Oracle Database Resource Manager (the Resource Manager) : 
enables you to optimize resource allocation among the many concurrent database sessions.

When database resource allocation decisions are left to the operating system, you may encounter the following problems:

  • Excessive overhead.

Excessive overhead results from operating system context switching between Oracle Database server processes when the number of server processes is high.

  • Inefficient scheduling

 The operating system deschedules database servers while they hold latches, which is inefficient.

  • Inappropriate allocation of resources

The operating system distributes resources equally among all active processes and is unable to prioritize one task over another.

  • Inability to manage database-specific resources, such as parallel execution servers and active sessions

Check The Resource Manager On your Database :

SQL> show parameter resource_manager_plan

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
resource_manager_plan                string      SCHEDULER[0x3003]:DEFAULT_MAIN

All this is Introduction about the Error that appear when you upgrade to 11g , you will getting the following messages in the alert.log.

Setting Resource Manager plan SCHEDULER[0x51B5]:DEFAULT_MAINTENANCE_PLAN via scheduler window
Setting Resource Manager plan DEFAULT_MAINTENANCE_PLAN via parameter
Thu Feb 05 22:00:03 2009
Begin automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"
Thu Feb 05 22:00:39 2009
End automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"

Solution to  Disable the resource manager , Steps to Do this :

1-set the current resource manager plan to null

alter system set resource_manager_plan='' scope=both 

2-change the active windows to use the null resource manager plan 

    execute dbms_scheduler.set_attribute('WEEKNIGHT_WINDOW','RESOURCE_PLAN','');
    execute dbms_scheduler.set_attribute('WEEKEND_WINDOW','RESOURCE_PLAN','');

3-run :

SQL> execute dbms_scheduler.set_attribute('','RESOURCE_PLAN','');

You Can Also Disable SQL tuning By :

client_name => 'sql tuning advisor',
operation => NULL,
window_name => NULL);

Thank you
Osama Mustafa