ILOG CPLEX 11.0 User's Manual

Root Relaxation and Parallel MIP ProcessingIn some models, the continuous root relaxation takes significant solution time before parallelbranching begins. These models have potential for additional speed increases by running theroot relaxation step in parallel. If the root problem is an LP or QP and the Threadsparameter is set to a value greater than 1 (one), the concurrent optimizer is invoked bydefault. This provides a form of parallelism that assigns the available threads to multipleoptimizers. If the root problem is a QCP, the barrier optimizer alone is used.You can try a different form of parallelism at the root by selecting the barrier optimizerspecifically with the starting algorithm parameter:◆◆RootAlg in Concert Technology;CPX_PARAM_STARTALG in the Callable Library;◆ set mip strategy startalgorithm in the Interactive Optimizer.In such a case, the parallel threads will all be applied to the barrier algorithm.Memory Considerations and the Parallel MIP OptimizerBefore the parallel MIP optimizer invokes parallel processing, it makes separate, internalcopies of the initial problem. The individual processors use these copies during computation,so each of them requires an amount of memory roughly equal to the size of the presolvedmodel.Output from the Parallel MIP OptimizerThe parallel MIP optimizer generates slightly different output in the node log (see ProgressReports: Interpreting the Node Log on page 283) from the sequential MIP optimizer. Thefollowing paragraphs explain those differences.ILOG CPLEX prints a summary of timing statistics specific to the parallel MIP optimizer atthe end of optimization. You can see typical timing statistics in the following sample run.Problem 'fixnet6.mps.gz' read.Read time = 0.02 sec.CPLEX> oTried aggregator 1 time.MIP Presolve modified 308 coefficients.Aggregator did 1 substitutions.Reduced MIP has 477 rows, 877 columns, and 1754 nonzeros.Presolve time = 0.00 sec.Clique table members: 2.MIP emphasis: balance optimality and feasibility.Root relaxation solution time = 0.00 sec.NodesCuts/Node Left Objective IInf Best Integer Best Node ItCnt Gap* 0+ 0 97863.0000 3192.0420 60 96.74%* 0+ 0 4505.0000 3192.0420 60 29.14%498 ILOG CPLEX 11.0 — USER’ S MANUAL