ILOG CPLEX 10.2 User's Manual > Advanced Programming Techniques > Parallel Optimizers > Parallel MIP Optimizer > Output from the Parallel MIP Optimizer |
Output from the Parallel MIP Optimizer |
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The parallel MIP optimizer generates slightly different output in the node log (see Progress Reports: Interpreting the Node Log) from the serial MIP optimizer. The following paragraphs explain those differences.
If the MIP optimizer is running in parallel, it will display elapsed time for the MIP optimizer in wall-clock time, independent of the setting of the clock-type parameter (assuming MIP logging has not been turned off).
ILOG CPLEX prints a summary of timing statistics specific to the parallel MIP optimizer at the 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> o Tried aggregator 1 time. MIP Presolve modified 308 coefficients. Aggregator did 1 substitutions. Reduced MIP has 477 rows, 877 columns, and 1754 nonzeros. Presolve time = 0.02 sec. Clique table members: 2 MIP emphasis: balance optimality and feasibility Root relaxation solution time = 0.07 sec. Nodes Cuts/ Node Left Objective IInf Best Integer Best Node ItCnt Gap 0 0 3192.0420 12 3192.0420 8 * 0+ 0 0 4505.0000 3192.0420 8 29.14% 3384.5860 15 4505.0000 Cuts: 36 51 24.87% 3513.7923 17 4505.0000 Cuts: 25 92 22.00% 3530.1967 19 4505.0000 Flowcuts: 9 104 21.64% * 0+ 0 0 4471.0000 3530.1967 104 21.04% 3604.4590 17 4471.0000 Flowcuts: 10 124 19.38% 3607.9420 18 4471.0000 Flowcuts: 4 131 19.30% * 0+ 0 0 4448.0000 3607.9420 131 18.89% 3608.7548 20 4448.0000 Flowcuts: 3 136 18.87% 3617.6257 19 4448.0000 Flowcuts: 2 141 18.67% 3624.7454 19 4448.0000 Flowcuts: 2 150 18.51% 3627.2428 20 4448.0000 Flowcuts: 1 152 18.45% * 0+ 0 0 3994.0000 3627.2428 152 9.18% * 30+ 5 0 3985.0000 3736.8034 305 6.23% * 46 0 0 3983.0000 3972.7760 326 0.26% Root relaxation processing (before b&c): CPU time = 0.60 Parallel b&c, 2 threads: Real time = 0.72 Critical time (total) = 0.00 Spin time (average) = 0.01 ------- Total (sequential+parallel) = 1.32 sec. Cover cuts applied: 1 Flow cuts applied: 38 Gomory fractional cuts applied: 9 Integer optimal solution: Objective = 3.9830000000e+03 Solution time = 0.90 sec. Iterations = 328 Nodes = 47 CPLEX> q
The summary at the end of the sample says that 0.60 of a second was spent in the phase of processing the root relaxation, that is, all the combined steps (preprocessing, root relaxation solution, cutting planes, heuristic) that occur at the root before the first branch occurs. The parallel part of this sample run took 0.72 of a second of real time (that is, elapsed time for that phase).
Other parts of the sample report indicate that the processors spent an average of 0.01 of a second of real time spinning (that is, waiting for work while there were too few active nodes available). The real critical time was a total of 0.00 seconds, time spent by individual processors in updating global, shared information. Since only one processor can access the critical region at any instant in time, the amount of time spent in this region really is crucial: any other processor that tries to access this region must wait, thus sitting idle, and this idle time is counted separately from the spin time.
There is another difference in the way logging occurs in the parallel MIP optimizer. When this optimizer is called, it makes a number of copies of the problem. These copies are known as clones. The parallel MIP optimizer creates as many clones as there are threads available to it. When the optimizer exits, these clones and the memory they used are discarded.
If a log file is active when the clones are created, then ILOG CPLEX creates a clone log file for each clone. The clone log files are named cloneK.log
, where K
is the index of the clone, ranging from 0 (zero) to the number of threads minus one. Since the clones are created at each call to the parallel MIP optimizer and discarded when it exits, the clone logs are opened at each call and closed at each exit. (The clone log files are not removed when the clones themselves are discarded.)
The clone logs contain information normally recorded in the ordinary log file (by default, cplex.log
) but inconvenient to send through the normal log channel. The information likely to be of most interest to you are special messages, such as error messages, that result from calls to the LP optimizers called for the subproblems.
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