Re: Parallel queries in RAC environment

From: Ian Cary <ian.cary_at_ons.gsi.gov.uk>
Date: Wed, 15 Oct 2008 14:34:03 +0100
Message-ID: <OFC155E995.B55BB711-ON802574E3.0049C430-802574E3.004A87E6@ons.gsi.gov.uk>


You can use a combination of the init.ora parameters instance_groups and parallel_instance_groups. A simple example would be have the values equal to the instance_number i.e. instance_number=1 , instance_group=1, parallel_instance_group=1 etc.

In this example all parallel processes would be spawned on the node they were requested from.

The usage isn't limited to a one to one mapping i.e. nodes 1 and 2 could have instance_groups = 'node 1 and 2' and parallel_instance_groups= 'node 1 and 2' and the parallel processes would be spawned on nodes 1 and/or 2.

You can also control which instance_groups the parallel processes are pointed to by issuing an alter session set parallel_instance_groups = '....' command and I believe it is even hintable.

I also understand that in 11g these parameters have be supceded by the use of db_services which controls it automatically but I haven't tried this yet.

Hope this helps,

Cheers,

Ian

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4-node RAC, 10.2.0.3

I have a long-running batch process. I suspect that some of the queries it contains would run faster with multiple read processes, and I'm trying to test this out.

I've hinted the relevant queries with

/*+ parallel (table_alias 6) */

and I can see that six read processed do start. The problem is that they are all on another instance than the original batch process. This results in a lot of block swapping and I actually see a slight degradation in performance. Currently I'm starting the batch process as a scheduled job, and the job class it belongs to has a service that is available on only one instance. Still, the child processes are on another instance - they don't seem to inherit the service from the parent process.

How can I force all of the child query processes to be on the same instance as the parent process?

Cheers,
James

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Received on Wed Oct 15 2008 - 08:34:03 CDT

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