I haven't been able to find any useful documentation as to what the intended behaviour is for this setting, besides what the help files mention:
Batch Size: When enabled, indicates that this node should generate batch work items and sets the batch size to this parameter’s value.
More broadly however, I am hoping to figure out what is the best way to output multiple geometry files from a single HDA Processor.
For example, I have created a 1000x1000 terrain heightfield with noise on it. I then wish to split it up into 100 tiles by cropping the input heightfield, 100x100 each, run an operation on each tile and then recombine the full landscape. Using the HDA Processor, whether I increase the number of iterations or set up a wedge node to drive my tile generator parameters, the result is quite slow. It spins up separate processes for every single tile, which means it has to load up the full landscape 100 times and only then crop it.
By comparison, a ROP Geometry node can work much faster. If it's set to run sequentially over frames, you can batch multiple geometries to be output from a single session. I can define a frame range from 1 to 100, set the Frames Per Batch parameter to be 25, and I can get four simultaneous processes to crunch through 25 tiles each really quickly, without having to reload the landscape between exports.
It seems like the HDA Processor should be able to do this as well, and the batching controls suggest that kind of functionality. But I can't get it to output anything at all?