Comparisons of deferred and early paging space allocation

The operating system uses the PSALLOC environment variable to determine the mechanism used for memory and paging space allocation.

If the PSALLOC environment variable is not set, is set to null, or is set to any value other than early, the system uses the default deferred allocation algorithm.

The deferred allocation algorithm helps the efficient use of disk resources and supports applications that prefer a sparse allocation algorithm for resource management. This algorithm does not reserve paging space when a memory request is made; the disk block allocation of paging space is delayed until it is necessary to page out the requested page. Some programs allocate large amounts of virtual memory and then use only a fraction of the memory. Examples of such programs are technical applications that use sparse vectors or matrices as data structures. The deferred allocation algorithm is also more efficient for a real-time, demand-paged kernel such as the one in the operating system.

This paging space might never be used, especially on systems with large real memory where paging is rare. The deferred algorithm delays allocation of paging space until it is necessary to page out the requested page, which results in no wasted paging space allocation. This delayed allocation can result in a case where the deferred algorithm can try to allocate more paging space than the space available to the system. This situation is called an over-commitment of paging space.

In the over-commitment scenario, where paging space becomes exhausted and an attempt is made to allocate a paging space disk block to page out a page, a failure results. The operating system attempts to avoid complete system failure by killing processes affected by the paging space over-commitment. The SIGDANGER signal is sent to notify processes that the amount of free paging space is low. If the paging space situation reaches an even more critical state, selected processes that did not receive the SIGDANGER signal are sent a SIGKILL signal.

You can use the PSALLOC environment variable to switch to an early allocation algorithm, which allocates paging space for the executing process at the time the memory is requested. If there is insufficient paging space available at the time of the request, the early allocation mechanism fails the memory request.

If the PSALLOC environment variable is set to early, then every program started in that environment from that point on, but not including currently running processes, runs in the early allocation environment. In the early allocation environment, interfaces such as the malloc subroutine and the brk subroutine will fail if sufficient paging space cannot be reserved when the request is made.

Processes run in the early allocation environment mode are not sent the SIGKILL signal if a low paging space condition occurs.

There are different ways to change the PSALLOC environment variable to early, depending on how broadly you want to apply the change.

The following memory allocation interface subroutines are affected by a switch to an early allocation environment:

• malloc
• free
• calloc
• realloc
• brk
• sbrk
• shmget
• shmctl