In Squeak we have disabled processPreemptionYields to restore proper preemption semantics. That's why the above loop prevents the UI process from running.

Before, the VM always forced a process to yield if it was preempted, which made the scheduling non-deterministic. The time slice given to each process was random (whenever a higher-priority process became runnable). It did have the "advantage" of doing an implicit round-robin scheduling if multiple processes had the same priority. I guess that's what you're seeing in Pharo.

Cog supports both schemes via vmParameter 48.

- Bert -