feat: CBC-style threaded branch & bound (-threads N)#25
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Opportunistic parallel tree search as CBC's: N workers over a shared node heap, incumbent and pseudocost arrays, each worker on its own problem copy + LP (reduced-cost fixing mutates column bounds, and a factor's solve scratch belongs to the LP that built it). Nodes crossing the shared heap carry a Portable() basis — status/values only, no factorization — and the receiving worker refactorizes, the same contract as CBC handing CoinWarmStartBasis between threads. Each worker re-applies reduced-cost fixing when another worker tightens the incumbent. In-tree cut generation stays serial-only (local cut rows would be dangling row indices in other workers' problems). Serial (Threads<=1) runs the same treeSearch code path with uncontended locks: 018/020/021 keep bit-identical nodes, pivots and objectives vs main (7/31746, 774/182619, 3/62278). Benchmarks: 90x30 multi-knapsack 1.58s -> 0.19s at 8 threads (8.4x, same optimum); 020 10.4s -> 7.8s at 4 threads (1.33x) with the same node-inflation pattern real CBC shows there (271 -> 832 nodes, 1.25x). Race detector clean over unit tests and a full threaded 020 solve. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Model.Threads/ CLI-threads N: opportunistic parallel branch & bound modeled on CBC's.Design
State.Portable()carries status/basicOf/value only; the receiving worker'swarmSolverefactorizes on first use — the same contract as CBC handingCoinWarmStartBasisbetween threads. Within a plunge the full factorized state is kept as before.Threads<=1runs the identicaltreeSearchcode path with uncontended locks — 018/020/021 keep bit-identical nodes/pivots/objectives vs main (7/31746, 774/182619, 3/62278).Benchmarks
Real CBC on 020 shows the same shape: t=1→t=4 is 2.0s→1.6s (1.25×) with nodes inflating 271→832; opportunistic parallel B&B trades node count for wall clock, and dive-dominated trees gain little. Objectives across thread counts differ only within the configured gap (GapAbs 1e-5), as with real CBC.
Race detector clean over the unit tests and a full threaded 020 solve (96s under
-race, zero warnings).🤖 Generated with Claude Code