Reg: Error in Parallel Hint -

Question

Hi Experts,
I have a concern regarding the usage of parallel hint with DOP = "AUTO".
Getting these errors while the ETL job(s) are executed with /*+ PARALLEL(AUTO) */
JOB - 1
java.sql.SQLException: ORA-12853: insufficient memory for PX buffers: current 1159024K, max needed 11666304K
ORA-04031: unable to allocate 65560 bytes of shared memory ("large pool","unknown object","large pool","PX msg pool")
JOB - 2
java.sql.SQLException: ORA-12801: error signaled in parallel query server P256
ORA-12853: insufficient memory for PX buffers: current 1159024K, max needed 11666304K
ORA-04031: unable to allocate 65560 bytes of shared memory ("large pool","unknown object","large pool","PX msg pool")
I tried google but couldn't get an appropriate reason for this and also fix.
1- Do I need to increase some memory buffer related parameter?
2- Does "AUTO" option causing this? And will using a DOP = 3, 4, etc. guarantee to fix this?
(I will try this and check but want to know the exact reason, so that it doesn't cause any problem in Prod)
Any pointers regarding this.
-- Ranit
( on Oracle 11.2.0.3.0 )

Answer 1

ranit B wrote:
Getting these errors while the ETL job(s) are executed with /*+ PARALLEL(AUTO) */
JOB - 1
java.sql.SQLException: ORA-12853: insufficient memory for PX buffers: current 1159024K, max needed 11666304K
ORA-04031: unable to allocate 65560 bytes of shared memory ("large pool","unknown object","large pool","PX msg pool")
JOB - 2
java.sql.SQLException: ORA-12801: error signaled in parallel query server P256
ORA-12853: insufficient memory for PX buffers: current 1159024K, max needed 11666304K
ORA-04031: unable to allocate 65560 bytes of shared memory ("large pool","unknown object","large pool","PX msg pool")
I tried google but couldn't get an appropriate reason for this and also fix.
1- Do I need to increase some memory buffer related parameter?
2- Does "AUTO" option causing this? And will using a DOP = 3, 4, etc. guarantee to fix this?
(I will try this and check but want to know the exact reason, so that it doesn't cause any problem in Prod)
Any pointers regarding this.
-- Ranit
( on Oracle 11.2.0.3.0 )
The error message is pretty self-explanatory: The memory that is required for communication between PX Servers ("PX msg pool") can either be allocated from the Shared Pool or the Large Pool - depending on some other settings. In your case obviously the memory is taken from the Large Pool, and the error message says that it needs approx. 12GB of memory from the Large Pool which isn't available. I'm not sure however if that error message is really showing a reasonable "needed" value because 12GB for the "PX msg pool" is much more than what you should get for reasonable DOP values according to the underlying formula.
So you either need to change the settings (depends on how the memory is configured / static allocation or *target* parameters and the parallel_automatic_tuning (deprecated) parameter, internally the "_PX_use_large_pool" parameter) to move the allocation to the Shared Pool, or much simpler, increase the size of the Large Pool accordingly.
Although I doubt the "needed" value of 12GB you should nevertheless review your AUTO DOP parameter settings since the values shown for the "PX msg pool" could indicate that the DOP chosen seems to be pretty high. You can find that information in the "Notes" section when you get the corresponding plan from the Library Cache using "DBMS_XPLAN.DISPLAY_CURSOR", or using EXPLAIN PLAN and DBMS_XPLAN.DISPLAY, or may be even from AWR if you have the license using DBMS_XPLAN.DISPLAY_AWR.
For example it might make sense to lower your PARALLEL_DEGREE_LIMIT parameter value to some explicit value rather than using the default value of CPU. Also you should review your I/O calibration results in the DBA_RSRC_IO_CALIBRATE view - and there the MAX_PMBPS value. This is the relevant value that is used to determine the DOP - see my blog post Oracle related stuff: Cost Is Time: Next Generation
Randolf