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[SPARK-15926] Improve readability of DAGScheduler stage creation methods

Kay Ousterhout authored 8 years ago

## What changes were proposed in this pull request?

This pull request refactors parts of the DAGScheduler to improve readability, focusing on the code around stage creation.  One goal of this change it to make it clearer which functions may create new stages (as opposed to looking up stages that already exist).  There are no functionality changes in this pull request.  In more detail:

* shuffleToMapStage was renamed to shuffleIdToMapStage (when reading the existing code I have sometimes struggled to remember what the key is -- is it a stage? A stage id? This change is intended to avoid that confusion)
* Cleaned up the code to create shuffle map stages.  Previously, creating a shuffle map stage involved 3 different functions (newOrUsedShuffleStage, newShuffleMapStage, and getShuffleMapStage), and it wasn't clear what the purpose of each function was.  With the new code, a single function (getOrCreateShuffleMapStage) is responsible for getting a stage (if it already exists) or creating new shuffle map stages and any missing ancestor stages, and it delegates to createShuffleMapStage when new stages need to be created.  There's some remaining confusion here because the getOrCreateParentStages call in createShuffleMapStage may recursively create ancestor stages; this is an issue I plan to fix in a future pull request, because it's trickier to fix and involves a slight functionality change.
* newResultStage was renamed to createResultStage, for consistency with naming around shuffle map stages.
* getParentStages has been renamed to getOrCreateParentStages, to make it clear that this function will sometimes create missing ancestor stages.
* The only *slight* functionality change is that on line 478, updateJobIdStageIdMaps now uses a stage's parents instance variable rather than re-calculating them (I couldn't see any reason why they'd need to be re-calculated, and suspect this is just leftover from older code).
* getAncestorShuffleDependencies was renamed to getMissingAncestorShuffleDependencies, to make it clear that this only returns dependencies that have not yet been run.

cc squito markhamstra JoshRosen (who requested more DAG scheduler commenting long ago -- an issue this pull request tries, in part, to address)

FYI rxin

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #13677 from kayousterhout/SPARK-15926.

c8809db5

c8809db5 8 years ago

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Apache Spark

Spark is a fast and general cluster computing system for Big Data. It provides high-level APIs in Scala, Java, and Python, and an optimized engine that supports general computation graphs for data analysis. It also supports a rich set of higher-level tools including Spark SQL for SQL and structured data processing, MLlib for machine learning, GraphX for graph processing, and Spark Streaming for stream processing.

http://spark.apache.org/

Online Documentation

You can find the latest Spark documentation, including a programming guide, on the project web page and project wiki. This README file only contains basic setup instructions.

Building Spark

Spark is built using Apache Maven. To build Spark and its example programs, run:

mvn -DskipTests clean package

(You do not need to do this if you downloaded a pre-built package.) More detailed documentation is available from the project site, at "Building Spark with Maven".

Interactive Scala Shell

The easiest way to start using Spark is through the Scala shell:

./bin/spark-shell

Try the following command, which should return 1000:

scala> sc.parallelize(1 to 1000).count()

Interactive Python Shell

Alternatively, if you prefer Python, you can use the Python shell:

./bin/pyspark

And run the following command, which should also return 1000:

>>> sc.parallelize(range(1000)).count()

Example Programs

Spark also comes with several sample programs in the examples directory. To run one of them, use ./bin/run-example <class> [params]. For example:

./bin/run-example SparkPi

will run the Pi example locally.

You can set the MASTER environment variable when running examples to submit examples to a cluster. This can be a mesos:// or spark:// URL, "yarn-cluster" or "yarn-client" to run on YARN, and "local" to run locally with one thread, or "local[N]" to run locally with N threads. You can also use an abbreviated class name if the class is in the examples package. For instance:

MASTER=spark://host:7077 ./bin/run-example SparkPi

Many of the example programs print usage help if no params are given.

Running Tests

Testing first requires building Spark. Once Spark is built, tests can be run using:

./dev/run-tests

Please see the guidance on how to run all automated tests.

A Note About Hadoop Versions

Spark uses the Hadoop core library to talk to HDFS and other Hadoop-supported storage systems. Because the protocols have changed in different versions of Hadoop, you must build Spark against the same version that your cluster runs.

Please refer to the build documentation at "Specifying the Hadoop Version" for detailed guidance on building for a particular distribution of Hadoop, including building for particular Hive and Hive Thriftserver distributions. See also "Third Party Hadoop Distributions" for guidance on building a Spark application that works with a particular distribution.

Configuration

Please refer to the Configuration guide in the online documentation for an overview on how to configure Spark.