Solver island
A group of physics bodies linked by contacts or joints that must be solved together, while disjoint groups can be solved separately and put to sleep.
The constraint solver works on islands — connected components of the contact-and-joint graph. Two stacks of boxes that don't touch form two islands and can't influence each other within the step, so they can be solved independently and on separate threads. Islands are rebuilt or updated each step as contacts appear and vanish.
When every body in an island has stayed below a velocity threshold long enough, the whole island is put to sleep: it is skipped until a new contact or an external force wakes it. That is what lets a scene with thousands of settled objects cost almost nothing. Warm starting and sequential impulse do their work inside each island.