sklearn convergence warning

Below warning what i was getting while executing sk learn logistic regression. 

ConvergenceWarning: lbfgs failed to converge (status=1):

STOP: TOTAL NO. of ITERATIONS REACHED LIMIT.

Increase the number of iterations (max_iter) or scale the data as shown in:

    https://scikit-learn.org/stable/modules/preprocessing.html

Please also refer to the documentation for alternative solver options:

    https://scikit-learn.org/stable/modules/linear_model.html#logistic-regression

  n_iter_i = _check_optimize_result(

The warning means what it mainly says: Suggestions to try to make the solver (the algorithm) converges.

lbfgs stand for: "Limited-memory Broyden–Fletcher–Goldfarb–Shanno Algorithm". It is one of the solvers' algorithms provided by Scikit-Learn Library.

The term limited-memory simply means it stores only a few vectors that represent the gradients approximation implicitly.

It has better convergence on relatively small datasets.

But what is algorithm convergence?

In simple words. If the error of solving is ranging within very small range (i.e., it is almost not changing), then that means the algorithm reached the solution (not necessary to be the best solution as it might be stuck at what so-called "local Optima").

On the other hand, if the error is varying noticeably (even if the error is relatively small [like in your case the score was good], but rather the differences between the errors per iteration is greater than some tolerance) then we say the algorithm did not converge.

Now, you need to know that Scikit-Learn API sometimes provides the user the option to specify the maximum number of iterations the algorithm should take while it's searching for the solution in an iterative manner:

LogisticRegression(... solver='lbfgs', max_iter=100 ...)

As you can see, the default solver in LogisticRegression is 'lbfgs' and the maximum number of iterations is 100 by default.

Final words, please, however, note that increasing the maximum number of iterations does not necessarily guarantee convergence, but it certainly helps!

references:

https://stackoverflow.com/questions/62658215/convergencewarning-lbfgs-failed-to-converge-status-1-stop-total-no-of-iter