Github user BryanCutler commented on the issue:
https://github.com/apache/spark/pull/16774
> I think setting the default to match current behavior is best (i.e. 1).
I agree, just wanted to bring it up in case others had differing view since
it was a concern in the JIRA.
>I think it's important to note in the docs that this will actually only
really work as expected if the FAIR scheduler is enabled, otherwise I don't
think things will actually be executed concurrently.
I ran some tests with and without the FAIR scheduler enabled. Without FAIR
scheduler, if there are enough resources in the cluster (e.g. cores) then the
tasks are run concurrently. If not enough resources, then the jobs will wait.
With FAIR scheduler the tasks are run concurrently and share the available
cores. Both will complete model selection in about the same time, with equal
resources, so I think the main benefit of FAIR is for multi-users so that one
users jobs don't get starved.
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