Cross validation in spark, is process of running a machine learning pipeline with different combinations of parameters to find the optimal model. Often this operation is costly as spark needs to go through many combinations of parameters on large size of data.
Cross Validation before Spark 2.3
Till spark 2.3, cross validation was done serially. That means , for every combination of cross validation, spark runs one set of jobs before it moves on to next combination.
The below code show one example of such cross validation.
In above code, we are running an algorithm 3 times, logistic regression in this case ( not shown in the code), with different values of maxIter.
Result of Serial Cross Validation
When we see the Spark UI, we can observe that logistic regression is running one after another. The below screenshots confirm the same.
As you can see from above images, two jobs have one second of gap.Also from the job number, we can make out that there were many jobs ran before second logistic regression started.
Parallel Cross Validation in Spark 2.3
Every combination of cross validation are independent of each other. Which means we can run them parallely. This will increase resource usage, but it does get us results much faster.
So from Spark 2.3 version, there is an option to specify parallelism in cross validation. The below codes shows the same
In above code, we set the parallelism with setParallelism parameter.
Result of Parallel Cross Validation
When we observe the spark UI with cross validation enabled, we can see the below result.
As you can observe from above screenshot, all the 3 logistic regression have started together.
You can access complete code on github.