Machine Learning, Artificial Intelligence, Model Selection, Onepagecode, Scikit Learn.
Parallel training of models is defined by the parameter n_jobs. We told scikit-learn to use all cores by setting n_jobs to -1. Nevertheless, it only uses one core by default, since n_jobs is set to 1. We can see this by running the same GridSearchCV as in the solution, but with n_jobs=1, which