```r
# hyperparamter tuning
set.seed(02082018)
tune = tuneParams(learner = lrn_rf, 
                  task = task,
                  resampling = perf_level,
                  par.set = ps,
                  control = ctrl, 
                  measures = mlr::rmse)
#>...
#> [Tune-x] 49: mtry=3; sample.fraction=0.533; min.node.size=5
#> [Tune-y] 49: rmse.test.rmse=0.5636692; time: 0.0 min
#> [Tune-x] 50: mtry=1; sample.fraction=0.68; min.node.size=5
#> [Tune-y] 50: rmse.test.rmse=0.6314249; time: 0.0 min
#> [Tune] Result: mtry=4; sample.fraction=0.887; min.node.size=10 :
#> rmse.test.rmse=0.5104918
```