1.255 PARALLEL_DEGREE_LIMIT
PARALLEL_DEGREE_LIMIT
limits the degree of parallelism used by the optimizer to ensure that parallel server processes do not flood the system.
Property | Description |
---|---|
Parameter type |
String |
Syntax |
|
Default value |
|
Modifiable |
|
Modifiable in a PDB |
Yes |
Basic |
No |
With automatic degree of parallelism, Oracle automatically decides whether a statement should execute in parallel and what degree of parallelism the statement should use. The optimizer automatically determines the degree of parallelism for a statement based on the resource requirements of the statement. However, PARALLEL_DEGREE_LIMIT
enforces the limit for the degree of parallelism used by the optimizer.
Values
-
CPU
The maximum degree of parallelism is limited by the number of CPUs in the system. The formula used to calculate the limit is
PARALLEL_THREADS_PER_CPU
*CPU_COUNT
* the number of instances available (by default, all the opened instances on the cluster but can be constrained usingPARALLEL_INSTANCE_GROUP
or service specification). This is the default. -
AUTO
This value is equivalent to the
CPU
value. -
IO
The maximum degree of parallelism the optimizer can use is limited by the I/O capacity of the system. The value is calculated by dividing the total system throughput by the maximum I/O bandwidth per process. You must run the
DBMS_RESOURCE_MANAGER.CALIBRATE_IO
procedure on the system to use theIO
setting. This procedure will calculate the total system throughput and the maximum I/O bandwidth per process. -
integer
A numeric value for this parameter specifies the maximum degree of parallelism the optimizer can choose for a SQL statement when automatic degree of parallelism is active. Automatic degree of parallelism is only enabled if
PARALLEL_DEGREE_POLICY
is set toADAPTIVE
,AUTO
, orLIMITED
.
See Also:
-
Oracle Database VLDB and Partitioning Guide for information about automatic degree of parallelism
-
Oracle Database PL/SQL Packages and Types Reference for information on the
DBMS_RESOURCE_MANAGER.CALIBRATE_IO
procedure