PMI data classification

This topic describes the Performance Monitoring Infrastructure (PMI) data classification.

PMI provides server-side data collection and client-side API to retrieve performance data. Performance data has two components: static and dynamic.

The static component consists of a name, ID and other descriptive attributes to identify the data. The dynamic component contains information that changes over time, such as the current value of a counter and the time stamp associated with that value.

The PMI data can be one of the following statistic types (these statistic types follow the J2EE 1.4 Performance Data Framework):

Statistic type	Description	Example
CountStatistic	Represents a running count of a given value.	Number of Servlet requests
AverageStatistic	Represents a simple average. Keeps track of total, count, min, and max. The average can be derived by total and count. (This type is WebSphere extension to J2EE Performance Data Framework)	Average HttpSession size in bytes.
TimeStatistic	Same as AverageStatistic, except that the unit of measure is milliseconds or seconds.	Average Servlet response time.
RangeStatistic	Represents a time-weighted average. Keeps track of current, low water mark, high water mark, time-weight total, and integral.	Number of concurrent Servlet requests.
BoundedRangeStatistic	Same as RangeStatistic, with lower bound and upper bound.	JDBC connection pool size.

The following diagram shows the statistic class hierarchy:

Statistic

ID

A unique ID that identifies the Statistic within the given Stats (WebSphere PMI extension)

name

Statistic name

unit

Unit of measurement for the statistic

description

Textual description of the statistic

startTime

Time the first measurement was taken

lastSampleTime

Time the most recent measurement was taken

CountStatistic

count

Count since the measurement started

DoubleStatistic

count

Value since the measurement started

AverageStatistic

(WebSphere PMI extension. This is the same as the TimeStatistic defined in J2EE 1.4, except that it is used to track non-time-related measurements like byte size, etc.)

count

Number of measurements

total

Sum of the values of all the measurements

min

Minimum value

max

Maximum value

BoundaryStatistic

upperBound

Upper limit of this attribute

lowerBound

Lower limit of this attribute

RangeStatistic

current

Current value of this attribute

lowWaterMark

Lowest value of this attribute

upperWaterMark

Highest value of this attribute

integral

Time-weighted sum of this attribute [time-weighted average = integral / (lastSampleTime - startTime] (WebSphere PMI extension)

• Numeric: Maps to CountStatistic in the J2EE 1.4 specification. Holds a single numeric value that can either be a long or a double. This data type is used to keep track of simple numeric data, such as counts.

• Stat: Holds statistical data on a sample space, including the number of elements in the sample set, their sum, and sum of squares. You can obtain the mean, variance, and standard deviation of the mean from this data.

• Load: Maps to the RangeStatistic or BoundedRangeStatistic, based on J2EE 1.4 specification. This data type keeps track of a level as a function of time, including the current level, the time that level was reached, and the integral of that level over time. From this data, you can obtain the time-weighted average of that level. For example, this data type is used in the number of active threads and the number of waiters in a queue.

These PMI data types continue to be supported through the PMI client API. Statistical data types are supported through both the PMI API and Java Management Extension (JMX) API.

In WebSphere Application Server, Version 4 and Version 5, CountStatistic data require a low monitoring level, and TimeStatistic data require a medium monitoring level. RangeStatistic and BoundedRangeStatistic require a high monitoring level. There are a few counters that are exceptions to this rule. The average method response time, the total method calls, and active methods counters require a high monitoring level. The Java virtual machine Profiler Interface (JVMPI) counters, SerializableSessObjSize, and data tracked for each individual method (method level data) require a maximum monitoring level. Also, the level maximum enables synchronized update to all the statistic types.

WebSphere Application Server, V6 deprecates the monitoring levels (Low, Medium, High, and Max) and introduces fine-grained control to enable/disable statistics individually. The fine-grained control is available under the custom option. Refer to Enabling PMI using the administrative console for more details.

In order to reduce the monitoring overhead, updates to CountStatistic, DoubleStatistic, AverageStatistic, and TimeStatistic are not synchronized. Since this data tracks the total and average, the extra accuracy is generally not worth the performance cost. RangeStatistic and BoundedRangeStatistic are very sensitive; therefore, they are always synchronized. To enable synchronized updates for all the statistic types enable the 'Use sequential update' option. Refer to Enabling PMI using the administrative console for details.

Related concepts

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