The docs for parallel_tuple_cost are quite terse, as the reference section of the docs usually are:
"Sets the planner's estimate of the cost of transferring one tuple from a parallel worker process to another process. The default is 0.1."
Usually you can find more extensive discussion of such settings in informal resources like blog posts or mailing lists, but Googling the name I don't find much for this setting. Is there good information out there somewhere?
If you were take the doc description literally, then the default value seems much too high, as it doesn't take 10x the (default) cpu_tuple_cost to transfer a tuple up from a parallel worker. On the other hand, you probably don't want a query which consumes 8x the CPU resources just to finish only 5% faster (on an otherwise idle server with 8 CPUs). Maybe this Amdahl factor is what inspired the high default value?
Cheers,
Jeff