Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 4 ■ MEMORY STRUCTURES 151 The Large pool is used specifically by • Shared server connections, to allocate the UGA region in the SGA • Parallel execution of statements, to allow for the allocation of interprocess message buffers, which are used to coordinate the parallel query servers • Backup for RMAN disk I/O buffers in some cases As you can see, none of these memory allocations should be managed in an LRU buffer pool designed to manage small chunks of memory. With shared server connection memory, for example, once a session logs out, this memory is never going to be reused, so it should be immediately returned to the pool. Also, shared server UGA memory allocation tends to be “large.” If you review the earlier examples with the SORT_AREA_RETAINED_SIZE or PGA_ AGGREGATE_TARGET, the UGA can grow very large and is definitely bigger than 4KB chunks. Putting MTS memory into the Shared pool causes it to fragment into odd-sized pieces of memory and, furthermore, you will find that large pieces of memory that will never be reused will age out memory that could be reused. This forces the database to do more work to rebuild that memory structure later. The same is true for parallel query message buffers, since they are not LRU manageable. They are allocated and cannot be freed until they are done being used. Once they have delivered their message, they are no longer needed and should be released immediately. With backup buffers, this applies to an even greater extent—they are large, and once Oracle is done using them, they should just “disappear.” The Large pool is not mandatory when using shared server connections, but it is highly recommended. If you do not have a Large pool and use a shared server connection, the allocations come out of the Shared pool as they always did in Oracle 7.3 and before. This will definitely lead to degraded performance over some period of time and should be avoided. The Large pool will default to some size if the parameter DBWR_IO_SLAVES or PARALLEL_MAX_ SERVERS is set to some positive value. It is recommended that you set the size of the Large pool manually if you are using a feature that uses it. The default mechanism is typically not the appropriate value for your situation. Java Pool The Java pool was added in version 8.1.5 of Oracle to support running Java in the database. If you code a stored procedure in Java, Oracle will make use of this chunk of memory when processing that code. The parameter JAVA_POOL_SIZE is used to fix the amount of memory allocated to the Java pool for all session-specific Java code and data. The Java pool is used in different ways, depending on the mode in which the Oracle server is running. In dedicated server mode, the Java pool includes the shared part of each Java class, which is actually used per session. These are basically the read-only parts (execution vectors, methods, etc.) and are about 4KB to 8KB per class. Thus, in dedicated server mode (which will most likely be the case for applications using purely Java stored procedures), the total memory required for the Java pool is quite modest and can be determined based on the number of Java classes you will be using. It should be noted that none of the per-session state is stored in the SGA in dedicated server mode, as this information is stored in the UGA and, as you will recall, the UGA is included in the PGA in dedicated server mode.
152 CHAPTER 4 ■ MEMORY STRUCTURES When connecting to Oracle using a shared server connection, the Java pool includes both of the following: • The shared part of each Java class. • Some of the UGA used for per-session state of each session, which is allocated from the JAVA_POOL within the SGA. The remainder of the UGA will be located as normal in the Shared pool, or if the Large pool is configured, it will be allocated there instead. As the total size of the Java pool is fixed in Oracle9i and before, application developers will need to estimate the total requirement of their applications and multiply this estimate by the number of concurrent sessions they need to support. This number will dictate the overall size of the Java pool. Each Java UGA will grow or shrink as needed, but bear in mind that the pool must be sized such that all UGAs combined must be able to fit in it at the same time. In Oracle 10g and above, this parameter may be modified, and the Java pool may grow and shrink over time without the database being restarted. Streams Pool The Streams pool is a new SGA structure starting in Oracle 10g. Streams itself is a new database feature as of Oracle9i Release 2 and above. It was designed as a data sharing/replication tool and is Oracle’s stated direction going forward for data replication. ■Note The statement that Streams “is Oracle’s stated direction going forward for data replication” should not be interpreted as meaning that Advanced Replication, Oracle’s now legacy replication feature, is going away anytime soon. Rather, Advanced Replication will continue to be supported in future releases. To learn more about Streams itself, see the Streams Concepts Guide available on http://otn.oracle.com in the Documentation section. The Streams pool (or up to 10 percent of the Shared pool if no Streams pool is configured) is used to buffer queue messages used by the Streams process as it is moving/copying data from one database to another. Instead of using permanent disk-based queues, with the attendant overhead associated with them, Streams uses in-memory queues. If these queues fill up, they will spill over to disk eventually. If the Oracle instance with the memory queue fails for some reason, due to an instance failure (software crash), power failure, or whatever, these in-memory queues are rebuilt from the redo logs. So, the Streams pool will only be important in systems using the Streams database feature. In those environments, it should be set in order to avoid “stealing” 10 percent of the Shared pool for this feature. Automatic SGA Memory Management Just as there are two ways to manage PGA memory, there are two ways to manage SGA memory starting in Oracle 10g: manually, by setting all of the necessary pool and cache parameters:
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xxiii Next up are t
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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