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Oracle library cache contention tips

Oracle Database Tips by Donald BurlesonAugust 31, 2015


The Oracle shared pool contains Oracle's library cache, which is responsible for collecting, parsing, interpreting, and executing all of the SQL statements that go against the Oracle database. Hence, the shared pool is a key component which can experience frequent contention, so it's necessary for the Oracle database administrator to check for shared pool contention.
Oracle library cache contention is easy to understand once you know how Oracle processes SQL statements. The library cache is used like the data buffers, a place to store ready-to-run SQL which has already been parsed, semantically checked, optimized and has a load-and-go execution plan in-place.
Library cache contention occurs when Oracle places a "latch" on a SQL statement in order to manage concurrency. There is a dedicated latch-related wait event for the more popular latches that often generate significant contention. For those events the name of the latch appears in the name of the wait event; such as latch: library cache or latch: cache buffers chains. This enables you to quickly figure out if a particular type of latch is responsible for most of the latch-related contention.
Oracle library cache pin wait contention is caused by contention with the library cache, the area used to store SQL executables for re-use.  The library cache pin wait usually happens when you are compiling or parsing a PL/SQL object or a view.
The following query can show whether Oracle has contention for common library cache activities:
select
   sid,
   event,
   p1raw,
   seconds_in_wait,
   wait_time
from
   v$session_wait
where
   event = 'library cache pin'
and
   state = 'WAITING';

Library cache latches function to protect all cached SQL statements as well as the associated object definitions contained within the library cache region in the shared pool of the Oracle SGA. Whenever new statements are added to the library cache area, this type of latch must be acquired by Oracle as part of the operation. Oracle scans the library cache area during the parse phase for matching SQL statements. If one is not found, then Oracle will complete the task, obtain the library cache latchfor the SQL statement and then insert it into the library cache area for future usage by the Oracle database.

There is a hidden Oracle database initialization parameter called kgl_latch_countwhich controls behavior in terms of the number of library cache latches created within the Oracle database. By default, this value should not be changed as it is sufficiently set in most cases. In the event that there should be a library cache contention issue and no other recourse is available to resolve the contention with the library cache latch, this value can be increased.

By the way, the default value for kgl_latch_countis set to the following prime number plus value of the database initialization parameter for cpu_count. The maximum allowable value for kgl_latch_count is no greater than 66 according to Oracle support per Bug # 1381824. Now move on to learning more about shared pool latches within Oracle.


5.      dba_hist_librarycache


 

 

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