SpacePartitioner distributes everything to just a few partitions/executors

Describe the bug

I might just be using this wrong, because I can't find much documentation at all on how to use the SpacePartitioner. I'm trying to use the SpacePartitioner on an RDD of type RDD[(SpatialKey, geotrellis.vector.Feature)]. I've created a LayoutDefinition that covers the entire pan-tropics (so 180E-180W and 30S-30N), and vector features cover are land in those bounds, so I just set the KeyBounds to the entire layout.

But even though I'm running with ~700 executors, it's giving everything to just 3 executors (vs. when I use HashPartitioner, where it distributes to pretty much all executors). I'm not sure if I'm just using it incorrectly (my bounds seem wrong) or it just doesn't support global-scale partitioning that well (this post makes it seem like it's intended for more local areas with clear bounding boxes).

Environment

• Scala version: 2.11.12
• Spark version: 2.4.0
• GeoTrellis version: 2.2.0

Start by reviewing how SpacePartitioner uses the LayoutDefinition and full-layout KeyBounds for the RDD[(SpatialKey, geotrellis.vector.Feature)] case. Compare its distribution with HashPartitioner at the reported global bounds and consult issue #1206; done means determining whether the behavior is incorrect or documenting the supported usage and expected partition distribution.