VMSTAT memory
The information under the memory heading provides information about real
and virtual memory.
avm
The avm column gives the average number of pages allocated to paging space.
(In AIX, a page contains 4096 bytes of data.) When a process executes,
space for working storage is allocated on the paging devices (backing store).
This can be used to calculate the amount of paging space assigned to executing
processes. The number in the avm field divided by 256 will yield the number
of megabytes (MB), systemwide, allocated to page space.
The lsps -a command also provides information on individual paging space.
It is recommended that enough paging space be configured on the system
so that the paging space used does not approach 100 percent. When
fewer than 128 unallocated pages remain on the paging devices, the
system will begin to kill processes to free some paging space.
Versions of AIX before 4.3.2 allocated paging space blocks for pages
of memory as the pages were accessed. On a large memory machine,
where the application set is such that paging is never or rarely
required, these paging space blocks were allocated but never needed.
AIX Version 4.3.2 implements deferred paging space allocation, in
which the paging space blocks are not allocated until paging is necessary,
thus, helping reduce the paging space requirements of the system.
The avm value in vmstat indicates the number of virtual memory
(working storage) pages that have been accessed but not necessarily
paged out. With the previous policy of "late page space allocation",
avm had the same definition. However, since the VMM
would allocate paging space disk blocks for each working page that
was accessed, the paging space blocks was equal to the avm. The reason
for the paging space blocks to be allocated at the time the working pages
are accessed is so that if the pages had to be paged out of memory,
there would be disk blocks on the page space lv's available for the
in-memory pages to go. On systems that never page-out to page-space,
it's a waste of disk space to have as many page space disk blocks as
there is memory. With deferred policy, the page space disk blocks
are only allocated for the pages that do need to be paged out.
The avm number will grow as more processes get started and/or
existing processes use more working storage. Likewise, the
number will shrink as processes exit and/or free working storage.
fre
The fre column shows the average number of free memory frames.
A frame is a 4096-byte area of real memory.
The system maintains a buffer of memory frames, called the free list,
that will be readily accessible when the VMM needs space. The nominal
size of the free list varies depending on the amount of real memory installed.
On systems with 64MB of memory or more, the minimum value (MINFREE)
is 120 frames. For systems with less than 64MB, the value is two
times the number of MB of real memory, minus 8. For example, a system
with 32MB would have a MINFREE value of 56 free frames.
If the fre value is substantially above the MAXFREE value (which is
defined as MINFREE plus 8), then it is unlikely that the system is thrashing
(continuously paging in and out). However, if the system is thrashing, be
assured that the fre value is small. Most UNIX and AIX operating
systems will use nearly all available memory for disk caching, so
you need not be alarmed if the fre value oscillates between MINFREE
and MAXFREE.
