Oracle Library Cache�Part I

Thursday Dec 20th 2007 by James Koopmann

Gaining an understanding of internal Oracle structures is essential to becoming better DBAs and servicing our end user community. Oracle's library cache is one such internal structure that can help eliminate some very nasty denial of service requests originating from application users. Read on to learn more.

Gaining an understanding of internal Oracle structures is essential to becoming better DBAs and servicing our end user community. Oracle's library cache is one such internal structure that, after learning about, can help eliminate some very nasty denial of service requests originating from application users.

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. When applications run and reference code, Oracle 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). There are various criteria as to whether code being requested actually matches code already in the library cache but that is beyond the scope of this article. Just be aware that a configured library cache area, since it is allocated a specific amount of memory, can actively only hold so much code before it must age out some to make room for code that is required by applications. This is not necessarily a bad thing but we must be aware of the size of our library cache as well as how many misses or hard parses that are occurring. If there are too many, we may need to increase the amount of memory allocated to the library cache.

To monitor and gain an understanding of how your current library cache has performed since your last startup of the Oracle database you can issue the following SQL. Obviously if you are experiencing immediate performance problems, you will want to look at the results of this query over a period of time. Regardless, each of the rows returned represents a specific type of code (namespace) kept in the library cache and their particular performance activity.

SQL> select namespace,
            pinhitratio * 100                     pinhitratio
       from v$librarycache;

--------------- ---------- ---------- ---------- ------------- -----------
SQL AREA           9510054    9475115       1920          1731  99.6326099
TABLE/PROCEDURE    2173571    2158422       2525             0  99.3030363
BODY                977001     976796         16             0  99.9790174
TRIGGER              28566      28491         11             0  99.7374501
INDEX                 9735       8359          5             0   85.865434
CLUSTER               2294       2281          2             0  99.4333043
OBJECT                   0          0          0             0         100
PIPE                     0          0          0             0         100
JAVA SOURCE              0          0          0             0         100
JAVA RESOURCE            0          0          0             0         100
JAVA DATA              493        489          0             0   99.188641

The way to look at these results is to first look at how many times a particular namespace was executed (PINS), then take a look at how many times something tried to execute but wasn't in the library cache (RELOADS). Another very important statistic is the number of INVALIDATIONS that have occurred for a particular namespace. INVALIDATIONS are those pieces of code that for some reason, typically through a DDL operation, have become invalid and required a reparse. All of this can be summarized in the hit ratio (PINHITRATIO). So, in our example SQL above we can see that our particular library cache seems to be doing quite well and our applications are reusing SQL quite effectively.

You can also get an overall feel for the library cache with the following SQL. Again, we can see that our library cache is performing well.

select sum(pins) pins,
       sum(pinhits) pinhits,
       sum(reloads) reloads,
       sum(invalidations) invalidations,
       100-(sum(pinhits)/sum(pins)) *100 reparsing
 from v$librarycache;

---------- ---------- ---------- ------------- ----------
  12703182   12651415       4479          1731 .407512071

It is easy to say, when code is required by an application, “just put the code in the library cache”. It is another thing to actually have it done. There are internal locking mechanisms that must be adhered to for all this to happen. All in the name of making sure that the queries and code that are executed are actually valid and are referencing valid objects. In a nutshell, those locks, and subsequent wait events, are the following:

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. This lock is always obtained in an EXCLUSIVE mode so that no one else can load the same object at the same time. If your session is trying to obtain this lock and someone else has already obtained the lock, you will wait until they are done loading that object in the library cache.

Library cache lock

Once objects have been loaded into the library cache this lock controls the concurrent access to objects between clients. This lock is acquired on an object for the serialization of access and allows for a single application to use an object. The length of this lock is dependent on what is being done by an application or user. This lock is also obtained to simply locate an object in the library cache.

Library cache pin

The library cache pin event is responsible for concurrent access within the library cache. The acquisition of a library cache pin is required to load an object's heap to be loaded into memory. In addition, if someone wants to modify or examine an object they must acquire this lock.

In order for Oracle to move objects into the library cache it uses both locks and pins to access or load object handles and heaps. Locks are used to manage the serialization or concurrency between different users or application processes. This means that, if needed, a process can lock out and prevent other processes from accessing an object. Locks must also be acquired to locate an object in the cache. After acquiring a lock on the handle of an object, if the process actually wants to examine or modify the object then it must acquire a pin on the object. The pinning of an object results in the objects heap being loaded into memory if it is not already there. Both locks and pins are required if compilation or parsing of code is ever going to happen—all for the purpose of making sure that no changes to an object's definition occurs. Therefore, for any type of code that needs to be loaded into the library cache, the session must first acquire a library cache lock on the objects being queried. After the library cache lock the session must acquire a library cache pin to pin the object heap into the library cache.

The monitoring of Oracle's library cache is essential to making sure that objects and SQL code are executing efficiently and available for subsequent applications to execute. The library cache could be under stress if specific wait events are occurring and limiting the concurrent access to code. Most of the problems that occur in the library cache are application or user induced—but that will be addressed next time. Until then, run the fore-mentioned SQL and get an idea of the overall and general health of your library cache. Next time we will look at where an issue can exist, how to find them, and obviously offer solutions to fixing them.

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