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Linear & Integer Programming: A Decade of Computation Robert E. Bixby ILOG, Inc. and Rice University Co-Authors Mary Fenelon, Zongao Gu, Irv Lustig, Ed Rothberg, Roland Wunderling ILOG, Inc

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Machine Comparison Model: PILOTS constraints: 1141, variables: 3652, nonzeros: Machine Seconds Sun 3/ Pentium PC (60 MHz) IBM 590 Powerstation 65.0 SGI R8000 Power Challenge 44.8 Athlon (650 MHz) 22.2 Compaq (667 Mhz EV67) 6.8 ~5000x improvement

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Outline Linear programming (LP) –Introduction –Computational history 1947 to late 1980s –The last decade Mixed-integer programming (MIP) – Closing the gap between theory and practice –Introduction –Computation history 1954 to late 1990s –Features in modern codes –The last year –A new kind of cutting planes The future

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LP

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A linear program ( LP ) is an optimization problem of the form Minimize c T x Subject to Ax = b l x u

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Computational History: 1950 – Dantzig invents simplex method 1951 SEAC ( Standards Eastern Automatic Computer ) 48 cons. & 72 vars. –“One could have started an iteration, gone to lunch and returned before it finished” William Orchard-Hays, IBM 7090, LP cons. –Oil companies used LP IBM 360, MPSX & MPSIII cons. –These codes lasted into the mid 80s with little fundamental change. Mid 1980s – 1990 –1984 Karmarkar, Interior-point (barrier) methods –1991 Grötschel: “Some linear programs were hard to solve, even for highly praised commercial codes like IBM’s MPSX” – Late 1980s: OSL, XPRESS, CPLEX …

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1989 – 1998 An Example

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LAU2 : A Fleet Assignment Model (1989) 4420 cons, 6711 vars, nzs (Tests run on 500 MHz EV6 & Cray Y/MP) Idea 0: Run CPLEX –7 hours on a Cray Y/MP, stalled in phase I. Idea 1: Handling degeneracy: Perturbation –12332 secs, itns Idea 2: Barrier methods – 656 secs (Cray Y/MP) Idea 3: Better pricing: hybrid = partial pricing + devex (1973) – 307 secs Idea 4: Use the dual –Explicit dual: 103 secs –Dual simplex: 345 secs Idea 5: Steepest edge –Dual steepest edge: 21 secs

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Examine larger models: rows Bottleneck: Linear solves for systems with very sparse input and very sparse output (BTRAN, FTRAN) Eliminating the Bottleneck: These solves can be done in “linear time” (Linear-Algebra folklore: reachability) 1999 – 2000

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Bottleneck Removed: Exploiting It Dual: Variables with two finite bounds usually do not need to be binding in the ratio test. Sparse pricing: Fast updates for minimum when few reduced-costs change. …

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BIG Test Set

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LP Performance Improvement 1999 – 2000 Barrier: All 3.6

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Algorithm Comparison Dual vs. Primal: 2.6x (7 not solved by primal) Dual vs. Barrier: 1.2x And barrier is now faster anyway: CPLEX 7.0 barrier 1.6x faster

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MIP: Closing the Gap between Theory and Practice

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A mixed-integer program ( MIP ) is an optimization problem of the form Minimize c T x Subject to Ax=b l x u some or all x j integral

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Branch and Bound (Cut) Tree Root Integer v 3 v 4 x 2 x 3 y 0 y 1 z 0 z 1 Lower Bound Integer Upper Bound Infeas z 0 z 1 GAPGAP Remarks: (1) GAP = 0 Proof of optimality (2) Practice: Often good enough to have good Solution Solve LP relaxation: v=3.5 (fractional)

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MIP really is HARD A Customer Model: 44 cons, 51 vars, 167 nzs, maximization 51 general integer variables Branch-and-Cut: Initial integer solution Initial upper bound …after 120,000 seconds, 370,000,000 B&C nodes, 45 Gig tree Integer solution and bound: UNCHANGED Electrical Power Industry, ERPI GS-6401, June 1989: Mixed-integer programming is a powerful modeling tool, “They are, however, theoretically complicated and computationally cumbersome”

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UNITCAL_7: cons, vars (2856 binary) California Unit Commitment: 7 Day Model Previous attempts (by model formulator) 2 Day model: 8 hours, no progress 7 Day model: 1 hour to solve initial LP CPLEX 7.0 on 667 MHz Compaq EV67 Running defaults...

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Problem 'unitcal_7.sav.gz' read. Reduced MIP has rows, columns, and nonzeros. Root relaxation solution time = 5.22 sec. Nodes Cuts/ Node Left Objective IInf Best Integer Best Node ItCnt Gap e e e Cuts: e Cuts: e Cuts: e Flowcuts: e Cuts: e e * e e e % * e e e % GUB cover cuts applied: 2 Cover cuts applied: 7 Implied bound cuts applied: 1186 Flow cuts applied: 1120 Flow path cuts applied: 7 Gomory fractional cuts applied: 147 Integer optimal solution: Objective = e+07 Solution time = sec. Iterations = Nodes = 6718 Model: UNITCAL_7

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Computational History: 1950 – Dantzig, Fulkerson, S. Johnson: 49 city TSP –Solved to optimality using cutting planes and solving LPs by hand 1957 Gomory –Cutting plane algorithm: A complete solution 1960 Land, Doig –B&B 1966 Beale, Small –B&B, Depth-first-search 1972 UMPIRE, Forrest, Hirst, Tomlin –SOS, pseudo-costs, best projection, … 1972 – 1998 Good B&B remained the state-of-the-art in commercial codes, in spite of –1983 Crowder, Johnson, Padberg: PIPX, pure 0/1 MIP –1987 Van Roy and Wolsey: MPSARX, mixed 0/1 MIP

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1998… A new generation of MIP codes Linear programming –Usable, stable, robust performance Variable selection –pseudo-costs, strong branching Primal heuristics –5 different tried at root, one selected Node presolve –Fast, incremental bound strengthening Probing –Three levels Auto disaggregation – x j ( u j ) y, y = 0/1 preferred Cutting planes –Gomory, knapsack covers, flow covers, mix-integer rounding, cliques, GUB covers, implied bounds, path cuts, disjunctive cuts

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y 3.5 y 3.5 (0, 3.5) x + y 3.5, x 0, y 0 integral

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y 3.5 y 3.5 CUT: 2x + y 4 x + y 3.5, x 0, y 0 integral

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Gomory Mixed Cut Given y, x j Z +, and y + a ij x j = d = d + f, f > 0 Rounding: Where a ij = a ij + f j, define t = y + ( a ij x j : f j f) + ( a ij x j : f j > f) Z Then (f j x j : f j f) + (f j -1)x j : f j > f) = d - t Disjunction: t d (f j x j : f j f) f t d ((1-f j )x j : f j > f) 1-f Combining: ((f j /f)x j : f j f) + ([(1-f j )/(1-f)]x j : f j > f) 1

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BIGTEST – 80 Models CPLEX 6.0 versus CPLEX 7.0 Running defaults (7200 second limit) –CPLEX 6.0 FAILS on 31 –CPLEX 7.0 FAILS on 2 CPLEX 6.0 tuned versus CPLEX 7.0 defaults –CPLEX x faster

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Performance Impact: Individual Cuts Cliques - 3 % Implied - 1 % Path 0 % GUB covers 14 % Disjunctive 17 % Flow covers 41 % MIR 56 % Covers 56 % Gomory 114 %

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Local Cuts for MIP: An idea from the TSP (Applegate, Chvátal, Cook, Bixby)

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Membership Problem Given: a. A polytope P R n. b. An oracle for optimizing linear objectives over P. c. x* R n Question: Find p 1,…,p k P giving x* as a convex combination, or find (a, ) such that a T x* > and a T x x P

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Solving Membership: Delayed Column Generation Step 1: Find 1,…, k such that j p j = x* j = 1 j 0 (all j) Succeed: Done Fail: LP code gives (a, ) such that a T x* > and a T p j for j = 1,…,k. Step 2: Use the oracle to solve = Maximize a T y Subject to y P Let y* P satisfy = a T y*. Case 1: done Case 2: Append y* to the list p 1,…,p k and return to Step 1.

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Application to MIP For the given MIP: Let x* be a solution of an LP relaxation minimize {c T x: Ax=b, l x u} and assume x* is not integral feasible. Let P = polyhedron defined by a subset of Ax=b together with l x u. Prototypical choice: A single constraint. Apply delayed column generation using MIP code itself as the optimization oracle.

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Future? Linear programming –The improvements in the last 2 years were completely unanticipated! –Examine behavior of even larger and more-difficult models. Mixed-integer programming –Increasingly exploit special structure.

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