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How does PRTG compute CPU Index, Traffic Index and Response Time Index?

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I need more information on how these graphs / indicators are computed, ideally on the basis of a specific example.

cpu-index response-time-index traffic-index

Created on Feb 2, 2010 4:11:45 PM by  Roland Grau [Paessler Support]

Last change on Jul 19, 2016 7:57:27 AM by  David Fabian [Paessler Support] (49) 3 1



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How does PRTG compute CPU Index, Traffic Index and Response Time Index?

In order to provide graphs that show a quick status overview of your complete network (or a part of it) PRTG computes so-called "Index" values based on the measurements of all sensors. The "Index" graphs are synthetic values ranging from 0% and 100% based on current sensor measurements and their historic peak values. The index calculation works similarly to a stock index which is the computed mean value of a stock selection.

For each group and device PRTG shows four values in a graph.

Screenshot

They are based on the measurements of all sensors in that group (or device). The "Alarms" graph simply shows the number of alarms at a given moment in history. Then there are three index values:

  • Response Time Index
  • CPU Load Index
  • Traffic Index

Examples

A CPU Load Index value of 10% for a group means that the average CPU load for all CPU sensors of this group currently is at 10% of the CPU peak (historic maximum) of that group. Usually, for response times the historic maximums are much higher than the average value. E.g. for pings in a LAN a "normal" time might be 2-10ms while a maximum of several hundred Milliseconds is not unusual. For this reason, most Response Time Index readings are usually between 10% and 20%.

The following two charts show the traffic index for two switches in 48 hour timespan:

Screenshot

The upper graph shows the traffic on one of our workstation switches. During the night the graph drops to a flat zero line - nobody was in the office. The lower graph shows the traffic on our core network switch. It has been busy all day and night because we run a lot of backup tasks during the night.

How does it work?

  • During the regular network monitoring process PRTG records the highest value ever measured for each sensor
  • The currently measured value is weighted against this upper liminal value to compute an index value for a sensor between 0% and 100%
  • For each device the index values for CPU load sensors, traffic sensors and sensors that measure response times are combined
  • For each group the index values of all devices are combined

Keep in mind that changing the sensor setup (i.e. the number and types of sensors) will also inevitably change the index values because the numerical base for the calculation changes. This means that values before and after a configuration change may not be comparable at all. The same applies to situations where one or more sensors can not be monitored (e.g. when a probe is disconnected).

Created on Feb 2, 2010 4:12:27 PM by  Roland Grau [Paessler Support]

Last change on Jul 19, 2016 8:16:23 AM by  David Fabian [Paessler Support] (49) 3 1



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Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.