1.248 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

PARALLEL_DEGREE_LIMIT = { CPU | IO | integer }

Default value

CPU

Modifiable

ALTER SESSION, ALTER SYSTEM

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 using PARALLEL_INSTANCE_GROUP or service specification). This is the default.

  • 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 the IO 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 to ADAPTIVE, AUTO, or LIMITED.

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