The SHARED_POOL_SIZE parameter specifies the memory size of the shared pool.
Parallel execution requires memory resources in addition to those required by serial SQL execution. Additional memory is used for communication and passing data between query server processes and the query coordinator.
Oracle Database allocates memory for query server processes from the shared pool. Tune the shared pool as follows:
Allow for other clients of the shared pool, such as shared cursors and stored procedures.
Remember that larger values improve performance in multiuser systems, but smaller values use less memory.
You can then monitor the number of buffers used by parallel execution and compare the shared pool PX msg pool to the current high water mark reported in output from the view V$PX_PROCESS_SYSSTAT.
Note:
If you do not have enough memory available, error message 12853 occurs (insufficient memory for PX buffers: current stringK, max needed stringK). This is caused by having insufficient SGA memory available for PX buffers. You must reconfigure the SGA to have at least (MAX - CURRENT) bytes of additional memory.
By default, Oracle Database allocates parallel execution buffers from the shared pool.
If Oracle Database displays the following error on startup, you should reduce the value for SHARED_POOL_SIZE low enough so your database starts:
ORA-27102: out of memory SVR4 Error: 12: Not enough space
After reducing the value of SHARED_POOL_SIZE, you might see the error:
ORA-04031: unable to allocate 16084 bytes of shared memory
("SHARED pool","unknown object","SHARED pool heap","PX msg pool")
If so, execute the following query to determine why Oracle Database could not allocate the 16,084 bytes:
SELECT NAME, SUM(BYTES) FROM V$SGASTAT WHERE UPPER(POOL)='SHARED POOL' GROUP BY ROLLUP (NAME);
Your output should resemble the following:
NAME SUM(BYTES)
-------------------------- ----------
PX msg pool 1474572
free memory 562132
2036704
If you specify SHARED_POOL_SIZE and the amount of memory you specify to reserve is bigger than the pool, Oracle Database does not allocate all the memory it can get. Instead, it leaves some space. When the query runs, Oracle Database tries to get what it needs. Oracle Database uses the 560 KB and needs another 16 KB when it fails. The error does not report the cumulative amount that is needed. The best way of determining how much more memory is needed is to use the formulas in Additional Memory Requirements for Message Buffers.
To resolve the problem in the current example, increase the value for SHARED_POOL_SIZE. As shown in the sample output, the SHARED_POOL_SIZE is about 2 MB. Depending on the amount of memory available, you could increase the value of SHARED_POOL_SIZE to 4 MB and attempt to start your database. If Oracle Database continues to display an ORA-4031 message, gradually increase the value for SHARED_POOL_SIZE until startup is successful.
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