Secrets of Oracle's Automatic Degree of Parallelism
Automatic degree of parallelism, or Auto DOP, is a new feature in 11gR2 that promises to help manage systems where large subset of the workload runs with parallel processing. In this post I'll introduce the feature and give very useful tips I got from Oracle's Real World Performance expert Greg Rahn on how to use it. So this is worth reading even if you are familiar with the feature. The problem is fairly well known - you system only has finite amount of resources. Only so many CPUs, only so many disks capable of delivering only so many IO/s and MB/s. A certain query may have amazing performance when running with 32 parallel processes all alone on your test system. When 5 people need to run it at once, and at the same time there are two scheduled jobs running each with its own parallel processes, there are two likely outcomes:
- You will run more parallel processes than your system is capable of serving. Resulting in long queues on the CPU and storage, and overall performance degradation.
- You limit the maximum number of parallel processes to protect the database resources, and some of the queries degrade. If you don't detect it, the ETL process that should have finished in two hours takes 24, which means that the daily report sent to the CEO is missing some of the data. Ouch.
- Limited - when accessing tables and indexes that have been declared with parallel clause, Oracle will decide on the degree of parallelism based on the query and system resources.
- Auto - Oracle will decide on degree of parallelism for every query. In addition two exciting new features are enabled: parallel statement queuing and in-memory parallel execution.