Oracle SGA - System Global Area

• SGA is dynamic and sized using SIZE_MAX_SIZE
•  SGA memory allocated and tracked in granules by SGA components
  – Contiguous virtual memory allocation
   – Size based on SGA_MAX_SIZE                                                                                                 
  
  
  The SGA consists of several memory structures
  – Shared pool
  – Database buffer cache
  – Redo log buffer

There are two optional memory structures that can be configured within the SGA:
– Large pool
– Java pool

Shared Pool
The shared pool environment contains both fixed and variable structures. The fixed structures remain relatively the same size, whereas the variable structures grow and shrink based on user and program requirements. The actual sizing for the fixed and variable structures is based on an initialization parameter and the work of an Oracle internal algorithm.
It consists of two key performance-related memory structures:
– Library cache
– Data dictionary cache

  a) Library Cache
The library cache size is based on the sizing defined for the shared pool. Memory is allocated when a statement is parsed or a program unit is called. If the size of the shared pool is too small, statements are continually reloaded into the library cache, which affects performance.
The library cache is managed by a least recently used (LRU) algorithm. As the cache fills, less recently used execution paths and parse trees are removed from the library cache to make room for the new entries. If the SQL or PL/SQL statements are not reused, they eventually are aged out.

The library cache consists of two structures:
• Shared SQL: The Shared SQL stores and shares the execution plan and parse tree for SQL statements run against the database. The second time that an identical SQL statement is run, it is able to take advantage of the parse information available in the shared SQL to expedite its execution. To ensure that SQL statements use a shared SQL area whenever possible, the text, schema, and bind variables must be exactly the same.
• Shared PL/SQL: The shared PL/SQL area stores and shares the most recently executed
PL/SQL statements. Parsed and compiled program units and procedures (functions,
packages, and triggers) are stored in this area.

b) Data Dictionary Cache
The data dictionary cache is also referred to as the dictionary cache or row cache. Caching data dictionary information into memory improves response time. Information about the database (user account data, data file names, segment names, extent locations, table descriptions, and user privileges) is stored in the data dictionary tables. When this information is needed by the database, the data dictionary tables are read, and the data that is returned is stored in the data dictionary cache.


Database Buffer Cache
When a query is processed, the Oracle server process looks in the database buffer cache for any blocks it needs. If the block is not found in the database buffer cache, the server process reads the block from the data file and places a copy in the database buffer cache. Because subsequent requests for the same block may find the block in memory, the requests may not require physical reads. The Oracle server uses a least recently used algorithm to age out buffers that have not been accessed recently to make room for new blocks in the database buffer cache.

Redo Log Buffer Cache
The redo log buffer cache is a circular buffer that contains changes made to data file blocks.This information is stored in redo entries. Redo entries contain the information necessary to recreate the data prior to the change made by INSERT, UPDATE, DELETE, CREATE,ALTER, or DROP operations.

Large Pool
When users connect through the shared server, Oracle needs to allocate additional space in the shared pool for storing information about the connections between the user processes, dispatchers, and servers. The large pool relieves the burden on areas within the shared pool. The shared pool does not have to give up memory for caching SQL parse trees in favor of shared server session information, I/O, and backup and recover processes. The performance gain is from the reduction of overhead from increasing and shrinkage of the shared SQL cache.

Java Pool
The Java pool is an optional setting but is required if installing and using Java. Its size is set, in bytes, using the JAVA_POOL_SIZE parameter.