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Question and Answer

Connor McDonald

Thanks for the question.

Asked: October 11, 2019 - 9:10 pm UTC

Last updated: October 22, 2019 - 2:58 am UTC

Version: 11.2.0.4

Viewed 1000+ times

You Asked

Hi ASKTOM team,
I am not very good with parallelism, so have a question about DW database. These are my current settings:

parallel_adaptive_multi_user    boolean TRUE   
parallel_automatic_tuning       boolean FALSE  
parallel_degree_limit           string  CPU    
parallel_degree_policy          string  MANUAL 
parallel_execution_message_size integer 16384  
parallel_force_local            boolean FALSE  
parallel_instance_group         string         
parallel_io_cap_enabled         boolean FALSE  
parallel_max_servers            integer 1120   
parallel_min_percent            integer 0      
parallel_min_servers            integer 0      
parallel_min_time_threshold     string  AUTO   
parallel_server                 boolean TRUE   
parallel_server_instances       integer 4      
parallel_servers_target         integer 448    
parallel_threads_per_cpu        integer 2


These are all at default level, nothing specified in SPFILE. The tables have DEGREE=2 or 4. Some of the bigger indexes have DEGREE=DEFAULT. But what I am seeing is that even without any hint, many users are being allocated 168 or 190 or more parallel slaves. This is causing major problems because CPU goes to 90%+. What I want to do is to set a parameter (or parameters) at database level, so no matter what the DEGREE is on the table/index or what a user specifies in a hint, any user whose query runs with parallelism, gets a max of 8 parallel query slaves. Is that possible?

Thanks

and Connor said...

Check out parallel_degree_limit. From the docs:

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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Confused

A reader, October 22, 2019 - 5:15 am UTC

"Automatic degree of parallelism is only enabled if PARALLEL_DEGREE_POLICY is set to ADAPTIVE, AUTO, or LIMITED. "

Requester posted his PARALLEL_DEGREE_POLICY as MANUAL.

So confusing why is happening such parallelism??
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