Reporting Discrepancies Between DB2 PM and Detector

Products

• Database Management for DB2 for z/OS - Performance Suite
• Database Management for DB2 for z/OS - SQL Performance Suite
• Insight Performance Monitor for DB2 UDB for z/OS
• Detector for DB2 for z/OS

Issue/Introduction

There is a significant difference in CPU reporting between DB2 PM and Detector.

 

 

Environment

Release : 20.0

Component : CA Detector for DB2 for z/OS

Resolution

It is expected that the DB2 PM values for INDB2_CPU and INDB2_TIME would be higher than Detector values. Detector sources most of its statisctics the same as DB2 PM and other monitors, however not INDB2_CPU and INDB2_TIME. For those Detector calculates its own values. So for Detector, thse values include only the cost of executing the SQL-and doesn't include Detector overhead and maybe a few other elements of the overall package CPU TCB time. At this point the Detector's highest overhead occurs on INSERT statements(and cursors with FETCH only one row), and the FBCRP90 package does a significantly high portion of INSERTs.

Detector only captures INDB2_CPU for SQL statements so there are other activities which contribute to the Class 7 CPU, which Detector doesn't account for (Plan/Package allocate, thread termination processing, Syncpoints etc...) as well as Detector overhead - which likely accounts for the majority of the discrepancy.