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Methodology

Since the focus of this book is using the Wait Interface and Wait Events to resolve performance problems, it is important to review the methodology that ADDM uses for identifying resolutions to problems.  Looking at the ADDM information for tuning allows it to be viewed from several different perspectives:

  • Top SQL statements

  • Top sessions

  • CPU/Wait times

While there may be times when other perspectives have merit, the focus will be on the CPU and wait time approach.  Wait information is grouped by wait classes.  These classes are groupings of waits by areas of commonality.  For example, db file scattered read and db file sequential read are both part of the I/O class. 

The goal of the analysis that ADDM performs is to minimize the metric called DB time.  DB time is the total amount of time spent by the database server on user requests.  It includes wait time and CPU time of all non-idle user sessions.  DB time is displayed in the v$sess_time_model and v$sys_time_model views containing data for individual sessions and system rollup numbers respectively.

This metric is cumulative, that is it continues to grow as long as the session is connected for v$sess_time_model; or as long as the instance has been running for v$sys_time_model.  This means that a particular value for DB time is irrelevant unless used in comparison to an earlier or later value.

Components

ADDM gathers data and issues alerts about performance issues.  The following sections introduce several components that make use of the data gathered by the AWR.

Data Dictionary Objects

The data dictionary objects will be examined first.  They will be briefly introduced with regard to their purpose, and then their use will be demonstrated. 

The dba_advisor_objects view displays information about all database objects currently referenced by the advisors.  Each row in this view pertains to a specific advisor tuning object.  Figure 10.1 shows the structure of this view.


The above book excerpt is from:

Oracle Wait Event Tuning

High Performance with Wait Event Iinterface Analysis 

ISBN 0-9745993-7-9  

Stephen Andert 

http://www.rampant-books.com/book_2004_2_wait_tuning.htm

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