Generally, the higher parallelism, the higher heap usage will be. To raise the heap size, use the -Xmx Java option (e.g. -Xmx16g), or -J=-Xmx8g when passing the option through the Python programs.

In higher-level languages like Python or Java, these are hidden from you. In day-to-day work, that's terrific, but when you're learning how these low-level data structures are built, it's great to ...