Objects that are unmarked at the end of the mark phase are unreachable by the application (dead) because there's no path from the root set through any series of references to find them.

As with the mark phase, the cost of sweeping dead objects back into the free memory pool can't be completely predicted.

But look carefully, there are no lurches, pauses, hitches or dead spots in their stroke technique - they have excellent rhythm and timing, moving seamlessly from one phase of the stroke to the other.