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Oracle 10g release 2 changes to pga_aggregate_target limits

In a paper titled “Advanced Management Of Working Areas in Oracle 9i/10g”, author Joze Senegacnik delivers a must-read paper on internals for Oracle PGA management in 10g release 2, including important changes to the sizing rules for pga_aggregate_target:.

(download password is “network”)

“In the latest release 10.2 Oracle changed these default values. The memory allocated to a single SQL operator  (_smm_max_size) is limited to:

  • for P_A_T <= 500MB   the parameter _smm_max_size = 20% of P_A_T
     

  • for P_A_T between 500MB and 1000MB the  parameter _smm_max_size = 100M
     

  • for P_A_T betweeen 1001MB and 2560MB (2.5GB) the parameter _smm_max_size =  10% of P_A_T
     

  • for P_A_T > 2560MB (2,5GB) the parameter _smm_max_size =  262,060 MB (~0,25GB)
     

  • I have seen even cases when these values were even bigger after the instance was restarted with P_A_T set at 4GB.
     

  • The maximum value for parallel operations changed from 30% to 50% PGA_AGGREGATE_TARGET/DOP. Also the DOP changed. When DOP <=5 then _smm_max_size is used, otherwise _smm_px_max_size/DOP limits the maximum memory usage. . .

Obviously the default value for PGA maximum size of 200M and 50% of that value as maximum value for the single SQL operator were causing performance bottlenecks in previous releases of the Oracle database. Therefore Oracle changed the defaults in release 10.2 and made them much more aggressive.

  • In Oracle9i/10gR1 when one has to perform a big sort or hash-join or bitmap operation, he can change the hidden parameters like _PGA_MAX_SIZE, _SMM_PX_MAX_SIZE for parallel execution when DOP > 6 or _SMM_MAX_SIZE to be able to use the available, still unused memory on the system for large SQL operators.

It is important to recall that _PGA_MAX_SIZE parameter defines the maximum size of  process’ global area (PGA). Because an SQL statement can have more than one SQL operator like sort, group-by, hash-join, etc. the size of working area for single SQL operator is limited by default to 50% of _PGA_MAX_SIZE.


 

 
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