Miscellaneous tunable parameters

Possible Values: ON (enables parameter)

Changes take effect for all processes that later start from this shell. Informs an application to always use the initial starting value of the TZ variable. This process improves performance if frequent timezone lookups are called by an application. For example, if an application frequently checks the local time. However, any changes to the TZ variable are not recognized when the application has already started.

Possible Values: ON, 1SEG, MSEG

Change takes effect immediately in this shell. Change is effective until logging out of this shell. Permanent change is made by adding EXTSHM=ON , EXTSHM=1SEG, or EXTSHM=MSEG command to the /etc/environment file.

Setting EXTSHM to ON has the same effect as setting the variable to 1SEG. With either setting, any shared memory less than 256 MB is created internally as a mmap segment, and thus has the same performance implications of mmap. Any shared memory greater or equal to 256 MB is created internally as a working segment.

If EXTSHM is set to MSEG, all shared memory is created internally as a mmap segment, allowing for better memory utilization.

Possible Values: PREREAD_SHLIB, LOADPUBLIC, IGNOREUNLOAD, USERREGS, MAXDATA, MAXDATA32, MAXDATA64, DSA, PRIVSEG_LOADS, DATA_START_STAGGER, LARGE_PAGE_TEXT, LARGE_PAGE_DATA, HUGE_EXEC, NAMEDSHLIB, SHARED_SYMTAB, or SED

Specifying the LOADPUBLIC option directs the system loader to load all modules requested by an application into the global shared library segment. If a module cannot be loaded publicly into the global shared library segment then it is loaded privately for the application.

Specifying the IGNOREUNLOAD option prevents the application from unloading libraries. This specification might prevent memory fragmentation and eliminate the overhead incurred when libraries are repeatedly loaded and unloaded. If you do not specify the IGNOREUNLOAD option, you might end up with two data instances of a module if the module was loaded at application load time and the module was then requested to be dynamically loaded and unloaded multiple times.

Specifying the USERREGS option tells the system to save all general-purpose user registers across system calls made by an application. This can be helpful in applications doing garbage collection.

Specifying the MAXDATA option sets the maximum heap size for a process, which includes overriding any maxdata value that is specified in the executable. The maxdata value is used to set the initial soft data resource limit of the process. For 32-bit programs, a non-zero maxdata value enables the large address-space mode, See Large Program Support. To disable the large address-space model, specify a maxdata value of zero by setting LDR_CNTRL=MAXDATA=0. For 64-bit programs, the maxdata value provides a guaranteed maximum size for the data heap of the program. The portion of the address space reserved for the heap cannot be used by the shmat() or mmap() subroutines, even if an explicit address is provided. Any value can be specified, but the data area cannot extend past 0x06FFFFFFFFFFFFFF regardless of the maxdata value specified.

The two additional maxdata options exist to allow finer control based on whether the process is 32-bit or 64-bit. These additional maxdata options override the MAXDATA option for the corresponding object mode. Specifying the MAXDATA32 option results in identical behavior to MAXDATA except that the value is ignored for 64-bit processes. Specifying the MAXDATA64 option results in identical behavior to MAXDATA except that the value is ignored for 32-bit processes.

Specifying the PRIVSEG_LOADS option directs the system loader to put dynamically loaded private modules into the process private segment. This specification might improve the availability of memory in large memory model applications that perform private dynamic loads and tend to run out of memory in the process heap. If the process private segment lacks sufficient space, the PRIVSEG_LOADS option has no effect. The PRIVSEG_LOADS option is only valid for 32-bit applications with a non-zero MAXDATA value.

Specifying the DATA_START_STAGGER=Y option starts the data section of the process at a per-MCM offset that is controlled by the data_stagger_interval option of the vmo command. The nth large-page data process executed on a given MCM has its data section start at offset (n * data_stagger_interval * PAGESIZE) % 16 MB. The DATA_START_STAGGER=Y option is only valid for 64-bit processes on a 64-bit kernel.

Specifying the LARGE_PAGE_TEXT=Y option indicates that the loader might attempt to use large pages for the text segment of the process. The LARGE_PAGE_TEXT=Y option is only valid for 64 bit processes on a 64 bit kernel.

Specifying the LARGE_PAGE_DATA=M option allocates only enough large pages for the data segment up to the brk value, rather than the entire segment, which is the behavior when you do not specify the LARGE_PAGE_DATA=M option. Changes to the brk value might fail if there are not enough large pages to support the change to the brk value.

Specifying the RESOLVEALL option forces the loader to resolve all undefined symbols that are imported at program load time or when the program loads the dynamic modules. Symbol resolution is performed in the standard AIX depth-first order. If you specify LDR_CNTRL=RESOLVEALL and the imported symbols cannot be resolved, the program or the dynamic modules fail to load.

Specifying the HUGE_EXEC option provides user control over the process address space location of the read-only segments for certain 32-bit executables. For more information see 32-bit Huge Executable.

Specifying the SHARED_SYMTAB=Y option causes the system to create a shared symbol table for a 64-bit program, if the program exports any symbols. If multiple instances of the program run concurrently, using a shared symbol table can reduce the amount of system memory that is required by the program.

Specifying the SHARED_SYMTAB=N option prevents the system from creating a shared symbol table for a 64-bit program. This option overrides the AOUT_SHR_SYMTAB flag in the XCOFF auxiliary header.

SED=system
SED=request
SED=exempt

echo $LDR_PRELOAD64

Resolves any symbols needed first from the libx.so shared object, then from the shr.o member of liby.a, and finally within the process' dependencies. All dynamically loaded modules (modules loaded with dlopen() or load()) will also be resolved first from the preloaded libraries listed by the variable.

Possible Value: True

Change takes effect immediately in this shell. Change is effective until logging out of this shell. Permanent change is made by adding NODISCLAIM=true command to the /etc/environment file.

Possible Values: bind, local, nis, bind4, bind6, local4, local6, nis4, or nis6

This is turned on internally, so the initial default value will not be seen with the echo command.

Change takes effect immediately in this shell. Change is effective until logging out of this shell. Permanent change is made by adding the NSORDER=value command to the /etc/environment file.

Possible Value: early

Change takes effect immediately in this shell. Change is effective until logging out of this shell.

Range: ON, OFF

Change takes effect immediately. Change is effective until next boot. Permanent change is made by adding the RT_GRQ={ON|OFF} command to the /etc/environment file.

Range: ON

Change takes effect immediately. Change is effective until next boot. Permanent change is made by adding the RT_MPC=ON command to the /etc/environment file.

Possible values: Olson time zone or POSIX time zone

Change takes effect immediately in the shell. Change is effective till you log out of the shell. Permanent change can be made by adding the TZ= value command to the /etc/environment file.

Possible Values: vmm_fork_policy, ESID_ALLOCATOR, SHM_1TB_SHARED, SHM_1TB_UNSHARED

Change takes effect immediately in this shell. Change is effective until you log out of this shell. Permanent change can be made by adding the VMM_CNTRL= environment variable to the /etc/environment file.

VMM_CNTRL=vmm_fork_policy=COW@SHM_1TB_SHARED=5

When you specify the vmm_fork_policy=COW option, the vmm uses the copy-on-write fork-tree policy whenever a process is forked. This is the default behavior. To prevent the vmm from using the copy-on-write policy, use the vmm_fork_policy=COR option. If the vmm_fork_policy option is specified, the global vmm_fork_policy tunable is ignored.

If ESID_ALLOCATOR option is specified, it controls the allocator from undirected shmat and mmap allocations. See 1 TB Segment Aliasing for detailed information.

If SHM_1TB_SHARED or SHM_1TB_UNSHARED is specified, it controls the use of 1 TB shared memory regions. See 1 TB Segment Aliasing for detailed information.

Possible values: Integer value that specifies the buffer size in bytes, KB, MB.

Changes take effect immediately in this shell. Change is effective until you log out of this shell. Permanent change to the buffer size can be made by adding the AIX_STDBUFSZ environment variable to the /etc/environment file.

Possible values: ON.

Changes take effect immediately in this shell. Change is effective until you log out of this shell. Permanent change to the system can be made by adding the AIX_LDSYM=ON environment variable to the /etc/environment file.

Use this parameter for applications which has 64 K text page size and needs source line information in its Lighweight_core file.