New performance-tuning features of Oracle Database 12c Release 12.1.0.0: Part 1

Originally published by TriCore: April 11, 2017

This two-part blog post series covers new performance-tuning features of Oracle® Database versions 12.1.0.1 and 12.1.0.2. Part 1 discusses the earlier version.

New features in Oracle Database 12.1.01

The following features are new in Oracle Database version 12.1.0.1:

• Real-Time Automatic Database Diagnostic Monitor (ADDM)
• Support for limiting the size of the Program Global Area (PGA)
• Active Session History (ASH) enhancements

Real-Time ADDM

Real-Time ADDM helps you analyze and resolve problems in a hung database without having to restart the database. This section describes Real-Time ADDM in detail.

Real-Time ADDM connection modes

Real-Time ADDM uses one of two connection modes to connect to a database through Oracle Enterprise Manager (OEM). The mode it uses depends on the database state:

• Normal connection: In this mode, Real-Time ADDM uses Java Database Connectivity (JDBC) to connect to the database. This mode conducts an extensive performance analysis of the database when connectivity is available.

• Diagnostic connection: In this mode, Real-Time ADDM performs a latch-less connection to the database. This mode is useful for extreme hang situations when a normal JDBC connection isn’t possible.

Real-Time ADDM Triggers

Real-Time ADDM runs automatically every three seconds and uses in-memory data to diagnose database performance issues. It automatically triggers an analysis when it detects a performance problem. This functionality involves the following steps:

1. Every three seconds, the manageability monitor (MMON) process performs an action to obtain performance statistics without lock or latch.

2. The MMON process checks these statistics and triggers a Real-Time ADDM analysis if it finds any of the issues that appear in Table 1.

3. The MMON slave process creates the report and stores it in the Automatic Workload Repository (AWR). For more information, you can check the DBA_HIST_REPORTS view and the DBA_HIST_REPORTS_DETAILS view.

You can also use the following command to generate a report manually:

SQL> select dbms_addm.real_time_addm_report() from dual;

Table 1: Performance issues and conditions that trigger a Real-Time ADDM analysis

Source: Database Performance Tuning Guide, Chapter 7: Automatic Performance Diagnostics: Real-Time ADDM Connection Modes

Real-Time ADDM trigger controls

To ensure that the automatic triggers don’t consume too many system resources, Real-Time ADDM uses the following controls:

• Duration between reports: If an automatic trigger created a Real-Time ADDM report in the past five minutes, then no new reports are generated.

• Oracle Real Application Clusters (RAC) control: Automatic triggers are local to the database instance. For Oracle RAC, only one database instance can create a Real-Time ADDM report at a time.

• Repeated triggers: An automatic trigger for any issue must have an impact of 100% or higher than the previous report that had the same triggering issue within the previous 45 minutes.

• Newly identified issues: If a new issue is detected that wasn’t already detected within the past 45 minutes, then a new report is generated.

Limiting the size of the PGA

Excessive PGA usage can lead to high rates of swapping. When this occurs, the system might become unresponsive and unstable. If this happens, consider using the PGA_AGGREGATE_LIMIT initialization parameter to limit overall PGA usage.

The following section describes how to limit the size of the PGA by using the PGA_AGGREGATE_LIMIT initialization parameter in Oracle Database 12c R1 (12.1.0.1) and higher.

About the PGA_AGGREGATE_LIMIT parameter

If the value defined in the PGA_AGGREGATE_LIMIT parameter is exceeded, Oracle Database aborts or terminates the sessions or processes that are consuming the most untunable PGA memory. Oracle Database performs the termination in the following order:

• Calls for sessions that are consuming the most untunable PGA memory and aborts them.
• If PGA memory usage is still over the PGA_AGGREGATE_LIMIT, then Oracle Database terminates the sessions and processes that are consuming the most untunable PGA memory.

Oracle Database treats parallel queries as a single unit. By default, the PGA_AGGREGATE_LIMIT parameter is set to the greater of 2 GB, 200% of the PGA_AGGREGATE_TARGET value, or 3 MB times the value of the PROCESSES parameter. However, it does not exceed 120% of the physical memory size minus the total System Global Area (SGA) size.

Setting the PGA_AGGREGATE_LIMIT parameter

The PGA_AGGREGATE_LIMIT initialization parameter can be set dynamically. The database doesn’t need to restart. You can set the value of PGA_AGGREGATE_LIMIT regardless of whether Oracle Database is using automatic memory management.

Changing the PGA_AGGREGATE_LIMIT parameter

Set the PGA_AGGREGATE_LIMIT initialization parameter to a new value in number of bytes. Setting the value to 0 disables the hard limit on PGA memory.

When the PGA_AGGREGATE_LIMIT is exceeded, Oracle Database performs the following actions:

• Aborts the calls that are associated with the sessions that are using the most untunable memory.
• If the total PGA memory usage is still over the limit, terminates the sessions that are using the most untunable memory.

sys.processes and background processes other than job queue processes aren’t affected. Instead, if they’re using the most untunable memory, they periodically write a brief summary of their PGA usage to a trace file.

ASH enhancements

As of Oracle 12c, you can access ASH data visualization through a new OEM page named ASH Analytics. This page enables you to drill down into logical dimensions. You can also send the reports to other users who can view them offline.

Image source: OCP 12C – Emergency Monitoring, Real-Time ADDM

Conclusion

These new features collectively help you troubleshoot and enhance database performance for Oracle Database 12c Release 1. Part 2 of this series covers more new features and changes to performance tuning in Oracle Database 12.1.0.2.

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References

The following sources were used as references for this blog post:

• Database Performance Tuning Guide, Chapter 7: Automatic Performance Diagnostics: Real-Time ADDM Connection Modes

• Database Performance Tuning Guide, Chapter 7: Automatic Performance Diagnostics: ADDM Analysis Results