问题
Spark SQL DataFrame/Dataset execution engine has several extremely efficient time & space optimizations (e.g. InternalRow & expression codeGen). According to many documentations, it seems to be a better option than RDD for most distributed algorithms.
However, I did some sourcecode research and am still not convinced. I have no doubt that InternalRow is much more compact and can save large amount of memory. But execution of algorithms may not be any faster saving predefined expressions. Namely, it is indicated in sourcecode of org.apache.spark.sql.catalyst.expressions.ScalaUDF, that every user defined function does 3 things:
- convert catalyst type (used in InternalRow) to scala type (used in GenericRow).
- apply the function
- convert the result back from scala type to catalyst type
Apparently this is even slower than just applying the function directly on RDD without any conversion. Can anyone confirm or deny my speculation by some real-case profiling and code analysis?
Thank you so much for any suggestion or insight.
回答1:
From this Databricks' blog article A Tale of Three Apache Spark APIs: RDDs, DataFrames, and Datasets
When to use RDDs?
Consider these scenarios or common use cases for using RDDs when:
- you want low-level transformation and actions and control on your dataset;
- your data is unstructured, such as media streams or streams of text;
- you want to manipulate your data with functional programming constructs than domain specific expressions;
- you don’t care about imposing a schema, such as columnar format, while processing or accessing data attributes by name or column;
- and you can forgo some optimization and performance benefits available with DataFrames and Datasets for structured and semi-structured data.
In High Performance Spark's Chapter 3. DataFrames, Datasets, and Spark SQL, you can see some performance you can get with the Dataframe/Dataset API compared to RDD
And in the Databricks' article mentioned you can also find that Dataframe optimizes space usage compared to RDD
来源：https://stackoverflow.com/questions/37533021/when-to-use-spark-dataframe-dataset-api-and-when-to-use-plain-rdd