Using the Gurobi solver

A number of modules in xserver use a so-called solver to perform complex mathematical calculations quickly. While a solid solver is already shipped with xserver at no extra cost, we support accelerating certain modules by using the commercial Gurobi solver instead.

Modules which can benefit from Gurobi are:

• xcluster

When is it worth considering an alternative solver?

• If you regularly experience excessive run-time or have throughput issue with the default solver.
• If you want to process problem instances of unusual size the default solver cannot cope with.

To find out whether you can benefit from switching to Gurobi, please contact our support.

What do I need to use Gurobi with xserver?

Gurobi is a third party product and has to be licensed separately of xserver.

• If you do not already possess a Gurobi license please contact our support. They will be able to help you decide on and procure a matching license for you.
• If you already have a Gurobi license (version 7 or later), you are able to re-use that existing license.

Be aware that the type of license can affect the number of queries a server can process in parallel. Please see the next section for more information.

How to configure xserver to use Gurobi

To enable Gurobi in xserver you need to perform the following steps:

• Ensure that a GRB_LICENSE_FILE environment variable exists that contains the full path including the file name and file extension of your gurobi license file.
• Edit conf/xserver.conf and set the solverType for all modules you want to use Gurobi with to Gurobi. E.g. for xcluster this would be:

  xcluster {
  	solverType = "Gurobi"
  }

• Ensure that your core modules configuration in conf/xserver.conf does not include Gurobi enabled modules in more back-end instances than your Gurobi license allows for. You can achieve this by creating a Gurobi specific modules definition. Assuming your license allows for no parallel use of Gurobi, but you still want to be able to handle parallel xroute and xdima requests at the same time, the core modules configuration could look like the following:

  core {
  	modules {
  		definitions {
  			default {
  				services = [xroute, xdima]
  			}
  
  			gurobiEnabled {
  				services = [xcluster]
  			}
  		}
  		instances {
  			default = 4 // Still allow four parallel xroute or xdima requests
  			gurobiEnabled = 1 // Only allow a single Gurobi related one
  		}
  	}
  	...
  }

  If you are unsure about the best configuration for your back-end instances, our support will be able to assist you.

• Restart the server and validate the modules are still functional.