AIX Version 4.3 Differences Guide
AIX Version 4.3 Differences Guide
AIX Version 4.3 Differences Guide
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In summary, Table 25 shows the different policies used in various <strong>AIX</strong> versions.<br />
Table 25. Paging Space Allocation Policies<br />
PSALLOC = early<br />
All <strong>AIX</strong> versions<br />
Paging space is allocated when memory is requested<br />
PSALLOC is not set<br />
or set to any value<br />
other than early<br />
Pre-<strong>AIX</strong> <strong>4.3</strong>.2 <strong>AIX</strong> <strong>4.3</strong>.2<br />
Paging space is allocated when<br />
the page in memory is accessed<br />
Paging space is allocated when<br />
the page in memory needs to be<br />
paged out<br />
Paging space slots are only released by process (not thread) termination or by<br />
the disclaim system call. They are not released by the free call.<br />
6.13.1.1 Early Paging Allocation Mode Considerations<br />
If the PSALLOC environment variable is set to early, then every program started<br />
in that environment from that point on, but not including currently running<br />
processes, will run in the early allocation environment. The early allocation<br />
algorithm causes the appropriate number of paging space slots to be allocated at<br />
the time the virtual-memory address range is allocated. For example, with malloc.<br />
Interfaces, such as the malloc subroutine and the brk subroutine, will fail if<br />
sufficient paging space cannot be reserved when the request is made.<br />
The early allocation algorithm guarantees as much paging space as requested by<br />
a memory allocation request. Thus, proper paging space allocation on the system<br />
disk is important for efficient operations. When available paging space drops<br />
below a certain threshold, new processes cannot be started, and currently<br />
running processes may not be able to get more memory. Any processes running<br />
under the default late allocation mode become highly vulnerable to the SIGKILL<br />
signal mechanism. In addition, since the operating system kernel sometimes<br />
requires memory allocation, it is possible to crash the system by using up all<br />
available paging space.<br />
Before you use the early allocation mode throughout the system, it is very<br />
important to define an adequate amount of paging space for the system. The<br />
paging space required for early allocation mode will almost always be greater<br />
than the paging space required for the default late allocation mode. How much<br />
paging space to define depends on how your system is used and what programs<br />
you run. A good starting point for determining the right mix for your system is to<br />
define a paging space four times greater than the amount of physical memory.<br />
Certain applications can use extreme amounts of paging space if they are run in<br />
early allocation mode. The <strong>AIX</strong>windows server currently requires more than 250<br />
MB of paging space when the application runs in early allocation mode. The<br />
paging space required for any application depends on how the application is<br />
written and how it is run.<br />
6.13.1.2 Late Paging Allocation<br />
If the environment variable PSALLOC is not set, is set to null, or is set to any<br />
value other than early, the default late paging space allocation policy is used, and<br />
a disk block is allocated only when a page is initially used, not when a memory<br />
request is made.<br />
138 <strong>AIX</strong> <strong>Version</strong> <strong>4.3</strong> <strong>Differences</strong> <strong>Guide</strong>
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