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PGA memory content explained

In a paper titled “Advanced Management Of Working Areas in Oracle 9i/10g”, author Joze Senegacnik notes internals for Oracle PGA management in 10g release 2.

http://conference.ukoug.org/display_presentation.asp?id=576#doc

This comprehensive and detailed paper on the latest PGA memory structures contains many details.

“The contents of the PGA varies depending on whether we are using DEDICATED or SHARED server mode, but generally we can use the following description of PGA parts:

·       

Session Memory

: The memory allotted to hold logon information and other session details. When a shared server model is used, this kind of information is stored in SGA because it needs to be persistent between calls.

·       

SQL Execution Memory

: The memory allotted for the execution of SQL statements The SQL Execution Memory has a persistent and a run time area:

o        The persistent area requires persisting across multiple executions of the same SQL statement and contains the information such as bind details, data type conversion, etc. This persistent area is de-allocated only when the cursor is closed. When the shared server processes model is used the persistent area is a part of the SGA (part of Large pool if properly configured).

The runtime area contains information used while a SQL statement is being executed. Its size depends on the number and size of rows being processed as well as the type and complexity of SQL statement as. It is de-allocated when the execution completes. For shared sever processes, the run time area is resides in the PGA for DML/DDL operation and in the SGA for queries.

The feedback loop is closed by the local memory manager. It uses the current value of the memory bound and the current profile of a work area to determine the correct amount of PGA memory, called

expected size

, which can be made available to this work area. The calculation of the expected work area size for each active operation is done based on the following rules:

·        The expected size can never be less than the minimum memory requirement of the operation and more than its cache size.

·        If the global memory bound is between minimum and cache requirement, the expected size will be equal to the bound.

·        The only exception to this rule is a sort operation. For a sort operation, the expected size under will be equal to one pass size if the bound is less than its cache size is less than the bound. This is due to the fact that the sort operation does not benefit from more than one pass memory unless the whole operation can be performed in cache, as explained earlier.

·        For parallel operations, the expected size is multiplied by the degree of parallelism.

·        Finally, no single operation will be allowed to “hog” all available memory. Therefore, the expected size can never be more than 5% of the overall target for serial and more than 30% for parallel operations.


 

 
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