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Posted Jan 3, 2008

Oracle Library Cache�Part II

By James Koopmann

What to do if the Library Cache gives you trouble.

In part one of this two part series, we took a look at what the Library Cache was all about.

To recap, Oracle’s library cache is nothing more than an area in memory, specifically one of three parts inside the shared pool. The library cache is composed of shared SQL areas, PL/SQL packages and procedures, various locks & handles, and in the case of a shared server configuration, stores private SQL areas. Whenever an application wants to execute SQL or PL/SQL (collectively called code), that code must first reside inside Oracle's library cache. In order for Oracle to use code, it will first search the library cache to see if that code already exists in memory. If the code already exists in memory then Oracle can reuse that existing code (also known as a soft parse). If the code does not exist Oracle must then load the code into memory (also known as a hard parse, or library cache miss).

The areas that give occasional trouble are various locks that occur, and subsequently cause the following wait events:

library cache load lock— As the name states, the library cache load lock is concerned with obtaining a lock for a database object so that it can load it into the library cache. When a user is experiencing this form of lock there is someone else that has the load lock for the object and thus the user must wait until the lock  becomes available.

Library cache lock— Once objects have been loaded into the library cache this lock controls the concurrent access to objects between clients. This allows one client to have sole access to an object and block out other clients, for instance, when two clients want to compile the same piece of code.

Library cache pin— The library cache pin event is responsible for concurrent access within the library cache. When a pin is acquired, it causes the object’s heap to be loaded into memory if it is not already there. A wait for a "library cache pin" implies some other session holds that PIN in an incompatible mode.

Regardless of the type of wait within the library cache, the diagnosis to determine who is waiting on a resource and what that resource is can be done through the querying of the V$SESSION_WAIT view. For instance if you wanted to find those sessions that were waiting for the “library cache pin” you could execute the following query. For a library cache pine, the key part of this query is the P1RAW column that gives the handle address of the object that is blocking this particular session. For other types of waits, you should reference Oracle documentation to find which P value corresponds to an object or resource being waited on.

SELECT sid, event, p1raw 
  FROM sys.v_$session_wait
 WHERE event = 'library cache pin'
   AND state = 'WAITING';

We can then execute the following query to find the library cache object being waited for. If you don’t currently have any particular waits occurring, this query is just fun to execute without the where clause to see what is in the library cache.

SELECT kglnaown AS owner, kglnaobj as Object
  FROM sys.x$kglob
 WHERE kglhdadr='&P1RAW';

To find those users that are waiting on the object in question you can use the DBA_WAITERS view and issue the following query. This is a very simple query but can be used to find the blocking session quite nicely. I basically try and find the waiting session that matches the above session we got from the V$SESSION_WAIT query and then see what the holding_session is. It is also good to note how many other sessions are waiting behind this blocking session. If there are a lot, you may need to take action quickly.

SELECT waiting_session, holding_session FROM dba_waiters;

If you want to investigate the actual SQL being done by the holding session, you can issue the following SQL statement.

select sesion.sid,
  from v$sqlarea sqlarea, v$session sesion
 where sesion.sql_hash_value = sqlarea.hash_value
   and sesion.sql_address    = sqlarea.address;

This is sometimes good to see what the originating issue really is. It might not be the mix of application code but actually one particular statement that is giving Oracle a problem. For instance, for me, I have had problems with SQL that reference objects through a view that had a database link and was querying an external non-Oracle database. This type of problem can only be seen and remedied through tracing the originating SQL statement.

Now that you have determined the actual session and object being waited on, along with the session causing the problem and its SQL, it is time to determine what to do. If the wait has been happening for quite some time you could be experiencing a bug or malfunction within the library cache. I have seen this happen occasionally and the only way to remedy was to kill off those processes that where holding the lock. To do this within Oracle you can issue the alter system kill session command. Depending on the connection type, this may or may not work. There have been too many times that the connection needed to be terminated through an operating system kill command, or shutting down a set of applications. You will have to investigate and see what works best for your system under a complete lockup in the library cache. The only advice I can give is always attempt the least obtrusive kills first before bringing down systems and databases.

When working with the library cache, just remember it is nothing more than a section of memory that Oracle must load code into before it can execute. Getting that code into the library cache can be limited and cause wait events that literally bring a system to a standstill. A quick determination of the SQL causing the hang is needed to take action through a kill of session or process or alteration of code, but don’t loose sight of the fact that this is memory and we might just need to re-allocate some to this cause to allow Oracle to work more efficiently.

» See All Articles by Columnist James Koopmann

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