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pga_aggregate_limit Tips

Oracle Database Tips by Donald BurlesonJune 26, 2015


Question:  What does the pga_aggregate_limit parameter do in Oracle 12c?

Answer:  One of the problems with pga_aggregate_target and pga_max_size was that it never aborted runaway tasks and only served as an upper "target" and not a form "limit".  It is important to have a firm limit on RAM usage and Oracle 12c and beyond introduces pga_aggregate_limit for the purpose of proper RAM sizing on Oracle servers.

The pga_aggregate_limit now sets a firm limit on PGA usage (for sorting and hash joins).  Without pga_aggregate_limit, Oracle can exceed the real amount of RAM, causing RAM buffer paging and RAC node eviction errors.

In order to make pga_aggregate_limit as confusing as possible, by default, the pga_aggregate_limit parameter is set to the greatest of:

     1:  2 gigabytes

     2:  Double the value of pga_aggregate_limit value

     3:  3 megabytes times the value of the processes parameter.

However, it will not exceed 120% of the physical memory size minus the total SGA size.

When using pga_aggregate_limit, whenever a process exceeds the limit, sessions with the highest PGA usage (see v$pgastat) are aborted until the bottleneck is cleared.

pga_aggregate_limit and RAC


If the total PGA consumption on a server reaches the pga_aggregate_limit value, sessions consuming the most memory will have their current calls terminated. If this action does not reduce the total PGA consumption below the limit, those sessions will be wholly terminated.

Database administrators running Oracle RAC instances on nodes that do not have an abundance of physical memory may want to configure the pga_aggregate_limit  value so that parallel operations do not consume too much memory leading to swapping and slower performance.

However, you may want to implement the pga_aggregate_limit initialization parameter with a high degree of caution. If a node fails and connections are now hitting a surviving instance, the pga_aggregate_limit limit may be reached easily if not sized to handle the additional workload.

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