New performance-tuning features of Oracle Database 12c Release 188.8.131.52: Part 1
Originally published by TriCore: April 11, 2017
This two-part blog post series covers new performance-tuning features of Oracle® Database versions 184.108.40.206 and 220.127.116.11. Part 1 discusses the earlier version.
New features in Oracle Database 12.1.01
The following features are new in Oracle Database version 18.104.22.168:
- 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 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:
Every three seconds, the manageability monitor (MMON) process performs an action to obtain performance statistics without lock or latch.
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.
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_REPORTSview and the
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
|High load||Average active sessions are greater than 3x the number of CPU cores|
|I/O bound||I/O impact on active sessions is based on single block read performance|
|CPU bound||Active sessions are greater than 10% of total load and CPU utilization is greater than 50%|
|Over-allocated memory||Memory allocations are over 95% of physical memory|
|Interconnect bound||Based on single block interconnect transfer time|
|Session limit||Session limit is close to 100%|
|Process limit||Process limit is close to 100%|
|Hung session||Hung sessions are greater than 10% of total sessions|
|Deadlock detected||Any deadlock is detected|
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
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
(22.214.171.124) 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
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
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
PGA_AGGREGATE_LIMIT initialization parameter to a new value in
number of bytes. Setting the value to
0 disables the hard limit on PGA
PGA_AGGREGATE_LIMIT is exceeded, Oracle Database performs the
- 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.
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
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 126.96.36.199.
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The following sources were used as references for this blog post: