pdf for information on AIX v5.3 support of 64 KB and 16 GB pages.
Parent topic: Running Java applications