HP-UX Reference (11i v2 07/12) - 5 Miscellaneous (vol 9)

m
maxssiz(5) maxssiz(5)
(Tunable Kernel Parameters)
NAME
maxssiz, maxssiz_64bit - maximum size (in bytes) of the stack for any user process
VALUES
Default
32 bit default:
0x800000 (8MB)
64 bit default: 0x10000000 (256MB)
Allowed values
32 bit minimum:
0x40000
32 bit maximum: 0x17F00000
64 bit minimum: 0x40000
64 bit maximum: 0x80000000
DESCRIPTION
User programs on HP-UX systems are composed of five discrete segments of virtual memory: text (or code),
data, stack, shared, and I/O. Each segment occupies an architecturally defined range of the virtual address
space which sets the upper limit to their size. However, text, data and stack segments may have a smaller
maximum enforced via the maxtsiz, maxdsiz and maxssiz tunables.
maxssiz and maxssiz_64bit define the maximum size of the stack segment for 32-bit and 64-bit
processes. The stack segment contains the actual program stack and the storage space for registers on a
process or thread context switch.
Who is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect only for processes started after the modification. In addition, a process
which modifies it’s
rlimit for the stack segment propagates the modified limit to all child processes,
thereby exempting them from any future modification of maxssiz.
When Should the Value of This Tunable Be Raised?
maxssiz should be raised if user processes are generating the console error message:
Warning: maxssiz value too small.
Processes generating this error message will likely terminate with the segmentation violation error [SIG-
SEGV] and dump core.
What Are the Side Effects of Raising the Value?
Raising this tunable by definition allows larger stack segments for every process. This means that
maxdsiz and maxssiz function as limitations on the amount of swap space which can be reserved or
used by each process. Therefore, using more virtual address space does not translate directly to using more
physical address space because virtual pages can be swapped out.
If swap space on the machine is near capacity, raising this tunable will increase the amount of swap that is
reservable per process. This would allow a process with a memory leak, or written to use a great deal of
memory, to reserve more swap, and possibly exhaust the swap space on the system.
It is also important to realize that for 32 bit user processes, data and stack are located contiguously. Rais-
ing the amount of virtual address space reserved for the stack segment implies lowering the amount of vir-
tual address space for the data segment. In other words, raising maxssiz may cause user processes
which use all (or nearly all) of the previously available data area to fail allocation with the [ENOMEM]
error, even with maxdsiz set above the current amount of memory allocated for data by this process.
On Itanium-based systems, raising the value of maxssiz_64bit will cause an increase in the kernel
data structures used to represent the larger stack space. This may use enough additional swap space that
the user may see performance degradation or application failure due to lack of reservable swap space.
One method to minimize impact is to use a script which launches the applications needing maximum stack
size. Within the script, raise the value of
maxssiz_64bit, launch the application, and then lower
maxssiz_64bit to its previous value. This will allow the specific application to benefit from the
increased stack size but will not cause additional stack growth for applications which do not need it.
234 Hewlett-Packard Company − 1 − HP-UX 11i Version 2: December 2007 Update