Session Management - Oracle 10g/11g Data and Database Management Utilities - page 173

Connecting users using services adds another identification dimension and allows
the DBA to selectively allocate resources based on services. This can be achieved
using Resource Manager.
Resource Manager
Resource Manager is a tool that provides the DBA more control over the resource
allocation; this circumvents problems with inefficient OS resource allocation. The
OS allocates CPU resources based on OS priorities, Oracle processes have the
same priority against the OS scheduler, so it doesn't matter if you launch a CPU
consuming task, this process may become a CPU hog. As a DBA there is nothing
to do from the OS side as it is not advisable to change the process priority of any
Oracle process.
Other problems that may be found at the OS level are:
• An excessive amount of context switching, resulting in an overhead when a
high number of OS processes is found.
• Ineficient scheduling from the OS side: It may reschedule an Oracle server
process while it holds latches, resulting in a reduction in the latch hit ratio.
• Inadequate resource allocation: This happens because from the OS point of
view all processes are the same, and processes consuming a high amount of
OS resources will be treated the same as any other process in the OS.
• The OS is not capable of controlling the degree of parallelism an Oracle process
demands, resulting in an unbound resource allocation from a few processes
that may create resource starvation for all the other concurrent process.
Resource Manager was created to address these issues, allowing the DBA to control
specific Oracle resources from inside the instance; such as the degree of parallelism, the
relative CPU consumption, the maximum amount of I/O, Undo resources, maximum
number of sessions allowed to share a given resource, and execution and idle time.