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Pinning Objects in the Shared Pool
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As I have discussed, if code objects have to be repeatedly hard-parsed and executed, database performance will deteriorate eventually. Your goal should be to see that as much of the executed code remains in memory as possible so compiled code can be reexecuted. You can avoid repeated reloading of objects in your library cache by pinning objects using the DBMS_SHARED_POOL package. (The library cache is a component of the shared pool, as you ve seen earlier.) Listing 20-4 shows how you can determine the objects that should be pinned in your library cache (shared pool). Listing 20-4. Determining the Objects to Be Pinned in the Shared Pool SQL> 2 3 4 5 6* SELECT type, COUNT(*) OBJECTS, SUM(DECODE(KEPT,'YES',1,0)) KEPT, SUM(loads) - count(*) reloads FROM V$DB_OBJECT_CACHE GROUP BY type ORDER BY objects DESC;
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TYPE OBJECTS KEPT RELOADS ---------------------------- ---------- ---------- ---------CURSOR 41143 0 136621 NOT LOADED 37522 0 54213 TABLE 758 24 133742 PUB_SUB 404 0 135 SYNONYM 381 0 7704 JAVA CLASS 297 296 317 VIEW 181 0 11586 INVALID TYPE 139 48 11 PACKAGE 137 0 8352 TRIGGER 136 0 8515 PACKAGE BODY 121 0 218 SEQUENCE 81 0 3015 INDEX 61 7 0 PROCEDURE 41 0 219
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CH A PT ER 2 0 PERF O RMAN CE TUNI NG: TUN ING TH E I NS TA NCE
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FUNCTION NON-EXISTENT TYPE CLUSTER TYPE BODY LIBRARY RSRC CONSUMER GROUP QUEUE JAVA SHARED DATA JAVA SOURCE 24 rows selected. SQL>
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If the number of reloads in the output shown in Listing 20-4 is high, you need to make sure that the objects are pinned using the following command: SQL> EXECUTE SYS.DBMS_SHARED_POOL.KEEP(object_name,object_type); You can use the following statements to pin a package first in the shared pool and then remove it, if necessary: SQL> EXECUTE SYS.DBMS_SHARED_POOL.KEEP(NEW_EMP.PKG, PACKAGE); SQL> EXECUTE SYS.DBMS_SHARED_POOL.UNKEEP(NEW_EMP.PKG,PACKAGE); Of course, if you shut down and restart your database, the shared pool won t retain the pinned objects. That s why most DBAs use scripts with all the objects they want to pin in the shared pool and schedule them to run right after every database start. Most of the objects usually are small, so there s no reason to be too conservative about how many you pin. For example, I pin all my packages, including Oracle-supplied PL/SQL packages. Look at the following example, which gives you an idea about the total memory taken up by a large number of packages. This query shows the total number of packages in my database: SQL> SELECTCOUNT(*) 2 FROM V$DB_OBJECT_CACHE 3* WHERE type='PACKAGE'; COUNT(*) --------------167 SQL> The following query shows the total amount of memory needed to pin all my packages in the shared pool: SQL> SELECT SUM(sharable_mem) 2 FROM V$DB_OBJECT_CACHE 3* WHERE type='PACKAGE'; SUM(SHARABLE_MEM) ----------------4771127 SQL> As you can see, pinning every single package in my database takes up less than 5MB of a total of several hundred megabytes of memory allocated to the shared pool.
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CHAPTER 20 PERFOR MAN CE TUNING: TUNING THE INSTA NCE
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Tuning the Buffer Cache
When users request data, Oracle reads the data from the disks (in terms of Oracle blocks) and stores it in the buffer cache so it may access the data easily if necessary. As the need for the data diminishes, eventually Oracle removes the data from the buffer cache to make room for newer data. Note that some operations don t use the buffer cache (SGA); rather, they read directly into the PGA area. Direct sort operations and parallel reads are examples of such operations.
How to Size the Buffer Cache
As with the shared pool component, the best way to manage the buffer cache is to choose automatic SGA management. However, if you choose to manage the SGA manually, you can use a process of trial and error to set the buffer cache size. You assign an initial amount of memory to the pool and watch the buffer cache hit ratios to see how often the application can retrieve the data from memory, as opposed to going to disk. The terminology used for calculating the buffer hit ratio can be somewhat confusing on occasion. Here are the key terms you need to understand: Physical reads: These are the data blocks that Oracle reads from disk. Reading data from disk is much more expensive than reading data that s already in Oracle s memory. When you issue a query, Oracle always first tries to retrieve the data from memory the database buffer cache and not disk. DB block gets: This is a read of the buffer cache, to retrieve a block in current mode. This most often happens during data modification when Oracle has to be sure that it s updating the most recent version of the block. So, when Oracle finds the required data in the database buffer cache, it checks whether the data in the blocks is up to date. If a user changes the data in the buffer cache but hasn t committed those changes yet, new requests for the same data can t show these interim changes. If the data in the buffer blocks is up to date, each such data block retrieved is counted as a DB block get. Consistent gets: This is a read of the buffer cache, to retrieve a block in consistent mode. This may include a read of undo segments to maintain the read consistency principle (see 8 for more information about read consistency). If Oracle finds that another session has updated the data in that block since the read began, then it will apply the new information from the undo segments. Logical reads: Every time Oracle is able to satisfy a request for data by reading it from the database buffer cache, you get a logical read. Thus logical reads include both DB block gets and consistent gets. Buffer gets: This term refers to the number of database cache buffers retrieved. This value is the same as the logical reads described earlier. The following formula gives you the buffer cache hit ratio: 1 - ('physical reads cache') / ('consistent gets from cache' + 'db block gets from cache') You can use the following query to get the current values for all three necessary buffer cache statistics: SQL> SELECT name, value FROM v$sysstat WHERE where name IN ('physical reads cache', 'consistent gets from cache', 'db block gets from cache');
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