Try our new documentation site (beta).

Sifting Logging

Sifting will sometimes be used within the dual simplex method, either as a result of an automatic choice by the Gurobi Optimizer or because the user selected it through the Sifting parameter. The sifting log consists of three sections: the presolve section, the sifting progress section, and the summary section. The first and last are identical to those for simplex, so we'll only discuss the middle section here.

Sifting Progress Section

As we mentioned, output for sifting and dual simplex are indistinguishable until the progress section begins. For sifting, the progress section begins with a clear indication that sifting has been selected:

Starting sifting (using dual simplex for sub-problems)...
The sifting algorithm performs a number of major iterations, where each iteration solves a smaller LP sub-problem. It uses the result to update the current primal and dual solution. The sifting log prints one line per major iteration, with information on the current primal and dual objective values:
    Iter     Pivots    Primal Obj      Dual Obj        Time
       0          0     infinity      2.0000000e+01     11s
       1       4662   1.5220652e+03   2.7034420e+02     12s
       2       8917   1.3127217e+03   4.6530259e+02     13s
       3      16601   1.1651147e+03   6.4767742e+02     17s
       4      30060   1.0881514e+03   7.8842688e+02     29s
       5      45169   1.0618879e+03   8.8656855e+02     46s
       6      59566   1.0549766e+03   9.5404159e+02     64s
       7      73614   1.0540577e+03   1.0172213e+03     82s
The first column in the log gives the major iteration number. The second shows the total number of simplex iterations performed in solving the sifting sub-problems. The third and fourth columns show the primal and dual objective values for the current solution. The final column shows elapsed runtime.

The completion of sifting is indicated with the following message:

Sifting complete
The basis computed by sifting is then handed back to dual simplex, and the log from that point forward comes from the dual simplex algorithm.

Try Gurobi for Free

Choose the evaluation license that fits you best, and start working with our Expert Team for technical guidance and support.

Evaluation License
Get a free, full-featured license of the Gurobi Optimizer to experience the performance, support, benchmarking and tuning services we provide as part of our product offering.
Academic License
Gurobi supports the teaching and use of optimization within academic institutions. We offer free, full-featured copies of Gurobi for use in class, and for research.
Cloud Trial

Request free trial hours, so you can see how quickly and easily a model can be solved on the cloud.

Search

Gurobi Optimization