1.253 PARALLEL_MAX_SERVERS

PARALLEL_MAX_SERVERS specifies the maximum number of parallel execution processes and parallel recovery processes for an instance. As demand increases, Oracle Database increases the number of processes from the number created at instance startup up to this value.
Property Description

Parameter type

Integer

Default value

PARALLEL_THREADS_PER_CPU * CPU_COUNT * concurrent_parallel_users * 5

Modifiable

ALTER SYSTEM

Modifiable in a PDB

Yes

Range of values

0 to 32767

Basic

No

Oracle RAC

Multiple instances can have different values.

Note:

This parameter applies to parallel execution in exclusive mode as well as in a Real Application Clusters environment.

The number of concurrent parallel users running at default degree of parallelism on an instance depends on the memory initialization parameter settings for the instance. For example, if the MEMORY_TARGET or SGA_TARGET initialization parameter is set, then the number of concurrent_parallel_users = 4. If neither MEMORY_TARGET or SGA_TARGET is set, then PGA_AGGREGATE_TARGET is examined. If a value is set for PGA_AGGREGATE_TARGET, then concurrent_parallel_users = 2. If a value is not set for PGA_AGGREGATE_TARGET, then concurrent_parallel_users = 1.

The database system always reserves a certain number of reserved processes. Therefore, the following two values are taken into account when setting the PARALLEL_MAX_SERVERS initialization parameter:

  • The default value of PARALLEL_MAX_SERVERS determined using the calculation in the table above

  • The value of the PROCESSES initialization parameter minus the number of reserved processes

The lower of the two values is used as the default value of PARALLEL_MAX_SERVERS, and if you attempt to explicitly set PARALLEL_MAX_SERVERS to a value that is higher than either of the values, then the setting is adjusted to the lower of the two values.

When the PARALLEL_MAX_SERVERS parameter is set to a value lower than the value in the table above, the lower default value enables the database to start service processes and allows user processes to connect to the database.

The default value for PARALLEL_MAX_SERVERS for a PDB is determined using the calculation in the table above with the PDB’s CPU_COUNT value.

Note:

If you set this parameter too low, then some queries may not have a parallel execution process available to them during query processing. If you set it too high, then memory resource shortages may occur during peak periods, which can degrade performance.

See Also:

Oracle Database SQL Tuning Guide for more information about parallel execution