9.2 About Java Memory Usage
The typical and custom database installation process furnishes a database that has been configured for reasonable Java usage during development. However, run-time use of Java should be determined by the usage of system resources for a given deployed application. Resources you use during development can vary widely, depending on your activity. The following sections describe how you can configure memory, how to tell how much System Global Area (SGA) memory you are using, and what errors denote a Java memory issue:
9.2.1 Configuring Memory Initialization Parameters
You can modify the following database initialization parameters to tune your memory usage to reflect your application needs more accurately:
Shared pool memory is used by the class loader within the JVM. The class loader, on an average, uses about 8 KB of memory for each loaded class. Shared pool memory is used when loading and resolving classes into the database. It is also used when compiling the source in the database or when using Java resource objects in the database.
The memory specified in
SHARED_POOL_SIZEis consumed transiently when you use the
loadjavatool. The database initialization process requires
SHARED_POOL_SIZEto be set to 96 MB because it loads the Java binaries for approximately 8,000 classes and resolves them. The
SHARED_POOL_SIZEresource is also consumed when you create call specifications and as the system tracks dynamically loaded Java classes at run time.
Oracle JVM memory manager uses
JAVA_POOL_SIZEmainly for in-memory representation of Java method and class definitions, and static Java states that are migrated to session space at end-of-call in shared server mode. In the first case, you will be sharing the memory cost with all Java users. In the second case, the value of
JAVA_POOL_SIZEvaries according to the actual amount of state held in static variables for each session. But, Oracle recommends the minimum value as 50 MB.
This parameter lets you specify a soft limit on Java memory usage in a session, which will warn you if you must increase your Java memory limits. Every time memory is allocated, the total memory allocated is checked against this limit.
When a user's session Java state exceeds this size, Oracle JVM generates a warning that is written into the trace files. Although this warning is an informational message and has no impact on your application, you should understand and manage the memory requirements of your deployed classes, especially as they relate to usage of session space.
This parameter is applicable only to a shared-server environment.
If a Java program, which can be called by a user, running in the server can be used in a way that is not self-limiting in its memory usage, then this setting may be useful to place a hard limit on the amount of session space made available to it. The default is 4 GB. This limit is purposely set extremely high to be usually invisible.
When a user's session Java state attempts to exceeds this size, the application can receive an out-of-memory failure.
This parameter is applicable only to a shared-server environment.
126.96.36.199 Initializing Pool Sizes within Database Templates
You can set the defaults for the following parameters in the database installation template:
Figure 9-1 illustrates how the Database Configuration Assistant enables you to modify these values in the Memory section.
Figure 9-1 Configuring Oracle JVM Memory Parameters
Description of "Figure 9-1 Configuring Oracle JVM Memory Parameters"
9.2.2 About Java Pool Memory
Java pool memory is a subset of SGA, which is used exclusively by Java for memory that must be aligned pagewise. This includes the majority, but, not all of the memory used for the shared definitions of Java classes. Other uses of Java pool memory depend on the mode in which the Oracle Database server runs.
Java Pool Memory Used within a Dedicated Server
The following is what constitutes the Java pool memory used within a dedicated server:
Most of the shared part of each Java class in use.
This includes read-only memory, such as code vectors, and methods. In total, this can average about 4 KB to 8 KB for each class.
None of the per-session Java state of each session.
For a dedicated server, this is stored in the User Global Area (UGA) within the Program Global Area (PGA), and not within the SGA.
Under dedicated servers, the total required Java pool memory depends on the applications running and usually ranges between 10 and 50 MB.
Java Pool Memory Used within a Shared Server
The following constitutes the Java pool memory used within a shared server:
Most of the shared part of each Java class in use
This includes read-only memory, such as vectors and methods. In total, this memory usually averages to be about 4 KB to 8 KB for each class.
Some of the UGA for per session memory
In particular, the memory for objects that remain in use across Database calls is always allocated from Java pool.
Because the Java pool memory size is limited, you must estimate the total requirement for your applications and multiply by the number of concurrent sessions the applications want to create, to calculate the total amount of necessary Java pool memory. Each UGA grows and shrinks as necessary. However, all UGAs combined must be able to fit within the entire fixed Java pool space.
Under shared servers, Java pool could be large. Java-intensive, multiuser applications could require more than 100 MB.
If you are compiling code on the server, rather than compiling on the client and loading to the server, then you might need a bigger
JAVA_POOL_SIZE than the default 20 MB.
Reducing the Number of Java-Enabled Sessions
The top-level invocation of Java in the database is issued by a client-side application or utility. If each client has a dedicated server, then large-scale deployment involves significant consumption of resources on the database server and also leads to resource wastage. You can use Client-side connection pools or Database Resident Connection Pool (DRCP) to reduce the number of database processes and sessions.
Oracle Database JDBC Developer's Guide for more information about DRCP
9.2.3 Displaying Used Amounts of Java Pool Memory
You can find out how much of Java pool memory is being used by viewing the
V$SGASTAT table. Its rows include pool, name, and bytes. Specifically, the last two rows show the amount of Java pool memory used and how much is free. The total of these two items equals the number of bytes that you configured in the database initialization file.
SVRMGR> select * from v$sgastat; POOL NAME BYTES ----------- -------------------------- ---------- fixed_sga 69424 db_block_buffers 2048000 log_buffer 524288 shared pool free memory 22887532 shared pool miscellaneous 559420 shared pool character set object 64080 shared pool State objects 98504 shared pool message pool freequeue 231152 shared pool PL/SQL DIANA 2275264 shared pool db_files 72496 shared pool session heap 59492 shared pool joxlod: init P 7108 shared pool PLS non-lib hp 2096 shared pool joxlod: in ehe 4367524 shared pool VIRTUAL CIRCUITS 162576 shared pool joxlod: in phe 2726452 shared pool long op statistics array 44000 shared pool table definiti 160 shared pool KGK heap 4372 shared pool table columns 148336 shared pool db_block_hash_buckets 48792 shared pool dictionary cache 1948756 shared pool fixed allocation callback 320 shared pool SYSTEM PARAMETERS 63392 shared pool joxlod: init s 7020 shared pool KQLS heap 1570992 shared pool library cache 6201988 shared pool trigger inform 32876 shared pool sql area 7015432 shared pool sessions 211200 shared pool KGFF heap 1320 shared pool joxs heap init 4248 shared pool PL/SQL MPCODE 405388 shared pool event statistics per sess 339200 shared pool db_block_buffers 136000 java pool free memory 30261248 java pool memory in use 19742720 37 rows selected.
9.2.4 Correcting Out of Memory Errors
If you run out of memory while loading classes, then it can fail silently, leaving invalid classes in the database. Later, if you try to call or resolve any invalid classes, then a
NoClassDefFoundException instance will be thrown at run time. You would get the same exceptions if you were to load corrupted class files. You should perform the following:
Verify that the class was actually included in the set you are loading to the server.
loadjava -forceoption to force the new class being loaded to replace the class already resident in the server.
loadjava -resolveoption to attempt resolution of a class during the load process. This enables you to catch missing classes at load time, rather than at run time.
Double check the status of the newly loaded class by connecting to the database in the schema containing the class, and run the following:
SELECT * FROM user_objects WHERE object_name = dbms_java.shortname('');
STATUSfield should be
VALID. If the
loadjavatool complains about memory problems or failures, such as lost connection, then increase
JAVA_POOL_SIZE, and try again.
9.2.5 Displaying Java Call and Session Heap Statistics
Database performance view
v$sesstat records a number of Java memory usage statistics. These statistics are updated often during Java calls. The following example shows the Java call return and session heap statistics for the database session with SID=102.
SQL> select s.sid, n.name p_name, st.value from v$session s, v$sesstat st, v$statname n where s.sid=102 and s.sid=st.sid and n.statistic# = st.statistic# and n.name like 'java%'; SID P_NAME VALUE ---------- ---------------------------------------- ---------- 102 java call heap total size 6815744 102 java call heap total size max 6815744 102 java call heap used size 668904 102 java call heap used size max 846920 102 java call heap live size 667112 102 java call heap live size max 704312 102 java call heap object count 13959 102 java call heap object count max 17173 102 java call heap live object count 13907 102 java call heap live object count max 14916 102 java call heap gc count 432433 102 java call heap collected count 123196423 102 java call heap collected bytes 5425972216 102 java session heap used size 444416 102 java session heap used size max 444416 102 java session heap live size 444416 102 java session heap live size max 444416 102 java session heap object count 0 102 java session heap object count max 0 102 java session heap live object count 0 102 java session heap live object count max 0 102 java session heap gc count 0 102 java session heap collected count 0 102 java session heap collected bytes 0 24 rows selected.