An active node is another way of referring to a subproblem which has not been further split, and whose lower bound is less than the current upper bound. Every active node needs to be worked on, and any active node could be worked on next. The computation assigns the active nodes with the smallest lower bounds to the available CPUs. The computation on an active node will either result in its lower bound being increased to the current upper bound, or the problem will be split into two new subproblems (and hence two new active nodes). From another viewpoint, the active nodes are the leaves of the current branch and bound tree.

The plot below shows the Number of Active Nodes as a function of the cpu time used.