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  1. ROOT
  2. ROOT-10114

Continued work on TMVA Hyper parameter optimisation

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    • Type: Task
    • Status: Open (View Workflow)
    • Priority: Medium
    • Resolution: Unresolved
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: TMVA
    • Labels:
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    • Development:

      Description

      The below discussion was triggered by the following forum post: https://root-forum.cern.ch/t/proper-obtaining-and-comparison-of-results-from-hyperparameteroptimisation/33838/4

      Currently the hyper parameter optimisation use case is made available in TMVA through the OptimizeConfigParameter and HyperParameterOptimisation classes.

      OptimizeConfigParameter: The work horse of the HPO. Currently does a grid search on a pre-determied range, specialised for each method that supports it. (BDT, SVM, ...)

      HyperParameterOptimisation: Incomplete wrapper that aims to provide some additional functionality on top of OptimizeConfigParameter, namely handling of a three way split (train/valid/test), and support for uncertainty analysis (through multiple fold evaluations).

      Current situation
      I think the situation is thus: HPO is provided through OptimizeConfigParameters, with the notable problem that it provides only a non-user configurable search space and no possible way of splitting data into subsets beyond the TMVA standard training/test.

      The HyperParameterOptimisation class was introduced with the intent of addressing these shortcomings but, for whatever reason, was never finished. It currently support splitting the data into multiple sets (train/valid/test), but cannot access the test set, leaving the user in a state where it's difficult to evaluate the optimised parameters on independent data. This problem is compounded by the fact the class is separate from the Factory with its Train/Test/EvaluateAllMethods.

      Additionally the HyperParameterOptimisation class supports multiple optimisations on slightly decorrelated datasets using k-folds. (Note: This is not nested cross-validation.) It is unclear, however, how to use the results of this optimisation in the case of K>1. Also, the results are not actually saved (incomplete implementation).

      As such it is unclear if we want a separate class for HPO. Rather it should be considered part of the normal workflow, but instead of fixing some parameters you specify a range.

      Currently missing:

      • User-visible way for performing the 3-way split (currently provided by HyperParameterOptimisation class).
      • User-visible way of moving data from original training set and test set, back to the "active" training and test sets.
      • A user-visible way to specify parameter search space.
      • Reporting of relevant metrics to the user. E.g. return figure-of-merits for all points evaluated incl. the final, best, figure-of-merit.

      One potential way of integrating into the TMVA workflow could then be:

      TMVA::Factory factory{"<name of factory>", "<factory options>"};
      TMVA::DataLoader dataloader{"<name of dataset>"};
      /* set up dataloader */
       
      auto method = factory.BookMethod(&dataloader, TMVA::Types::kBDT,
                                       "<classifier name>", "<method options>");
       
      // Note: This approach would make HPO incompatible with CV, both would rely on
      //       the same dataset splitting mechanism, which does not support nesting.
      dataloader.PrepareDatasetForHPO(); // Fictional method
      method->OptimizeTuningParameters();
      dataloader.RestoreDatasetFromHPO(); // Fictional method
       
      factory.TrainAllMethods();
      factory.TestAllMethods();
      factory.EvaluateAllMethods();
      

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            • Assignee:
              moneta Lorenzo Moneta
              Reporter:
              kialbert Kim Albertsson
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