SMP causes long-running queries to complete faster but consume more resources. Therefore, SMP shouldn’t be used in all situations; consider carefully how SMP is applied in your environment.
By default, when a query is run, only a single process is used to run the query. Db2 Symmetric Multiprocessing (SMP) allows queries to use multiple, parallel processes when running a query in order to complete faster. This is achieved by allowing the database to use multiple CPUs and I/O processors to work on the query at the same time. SQL queries, OPNQRYF, Query/400, index creation and maintenance, and reorganizing of physical files can all take advantage of SMP. This powerful tool can result in these operations running more quickly but will cause increased consumption of main storage, CPU, and disk resources. Therefore, SMP isn’t always the best choice, and it’s important to choose wisely how and when to use SMP.
Long-running queries benefit the most from SMP. For a long-running query, such as one that utilizes OLAP functions, the additional overhead of running multiple, parallel tasks is small in comparison to the length of time it takes to run the query. However, in short-running queries, SMP may actually cause your query to perform worse because of the additional overhead. Therefore, new and enhanced controls were introduced in IBM i 7.5 to allow you to better control how SMP is used in your environment. These controls are set using the QAQQINI options file. These new and enhanced options are:
- PARALLEL_DEGREE: Control the maximum number of tasks allowed per query
- PARALLEL_MAX_SYSTEM_CPU: Dynamically adjust running queries to keep CPU usage below a safe threshold
- PARALLEL_MIN_TIME: Set the minimum time a query must run before it can use parallel processing
This QAQQINI option is used to control how much processing power the optimizer can consider for use during optimization. By default, the optimizer assumes it can use as many tasks as there are cores on the system. The PARALLEL_DEGREE option allows you to set the number of tasks allowed to either *NONE, *MAX, *NBRTASKS, or *OPTIMIZE. The *OPTIMIZE option allows the query optimizer to choose the best implementation and is the recommended option because it allows the optimizer the greatest flexibility in how a query is run.
In IBM i 7.5, the *OPTIMIZE [percentage] option was enhanced. The default value is 100%; however, you can change this default by setting the percentage to a number between 1 and 200. By setting the percentage to less than 100, you reduce the number of tasks the optimizer considers available for use. Previously, this setting only applied after the optimization finished but before the query executed. Now the SQL Query Engine (SQE) will apply the percentage during optimization, allowing the optimizer to make a better, more accurate decision on the number of tasks the query will run with.
PARALLEL_DEGREE can be set using the QAQQINI options file, the CHGQRYA CL command, or the system value QQRYDEGREE.
This QAQQINI option protects your system from being overloaded by dynamically adjusting queries to keep the CPU usage below a safe threshold. By default, when the CPU usage exceeds 80%, SMP enabled queries will experience a gradual reduction in the number of tasks used until the CPU usage falls back below 80%. There will always be at least one task running to process the query, which allows the queries to slow down but not stop processing. Once the CPU usage drops below 80%, any tasks that were suspended will be reinstated if there are additional resources available. In IBM i 7.5, the default value of 80% can now be modified using the QAQQINI option PARALLEL_MAX_SYSEM_CPU. This option is available only in IBM i 7.5 and is valid only when PARALLEL_DEGREE is set to *OPTIMIZE.
This QAQQINI option allows you to set the minimum time a query must run before it uses parallel processing. This allows you to only use SMP for the long-running queries that can benefit from using multiple parallel tasks. The optimizer starts out processing a query using only one task. If a query has not yet returned any data, and it runs longer than the time configured, then the query will be re-optimized to use SMP and the new plan swapped in. If a query has returned data, the query will run to completion and will be re-optimized with SMP the next time it is run. The default time before considering using SMP is 60 seconds. This default can now be modified to a different minimum time using the QAQQINI option PARALLEL_MIN_TIME. This option is available only in IBM i 7.5 and is valid only when PARALLEL_DEGREE is set to *OPTIMIZE.
In 2022, Db2 Symmetric Multiprocessing (SMP) switched to a no-charge option. Db2 Symmetric Multiprocessing is 5770SS1 option 26, and it must be ordered and installed separately. You can check to see if it is installed with the following query:
WHERE PRODUCT_ID = '5770SS1'
AND PRODUCT_OPTION = '26'
Find Out More
To learn how to set up and use the QAQQINI query options file, see https://www.ibm.com/docs/en/i/7.5?topic=attributes-qaqqini.
For more information about these improvements to SMP processing, see https://www.ibm.com/support/pages/node/6579159.