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KNITRO.jl

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KNITRO.jl is a wrapper for the Artelys Knitro solver.

It has two components:

KNITRO.jl v0.14.0 introduced a number of breaking changes to the low-level C API. See the Low-level wrapper section for details.

Affiliation

This wrapper is maintained by the JuMP community with help from Artelys.

Contact Artelys support if you encounter any problem with this interface or the solver.

License

KNITRO.jl is licensed under the MIT License.

The underlying solver is a closed-source commercial product for which you must purchase a license.

Installation

First, obtain a license and install a copy of KNITRO from Artelys.

Then, install KNITRO.jl using the Julia package manager:

import Pkg
Pkg.add("KNITRO")

If you are having trouble installing KNITRO.jl, here are several things to try:

  • Make sure that you have defined your global variables correctly, for example with export KNITRODIR="/path/to/knitro-vXXX-$OS-64" and export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$KNITRODIR/lib". You can check that KNITRO.jl sees your library with using KNITRO; KNITRO.has_knitro().
  • If KNITRO.has_knitro() returns false but you are confident that your paths are correct, try running Pkg.build("KNITRO") and restarting Julia. In at least one user's experience, installing and using KNITRO in a temporary Julia environment (activated with ] activate --temp) does not work and the need to manually build is likely the reason why.

Use with JuMP

To use KNITRO with JuMP, use KNITRO.Optimizer:

using JuMP, KNITRO
model = Model(KNITRO.Optimizer)
set_attribute(model, "outlev", 1)
set_attribute(model, "algorithm", 4)

Use with AMPL

To use KNITRO with AmplNLWriter.jl, use KNITRO.amplexe:

using JuMP
import AmplNLWriter
import KNITRO
model = Model(() -> AmplNLWriter.Optimizer(KNITRO.amplexe, ["outlev=3"]))

Use with other packages

A variety of packages extend KNITRO.jl to support other optimization modeling systems. These include:

MathOptInterface API

The Knitro optimizer supports the following constraints and attributes.

List of supported objective functions:

List of supported variable types:

List of supported constraint types:

List of supported model attributes:

Options

A list of available options is provided in the KNITRO reference manual.

Low-level wrapper

The complete C API can be accessed via KNITRO.KN_xx functions, where the names and arguments are identical to the C API.

See the KNITRO documentation for details.

As general rules when converting from Julia to C:

  • When KNITRO requires a Ptr{T} that holds one element, like double *, use a Ref{T}().
  • When KNITRO requires a Ptr{T} that holds multiple elements, use a Vector{T}.
  • When KNITRO requires a double, use Cdouble
  • When KNITRO requires an int, use Cint
  • When KNITRO requires a NULL, use C_NULL

Extensive examples using the C wrapper can be found in examples/.

Breaking changes

KNITRO.jl v0.14.0 introduced a number of breaking changes to the low-level C API. The main changes were:

  1. removing Julia-specific functions like KN_set_param. Use the C API functions like KN_set_int_param and KN_set_double_param_by_name.
  2. removing intermediate methods that tried to make the C API more Julia-like. For example, we have removed the KN_add_var method that returned the index of the variable. There is now only the method from the C API.

If you have trouble updating, please open a GitHub issue.

Multi-threading

Due to limitations in the interaction between Julia and C, KNITRO.jl disables multi-threading if the problem is nonlinear. This will override any options such as par_numthreads that you may have set.

If you are using the low-level API, opt-in to enable multi-threading by calling KN_solve(model.env) instead of KN_solve(model), where model is the value returned by model = KN_new(). Note that calling KN_solve(model.env) is an advanced operation because it requires all callbacks you provide to be threadsafe.

Read GitHub issue #93 for more details.