Monday, April 1, 2013

AIX memory usage/shortage determination.


Vmstat tool in AIX has PO and PI columns. PO and PI columns means swapout and swapin . Values of these columns should be zero, or near zero  most of the time. If nonzero values are displayed , there might be a memory bottleneck on the system. Note that,  Nonzero values does not always indicate a problem ,as pagein and pageout is the main principle of the virtual memory, and Virtual memory comes by design of the operating system.

Here is the detailed information from IBM;

Memory usage determination with the vmstat command
The vmstat command summarizes the total active virtual memory used by all of the processes in the system, as well as the number of real-memory page frames on the free list.

Active virtual memory is defined as the number of virtual-memory working segment pages that have actually been touched. For a more detailed definition, see Late page space allocation. This number can be larger than the number of real page frames in the machine, because some of the active virtual-memory pages may have been written out to paging space.

When determining if a system might be short on memory or if some memory tuning needs to be done,
run the vmstat command over a set interval and examine the pi and po columns on the resulting report.
These columns indicate the number of paging space page-ins per second and the number of paging space page-outs per second. If the values are constantly non-zero, there might be a memory bottleneck.
Having occasional non-zero values is not be a concern because paging is the main principle of virtual memory.
PI COLUMN:
The pi column details the number of pages paged in from paging space. Paging space is the part of virtual memory that resides on disk. It is used as an overflow when memory is over committed. Paging space consists of logical volumes dedicated to the storage of working set pages that have been stolen from real memory. When a stolen page is referenced by the process, a page fault occurs, and the page must be read into memory from paging space.
Due to the variety of configurations of hardware, software and applications, there is no absolute number to look out for. This field is important as a key indicator of paging-space activity. If a page-in occurs, there must have been a previous page-out for that page. It is also likely in a memory-constrained environment that each page-in will force a different page to be stolen and, therefore, paged out.

PO COLUMN:
The po column shows the number (rate) of pages paged out to paging space. Whenever a page of working storage is stolen, it is written to paging space,
if it does not yet reside in paging space or if it was modified. If not referenced again, it will remain on the paging device until the process terminates
or disclaims the space. Subsequent references to addresses contained within the faulted-out pages results in page faults, and the pages are paged in individually
by the system. When a process terminates normally, any paging space allocated to that process is freed. If the system is reading in a significant number of persistent pages, you might see an increase in po without corresponding increases in pi. This does not necessarily indicate thrashing, but may warrant investigation into data-access patterns of the applications.


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