Oracle Instance Concepts - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 109

Things to Remember Before Moving Ahead
1) Figure 4-6: Memory architecture of Oracle Instance. It's
the area of the computer memory (RAM) occupied by
Oracle Instance for requests processing. Every software
that you run under any OS occupies RAM (or Memory),
so does Oracle. SGA is the most important piece and it
stands for System Global Area. Non-shared memory is
not shared among all the users whereas shared is shared
among all. Database Buffer Cache has three sub-caches:
KEEP, RECYCLE, DEFAULT. Three different states of
buffers within Database Buffer Cache: Dirty Buffers,
Free Buffers, Pinned Buffers. Non-Shared part of Oracle
memory architecture is also called PGA or Program
Global Area. The utilization of PGA depends how you
configure the Oracle Server. Unlike SGA where RAM is
shared by all the background process, PGA is utilized by
server process for sorting, session information and as
stack space. Oracle Server can be configured to run as
Dedicated Server mode or Shared Server mode.
2) Figure 4-7: Background processes of Oracle Instance.
Oracle Instance running under Shared Server Mode is
shown. RECO, LGWR, SMON, PMON, ARCn etc. are all
background processes. SGA is the shared part of
memory architecture of Oracle. Oracle can be
configured to run as dedicated server mode or shared
server mode.
3) Figure 4-8: Background processes of Oracle Instance.
Oracle Instance running under Dedicated Server Mode
is shown. There would not be any Request and Response
Queues in a dedicated server configuration. There would
not be any dispatcher process in dedicated server mode.
For each connected user then there would one dedicated
server process. The idle time of server will be more but
this configuration has the highest performance factor.