Thread Limit

Type

Integer

Range

{-1 .. 2000000000}

Default

0

This option controls the number of threads to apply to parallel algorithms (parallel MIP, concurrent MIP, concurrent LP and parallel barrier). The default value of 0 is an automatic setting. It will generally use as many threads as there are virtual processors. The number of virtual processors may exceed the number of cores due to hyperthreading or other similar hardware features.

While you will generally get the best performance by using all available cores in your machine, there are a few exceptions. One is of course when you are sharing a machine with other jobs. In this case, you should select a thread count that doesn’t oversubscribe the machine.

Certain classes of MIP models benefit from reducing the thread count, often all the way down to one thread. Starting multiple threads introduces contention for machine resources. For classes of models where the first solution found by the MIP solver is almost always optimal, and that solution isn’t found at the root, it is often better to allow a single thread to explore the search tree uncontested.

Another situation where reducing the thread count can be helpful is when memory is tight. Each thread can consume a significant amount of memory.

Gurobi made the pragmatic choice to impose a soft limit of 32 threads for the automatic setting (0), because usually, Gurobi’s algorithms do not benefit from higher thread counts. Actually, higher thread counts may even hurt performance, because this will often saturate the memory system. If your machine has more virtual processors, and you find that using more threads increases performance, you should feel free to set this option to a larger value. Alternatively, you can use the value -1 to indicate that Gurobi should use all available virtual processors, even if the machine has more than 32.

Note

  • The algorithms for parallel MIP, parallel barrier and concurrent LP, if the option Method is set to ‘Deterministic concurrent’, are deterministic which means that multiple runs with the same model at the same option settings on the same platform will reproduce the same solution path and results.

  • This option also limits the number of threads used by the Concurrent MIP solver.

  • The special value -1 was added in Gurobi 13.0.

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