Hypergraph partitioning with fixed vertices [VLSI CAD]

We empirically assess the implications of fixed terminals for hypergraph partitioning heuristics. Our experimental testbed incorporates a leading-edge multilevel hypergraph partitioner and IBM-internal circuits that have recently been released as part of the ISPD-98 Benchmark Suite. We find that the presence of fixed terminals can make a partitioning instance considerably easier (possibly to the point of being "trivial"); much less effort is needed to stably reach solution qualities that are near best-achievable. Toward development of partitioning heuristics specific to the fixed-terminals regime, we study the pass statistics of flat FM-based partitioning heuristics. Our data suggest that more fixed terminals implies that the improvements within a pass will more likely occur near the beginning of the pass. Restricting the length of passes-which degrades solution quality in the classic (free-hypergraph) context-is relatively safe for the fixed-terminals regime and considerably reduces the run times of our FM-based heuristic implementations. The distinct nature of partitioning in the fixed-terminals regime has deep implications: (1) for the design and use of partitioners in top-down placement; (2) for the context in which VLSI hypergraph partitioning research is pursued; and (3) for the development of new benchmark instances for the research community.