Intel AS/400 RISC Server, 170 Servers, 7xx Servers manual Interactive Utilization, Example

and higher than normal CFINT values. The goal is to avoid exceeding the threshold (knee of the curve) value of interactive capacity.

2.8 Interactive Utilization

When the interactive CPW utilization is beyond the knee of the curve, the following formulas can be used to determine the effective interactive utilization or the available/remaining client/server CPW. These equations apply to all server models.

CPWcs(maximum) = client/server CPW maximum value

CPWint(maximum) = interactive CPW maximum value

CPWint(knee)	= interactive CPW at the knee of the curve
CPWint	= interactive CPW of the workload

X is the ratio that says how far into the overhead zone the workload has extended:

X = (CPWint - CPWint(knee)) / (CPWint(maximum) - CPWint(knee))

EIU = Effective interactive utilization. In other words, the free running, CPWint(knee), interactive plus the combination of interactive and overhead generated by X.

EIU = CPWint(knee) + (X * (CPWcs(maximum) - CPWint(knee)))

CPW remaining for batch = CPWcs(maximum) - EIU

Example 1:

A model 7xx server has a Processor CPW of 240 and an Interactive CPW of 70. The interactive CPU percent at the knee equals (70 CPW / 240 CPW) or 29.2%.

The maximum interactive CPU percent (7/6 of the Interactive CPW ) equals (81.7 CPW / 240 CPW) or 34%.

Now if the interactive CPU is held to less than 29.2% CPU (the knee), then the CPU available for the System, Batch, and Client/Server work is 100% - the Interactive CPU used.

If the interactive CPU is allowed to grow above the knee, say for example 32.1 % (110% of the knee), then the CPU percent remaining for the Batch and System is calculated using the formulas above:

X = (32.1 - 29.2) / (34 - 29.2) = .604

EIU = 29.2 + (.604 * (100 - 29.2)) = 71.9%

CPW remaining for batch = 100 - 71.9 = 28.1%

Note that a swing of + or - 1% interactive CPU yields a swing of effective interactive utilization (EIU) from 57% to 87%. Also note that on custom servers and 7xx models, environments that go beyond the interactive knee may experience erratic behavior.

Example 2:

A Server Model has a Client/Server CPW of 450 and an Interactive CPW of 50. The maximum interactive CPU percent equals (50 CPW / 450 CPW) or 11%.

The interactive CPU percent at the knee is 1/3 the maximum interactive value. This would equal 4%.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
© Copyright IBM Corp. 2008	Chapter 2 - Server Performance Behavior	25