AIX Tip of the Week

AIX Tip of the Week: Why AIX Memory Typically Runs Near 100% Utilization

Audience: AIX Administrators

Date: December 1998

Memory utilization on AIX systems typically runs around 100%. This is often a source of concern. However, high memory utilization in AIX does not imply the system is out of memory. By design, AIX leaves files it has accessed in memory. This significantly improves performance when AIX reaccesses these files because they can be reread directly from memory, not disk*. When AIX needs memory, it discards files using a "least used" algorithm. This generates no I/O and has almost no performance impact under normal circumstances.

Sustained paging activity is the best indication of low memory. Paging activity can be monitored using the "vmstat" command. If the "page-in" (PI) and "page-out" (PO) columns show non-zero values over "long" periods of time, then the system is short on memory. (All systems will show occasional paging, which is not a concern.)

Memory requirements for applications can be empirically determined using the AIX "rmss"command. The "rmss" command is a test tool that dynamically reduces usable memory. The onset of paging indicates an application's minimum memory requirement.

Finally, the "svmon" command can be used to list how much memory is used each process. The interpretation of the svmon output requires some expertise. See the AIX documentation for details.


To test the performance gain of leaving a file in memory, a 40MB file was read twice. The first read was from disk, the second was from memory. The first read took 10.0 seconds. The second read took 1.3 second: a 7.4x improvement.