## What changes were proposed in this pull request?
By default, the dynamic allocation will request enough executors to maximize the
parallelism according to the number of tasks to process. While this minimizes the
latency of the job, with small tasks this setting can waste a lot of resources due to
executor allocation overhead, as some executor might not even do any work.
This setting allows to set a ratio that will be used to reduce the number of
target executors w.r.t. full parallelism.
The number of executors computed with this setting is still fenced by
`spark.dynamicAllocation.maxExecutors` and `spark.dynamicAllocation.minExecutors`
## How was this patch tested?
Units tests and runs on various actual workloads on a Yarn Cluster
Author: Julien Cuquemelle <j.cuquemelle@criteo.com>
Closes#19881 from jcuquemelle/AddTaskPerExecutorSlot.
55c4ca88a3