Repeating the tuning steps

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A truism of performance analysis is that there is always a next bottleneck. Reducing the use of one resource means that another resource limits throughput or response time.

Suppose, for example, we have a system in which the utilization levels are as follows:

CPU: 90% Disk: 70% Memory 60%

This workload is CPU-bound. If we successfully tune the workload so that the CPU load is reduced from 90 to 45 percent, we might expect a two-fold improvement in performance. Unfortunately, the workload is now I/O-limited, with utilizations of approximately the following:

CPU: 45% Disk: 90% Memory 60%

The improved CPU utilization allows the programs to submit disk requests sooner, but then we hit the limit imposed by the disk drive's capacity. The performance improvement is perhaps 30 percent instead of the 100 percent we had envisioned.

There is always a new critical resource. The important question is whether we have met the performance objectives with the resources at hand.

Attention: Improper system tuning with the vmo, ioo, schedo, no, and nfso tuning commands might result in unexpected system behavior like degraded system or application performance, or a system hang. Changes should only be applied when a bottleneck has been identified by performance analysis.

Note: There is no such thing as a general recommendation for performance dependent tuning settings.