Enhancing Stability in a Limited Memory Condition

Scenario

A large amount of memory is required when Spark SQL executes a query, especially during Aggregate and Join operations. If the memory is limited, OutOfMemoryError may occur. Stability in a limited memory condition ensures queries to be run in limited memory without OutOfMemoryError.

Limited memory does not mean infinitely small memory, but ensures stable queries by using disks in a scenario where memory fails to store the data amount that is several times larger than the available memory size. For example, for queries involving Join, the data of the same key used for Join needs to be stored in memory. If the data amount is too large to be stored in the available memory, OutOfMemoryError occurs.

Stability in a limited memory condition involves the following sub-functions:

ExternalSort
If the memory is inadequate during sorting, partial data overflows to disks.
TungstenAggregate
By default, ExternalSort is used to sort data before data aggregation. Therefore, if the memory is inadequate, the data overflows to disks during sorting. The data has been properly sorted before aggregation and only aggregation results of the current key are remained, which use a small amount of memory.
SortMergeJoin and SortMergeOuterJoin
SortMergeJoin and SortMergeOuterJoinan are based on the equivalence join of sorted data. By default, ExternalSort is used to sort the data before the equivalence join. Therefore, if the memory is inadequate, the data overflows to disks during sorting. The data has been properly sorted before the equivalence join and only the data of the same key are remained, which uses a small amount of memory.

Configuration

Navigation path for setting parameters:

When submitting an application, set the following parameters using --conf or adjust the parameters in the spark-defaults.conf configuration file on the client.

**Table 1** Parameter description
Parameter	Scenario	Description	Default Value
spark.sql.tungsten.enabled	/	Type: Boolean If the value is true, tungsten is enabled. That is, the logic plan is equivalent to the codegeneration function, and the physical plan uses the corresponding tungsten execution plan. If the value is false, tungsten is disabled.	true
spark.sql.codegen.wholeStage	/	Type: Boolean If the value is true, codegeneration is enabled. That is, for some specified queries, the logic plan code will be generated dynamically when running. If the value is false, codegeneration is disabled and the existing static code is used.	true

To enable ExternalSort, you need to set spark.sql.planner.externalSort to true and spark.sql.unsafe.enabled to false or spark.sql.codegen.wholeStage to false.
To enable TungstenAggregate, use either of the following methods:
Set spark.sql.codegen.wholeStage and spark.sql.unsafe.enabled to true in the configuration file or CLI.

If neither spark.sql.codegen.wholeStage nor spark.sql.unsafe.enabled is true or either of them is true, TungstenAggregate is enabled as long as spark.sql.tungsten.enabled is set to true.