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
SIGSEGV
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. Raising the amount of
virtual address space reserved for the stack segment implies
lowering the amount of virtual 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.
When Should the Value of This Tunable Be Lowered?
This tunable should be lowered if swap space is at a premium
on the system, and several poorly written programs, or malicious
programs, are using swap space to the detriment of more critical
user processes.
For example, several students at a university might execute less than
production level code and leak memory all over the place.
What Are the Side Effects of Lowering the Value?
Lowering this tunable will limit the amount of memory available for
stack usage per process.
This could cause processes with large stack requirements to terminate
with a
SIGSEGV
error.
What Other Tunable Values Should Be Changed at the Same Time?
maxdsiz
should be considered because it performs a similar function of limiting
swap usage by process data segment.
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