Allocation and reclamation of paging space slots

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The operating system supports three allocation methods for working storage.

The three allocation methods for working storage, also referred to as paging-space slots, are as follows:

  • Late allocation
  • Early allocation
  • Deferred allocation
Note: Paging-space slots are only released by process (not thread) termination or by the disclaim() system call. The slots are not released by the free() system call

Late allocation algorithm

Many programs exploit late allocation by allocating virtual-memory address ranges for maximum-sized structures and then only using as much of the structure as the situation requires. The pages of the virtual-memory address range that are never accessed never require real-memory frames or paging-space slots.

This technique does involve some degree of risk. If all of the programs running in a machine happened to encounter maximum-size situations simultaneously, paging space might be exhausted. Some programs might not be able to continue to completion.

Early allocation algorithm

The second operating system's paging-space-slot-allocation method is intended for use in installations where this situation is likely, or where the cost of failure to complete is intolerably high. Aptly called early allocation, this algorithm causes the appropriate number of paging-space slots to be allocated at the time the virtual-memory address range is allocated, for example, with the malloc() subroutine. If there are not enough paging-space slots to support the malloc() subroutine, an error code is set. The early-allocation algorithm is invoked as follows:

# export PSALLOC=early

This example causes all future programs to be executed in the environment to use early allocation. The currently executing shell is not affected.

Early allocation is of interest to the performance analyst mainly because of its paging-space size implications. If early allocation is turned on for those programs, paging-space requirements can increase many times. Whereas the normal recommendation for paging-space size is at least twice the size of the system's real memory, the recommendation for systems that use PSALLOC=early is at least four times the real memory size. Actually, this is just a starting point. Analyze the virtual storage requirements of your workload and allocate paging spaces to accommodate them. As an example, at one time, the AIXwindows server required 250 MB of paging space when run with early allocation.

When using PSALLOC=early, the user should set a handler for the following SIGSEGV signal by pre-allocating and setting the memory as a stack using the sigaltstack function. Even though PSALLOC=early is specified, when there is not enough paging space and a program attempts to expand the stack, the program may receive the SIGSEGV signal.

Deferred allocation algorithm

The paging-space-slot-allocation method of the third operating system is the default behavior. Deferred Page Space Allocation (DPSA) policy delays allocation of paging space until it is necessary to page out the page, which results in no wastage of the paging space allocation. This method saves huge amounts of paging space that is the disk space.

On some systems, paging space might not ever be needed even if all the pages accessed have been touched. This situation is most common on systems with very large amount of RAM. However, this may result in overcommitment of paging space in cases where more virtual memory than available RAM is accessed.

To disable DPSA and preserve the Late Page Space Allocation policy, run the following command:

# vmo -o defps=0

To activate DPSA, run the following command:

# vmo -o defps=1

In general, system performance can be improved by DPSA, because the overhead of allocating page space after page faults is avoided the. Paging space devices need less disk space if DPSA is used.

For further information, see Page space allocation and Paging spaces placement and sizes.