maxssiz(5)

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

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:

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.

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.

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.