7th ICML Workshop on Automated Machine Learning (2020)

H2O AutoML: Scalable Automatic Machine Learning
E. LeDellerin@h2o.ai

H2O.ai, USA

S. Poirier

sebastien@h2o.ai

H2O.ai, USA

Abstract
H2O is an open source, distributed machine learning platform designed to scale to verylarge datasets, with APIs in R, Python, Java and Scala. We present H2O AutoML, ahighly scalable, fully-automated, supervised learning algorithm which automates the process of training a large selection of candidate models and stacked ensembles within a single
function. The result of the AutoML run is a “leaderboard”: a ranked list of models, all ofwhich can be easily exported for use in a production environment. Models in the leaderboard can be ranked by numerous model performance metrics or other model attributes
such as training time or average per-row prediction speed.
The H2O AutoML algorithm relies on the efficient training of H2O machine learning algorithms to produce a large number of models in a short amount of time. H2O AutoML uses
a combination of fast random search and stacked ensembles to achieve results competitive
with, and often better than, other frameworks which rely on more complex model tuning
techniques such as Bayesian optimization or genetic algorithms. H2O AutoML trains a variety of algorithms (e.g. GBMs, Random Forests, Deep Neural Networks, GLMs), yieldinga healthy amount of diversity across candidate models, which can be exploited by stacked
ensembles to produce a powerful final model. The effectiveness of this technique is reflectedin the OpenML AutoML Benchmark, which compares the performance of several of themost well known, open source AutoML systems across a number of datasets.

1. Introduction
There have been big strides in the development of user-friendly machine learning software
which features simple, unified interfaces to a variety of machine learning algorithms (e.g.
scikit-learn, H2O, caret, tidymodels, mlr). Although these tools have made it easy for nonexperts to train machine learning models, there is still a fair bit of expertise that is requiredin order to achieve state-of-the-art results. Automatic machine learning or “AutoML” tools
provide a simple interface to train a large number of models (or a powerful single model), canbe a helpful tool for either a novice or advanced machine learning practitioner. Simplifying
training and tuning of machine learning models by offering a single function to replace aprocess that would typically require many lines of code, frees the practitioner to focus onother aspects of the data science pipeline, such as data-preprocessing, feature engineeringand model deployment.
H2O AutoML (H2O.ai, 2017) is an automated machine learning algorithm included in
the H2O framework (H2O.ai, 2013) that is simple to use and produces high quality models
that are suitable for deployment in a enterprise environment. H2O AutoML supports supervised training of regression, binary classification and multi-class classification models on
tabular datasets. One of the benefits of H2O models is the fast scoring capabilities – many
©2020 E. LeDell and S. Poirier.

E. LeDell and S. Poirier

H2O models can generate predictions in sub-millisecond scoring times. H2O AutoML offers
APIs in several languages (R, Python, Java, Scala) which means it can be used seamlessly
within a diverse team of data scientists and engineers. It is also available via a point-andclick H2O web GUI called Flow1 , which further reduces the barriers to widespread use of
automatic machine learning. H2O also has tight integrations to big data computing platforms such as Hadoop2 and Spark3 and has been successfully deployed on supercomputers4
in a variety of HPC environments (e.g. Slurm).
The H2O AutoML algorithm was first released in June, 2017 in H2O v3.12.0.1 so it hasgone through many iterations of development over the past three years. It is widely used
in industry and academia and has many advocates in the open source machine learning
community. The algorithm is constantly evolving and improving in each new version of
H2O, so this paper serves as a snapshot of the algorithm in time (May, 2020). A full list of
in-development and planned improvements and new features is available on the H2O bug
tracker website.5

2. H2O AutoML
H2O AutoML is a fully automated supervised learning algorithm implemented in H2O, theopen source, scalable, distributed machine learning framework. H2O AutoML is available
in Python, R, Java and Scala as well as through a web GUI. Though the algorithm is fully
automated, many of the settings are exposed as parameters to the user, so that certain
aspects of the modeling steps can be customized.
2.1 Data pre-processing
H2O AutoML currently provides the same type of automatic data-preprocessing that’s
provided by all H2O supervised learning algorithms. This includes automatic imputation,
normalization (when required), and one-hot encoding for XGBoost models. H2O tree-basedmodels (Gradient Boosting Machines, Random Forests) support group-splits on categoricalvariables, so categorical data can be handled natively. In experimental versions of thealgorithm, we have benchmarked various automatic target encoding6 strategies for highcardinality features, though that’s not yet available in the current stable release (H2O
v3.30.0.3). Additional data pre-processing steps such as automatic text encoding using
Word2Vec7 , as well as feature selection and feature extraction for automatic dimensionality
reduction are all part of the H2O AutoML roadmap.

1. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/flow.html
2. https://hadoop.apache.org/
3. https://spark.apache.org/
4. https://scinet.usda.gov/user/geospatial/#tools-and-software
5. https://0xdata.atlassian.net/issues/?filter=21603
6. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-munging/target-encoding.html
7. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/word2vec.html

2

H2O AutoML

2.2 Models
2.2.1 Base Models
H2O AutoML includes XGBoost Gradient Boosting Machines (GBM)8 , as well as H2O Gradient Boosting Machines (GBM)9 , Random Forests10 (Default and Extremely Randomized
Tree variety), Deep Neural Networks11 and Generalized Linear Models (GLM)12 . H2O offers
a wrapper around the popular XGBoost software, so we are able to include this third-party
algorithm in H2O AutoML. This also allows GPU acceleration of training.
The current version of H2O AutoML (H2O v3.30.0.3) trains and cross-validates (whennfolds > 1) models: three pre-specified XGBoost GBM models, a fixed grid of H2O GLMs,
a default H2O Random Forest (DRF), five pre-specified H2O GBMs, a near-default H2O
Deep Neural Net, an H2O Extremely Randomized Trees (XRT) model, a random grid of
XGBoost GBMs, a random grid of H2O GBMs, and a random grid of H2O Deep Neural Nets.
For each algorithm, we identified which hyperparamters we consider to be most important,defined ranges for those parameters and utilize random search to generate models. A list of
the hyperparameters and the ranges we explored for each algorithm in the H2O AutoML
process is documented in the user guide.13 Which hyperparamters to consider, as well as
their ranges, were decided upon based on benchmarking as well as the experience of expert
data scientists (e.g. Kaggle Grandmasters14 ), and this is something we consistently try to
improve upon over time via extensive benchmarking.
The pre-specified models are included to give quick, reliable defaults for each algorithm.
The order of the algorithms, which can be customized by the user, is set to start with models
that consistently provide good results (pre-specified XGBoost models) across a wide variety
of datasets, followed by a tuned GLM for a quick reference point. From here we prioritize
increasing the diversity across our set of models (for the sake of the final Stacked Ensembles)by introducing a few Random Forests, (H2O) GBM and Deep Learning models. After thisset of prescribed models is trained and added to the leaderboard, we begin a random
search across those same algorithms. The proportion of time spent on each algorithm in
the AutoML run is explicitly defined to give some algorithms (e.g. XGBoost GBM, H2O
GBM) more time than others (e.g. H2O Deep Learning), according to our perceived or
estimated “value” of each task. In H2O v3.30.0.1, we introduced an experimental parameter,
exploitation ratio, which, if activated, fine-tunes the learning rate of the best XGBoost
GBM model and best H2O GBM model and if a better model is found, adds it to theleaderboard.
2.2.2 Stacked Ensembles
After training the base models, two Stacked Ensemble models are trained using H2O’s
Stacked Ensemble algorithm15 . Stacked Ensembles, also called Stacking or Super Learning,
8. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/xgboost.html
9. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/gbm.html
10. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/drf.html
11. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/deep-learning.html
12. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/glm.html
13. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/automl.html#random-grid-search-parameters
14. https://www.kaggle.com/rankings
15. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/stacked-ensembles.html

3

E. LeDell and S. Poirier

is a class of algorithms that involves training a second-level metalearner to find the optimal
combination of the base learners. Stacked Ensembles perform particularly well if the basemodels are individually strong and make uncorrelated errors. Random search across avariety of algorithm families produces a very diverse set of base models, and when pairedwith stacking, produces powerful ensembles.
The All Models ensemble contains all the models, and the Best of Family ensemble
contains the best performing model from each algorithm class/family. In particular, that
includes one XGBoost GBM, H2O Random Forest, H2O Extremely Randomized Tree Forest, H2O GBM, H2O Deep Learning, and H2O GLM model. The Best of Family ensemble
is optimized for production use cases since it only contains six (or fewer) base models andcan generate predictions rapidly compared the All Models ensemble. The Best of Family
ensemble will usually have slightly lower model performance, however it’s usually still ajump in performance over the top base model. Generally, both of the ensembles produce
better models than any individual model from the AutoML run, however in rare cases,
models such as a simple GLM can rise to the top of the leaderboard.
By default, the metalearner in the Stacked Ensemble will be trained using the k -foldcross-validated predictions from the base learners. This version of stacking is called theSuper Learner algorithm (Laan et al., 2007) and has been proven to represent an asymptotically optimal system for learning. In cases where the data is very large or there is atime-dependency across rows in the data, it is not advised to use cross-validation. Useof a holdout blending frame is the recommended way to train a Stacked Ensemble in this
scenario (described in Appendix C).
2.3 APIThe H2O AutoML interface is designed to have as few parameters as possible so that all
the user needs to do is point to their dataset, identify the response column and optionallyspecify a time constraint or limit on the number of total models trained.
Python and R interfaces:
aml = H2OAutoML(max_runtime_secs=3600)
aml.train(y="response_colname", training_frame=train)
aml <- h2o.automl(y = "response_colname", training_frame = train,
max_runtime_secs = 3600)
In both the R and Python API, H2O AutoML uses the same data-related arguments,
as the other H2O algorithms (e.g. x, y, training frame). The user can configure values
for max runtime secs and/or max models to set explicit time or number-of-model limits
on your run. The only required user-defined parameters of H2O AutoML are the data
specification. If a stopping criteria (max runtime secs and/or max models), is not set by
the user, the AutoML run will execute for 1 hour. For Python users, there is also scikit-learncompatible API for H2O AutoML16 included in the H2O python package, which exposes
the standard scikit-learn methods (e.g. fit, predict, score), and supports integration
into scikit-learn (Pedregosa et al., 2011a) pipelines.
16. https://github.com/h2oai/h2o-tutorials/blob/master/tutorials/sklearn-integration/README.md

4

H2O AutoML

2.4 Leaderboard
The AutoML object includes a leaderboard which ranks all models trained by model performance. They are ranked by cross-validated performance by default, unless a holdout frameto score and rank models for the leaderboard is provided via the leaderboard frame argument. An example leaderboard is shown in Table 1. Other performance metrics, such as
training time and per-row prediction speed can be computed and added to the leaderboard
via the H2OAutoML.get leaderboard() and h2o.get leaderboard() functions in Python
and R, respectively. In production use cases, the top performing model which can generatepredictions faster than a certain speed threshold might be chosen over the top model on
the leaderboard, ranked by model accuracy (which is usually a Stacked Ensemble).

3. Performance
3.1 Accuracy
The OpenML AutoML Benchmark (Gijsbers et al., 2019) is an ongoing, and extensible
benchmarking framework which follows best practices and avoids common mistakes in machine learning benchmarking. The 2019 benchmark compares four popular AutoML systems
across 39 classification datasets using the framework, and there is continued work to integrate more systems and datasets as well as extending the benchmark to include regression.
Open Source AutoML systems. Some of the most well known open source AutoML
systems for tabular data are Auto-WEKA (Thornton et al., 2013), auto-sklearn (Feurer
et al., 2015), TPOT (Olson et al., 2016) and H2O AutoML, which are the four systems
evaluated in the 2019 OpenML AutoML Benchmark. Some other projects, not part ofthe original benchmark, but worth mentioning are hyperopt-sklearn (Bergstra et al., 2015),
autoxgboost (Thomas et al., 2018), ML-Plan (Mohr et al., 2018), OBOE (Yang et al.,2018), GAMA (G. and V., 2019), TransmogrifAI (Salesforce.com, 2018), Auto-keras (Jinet al., 2019), and the newly released AutoGluon-Tabular (Erickson et al., 2020).
The 2019 edition of the benchmark, published in Gijsbers et al. (2019), concluded thatno one system consistently out-performed (based on accuracy) all the other systems on
small or medium-sized (≤ 50,000 rows) classification datasets on simple hardware (8 CPU
cores, 32G RAM). In Truong et al. (2019), another AutoML benchmark on 300 datasets
which focused on shorter runtimes (≤ 1 hour), concluded that H2O AutoML, Auto-keras
and auto-sklearn performed better than the other systems. In particular, they noted that
H2O AutoML “slightly outperforms the others for binary classification and regression, andquickly converges to the optimal results”.
We used the OpenML AutoML benchmark framework to run an updated version17 of
the benchmark, removing Auto-WEKA due to poor results on the previous benchmark
and adding AutoGluon. We added 5 datasets that were part of the “validation” set18 in
the benchmark, for a total of 44. We used the latest stable version of all the systems, asof May 2019. Figure 4 shows results which are very close among most tools for binaryclassification. In Figure 3, we can see that H2O AutoML performs favorably compared tomost algorithms in a variety of data types. AutoGluon showed strong results for multiclass
17. http://github.com/h2oai/h2o-automl-paper
18. https://github.com/openml/automlbenchmark/blob/master/resources/benchmarks/validation.yaml

5E. LeDell and S. Poirier

problems, however not as strong as stated in Erickson et al. (2020) because all non-default
options in AutoGluon were turned off, as required by the OpenML benchmark.
3.2 Scalability & Speed
Due to the efficient implementations of the algorithms and the distributed nature of the
H2O platform, H2O AutoML can scale to large datasets (e.g. 100M+ rows) as shownin Section E.1. Training of individual models is parallelized across CPU cores on a single
machine, or across a cluster of networked machines in a multinode setting. XGBoost models,
which are included in the AutoML algorithm by default, also support GPU acceleration forfurther speed-up in training. Since a significant portion of training is usually dedicated to
XGBoost models, H2O AutoML benefits from GPU acceleration. Appendix A includes amore detailed discussion about the architecture and scalability of the H2O platform.
One of the benefits of building an AutoML system on top of a fast, scalable machinelearning library, is that you can utilize speed and parallelism to train more models in thesame amount of time as compared to AutoML libraries that are built with slower or less scalable underlying algorithms. As demonstrated in the OpenML AutoML benchmark resultsin Figure 4, this allows us to use simple, straight-forward techniques like random search andstacking to achieve excellent performance in the same amount of time as algorithms which
use more complex tuning techniques such as Bayesian optimization or genetic algorithms.

4. Conclusion
We presented H2O AutoML, an algorithm for automatic machine learning on tabular data,
part of the H2O machine learning platform. H2O excels in the areas of usability andscalability and has a very active and engaged user base in the open source machine learning
community. Key aspects of H2O AutoML include its ability to handle missing or categorical
data natively, it’s comprehensive modeling strategy, including powerful stacked ensembles,
and the ease in which H2O models can be deployed and used in enterprise production
environments. The leaderboard features informative and actionable information such asmodel performance, training time and per-row prediction speed for each model trained in
the AutoML run, ranked according to user preference.
H2O models, including stacked ensembles, can be inspected further using model interpretability tools featured in H2O such as partial dependence plots19 , Shapley values20 , andcan also be used in conjunction with popular third-party interpretabilty tools such as lime21 .
H2O AutoML is a well-tested, widely-used, scalable, production-ready automatic machinelearning system with APIs in R, Python, Java and Scala, and a web GUI. With such wide
language support, it’s a tool that can be used across diverse teams of statisticians, data
scientists and engineers, making it a very practical choice in any heterogeneous team. Itsability to scale to very large datasets also means that it can handle a diverse set use-cases(both large and small data) across an organization, allowing consistent tooling and lessoverhead than relying on different tools for different problems.
19. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/performance-and-prediction.html#partial-dependenceplots
20. https://www.h2o.ai/blog/h2o-release-3-26-yau/
21. https://github.com/marcotcr/lime & https://github.com/thomasp85/lime

6

H2O AutoML

References
James Bergstra, Brent Komer, Chris Eliasmith, Dan Yamins, and David Cox. Hyperopt:
A python library for model selection and hyperparameter optimization. Computational
Science Discovery, 8:014008, 07 2015. doi: 10.1088/1749-4699/8/1/014008.
Stephen Boyd, Neal Parikh, Eric Chu, Borja Peleato, and Jonathan Eckstein. Distributed
optimization and statistical learning via the alternating direction method of multipliers.
Foundations and Trends® in Machine Learning, 3(1):1–122, 2011. ISSN 1935-8237. doi:
10.1561/2200000016. URL http://dx.doi.org/10.1561/2200000016.
Nick Erickson, Jonas Mueller, Alexander Shirkov, Hang Zhang, Pedro Larroy, Mu Li, andAlexander Smola. Autogluon-tabular: Robust and accurate automl for structured data.
arXiv preprint arXiv:2003.06505, 2020.
M. Feurer, A. Klein, K. Eggensperger, J. Springenberg, M. Blum, and F. Hutter. Efficientand robust automated machine learning. In Advances in Neural Information Processing
Systems, pages 2962–2970, 2015.
Pieter G. and Joaquin V. GAMA: Genetic automated machine learning assistant. Journalof Open Source Software, 4(33):1132, jan 2019. doi: 10.21105/joss.01132. URL https://doi.org/10.21105/joss.01132.
P. Gijsbers, E. LeDell, S. Poirier, J. Thomas, B. Bischl, and J. Vanschoren. An open
source automl benchmark. 6th ICML Workshop on Automated Machine Learning,2019. URL https://www.automl.org/wp-content/uploads/2019/06/automlws2019_
Paper45.pdf.
H2O.ai. H2O: Scalable Machine Learning Platform, 2013. URL https://github.com/
h2oai/h2o-3. First version of H2O was released in 2013.
H2O.ai. H2O AutoML, June 2017. URL http://docs.h2o.ai/h2o/latest-stable/
h2o-docs/automl.html. First released in H2O version 3.12.0.1.
M. Hall, E. Frank, G. Holmes, B. Pfahringer, P. Reutemann, and I.H. Witten. The weka
data mining software: An update. SIGKDD Explor. Newsl., 11(1):10–18, November 2009.
ISSN 1931-0145. doi: 10.1145/1656274.1656278. URL http://doi.acm.org/10.1145/
1656274.1656278.
Haifeng Jin, Qingquan Song, and Xia Hu. Auto-keras: An efficient neural architecture searchsystem. Proceedings of the 25th ACM SIGKDD International Conference on Knowledge
Discovery Data Mining, pages 1946–1956, 2019. URL https://dl.acm.org/doi/10.
1145/3292500.3330648.
Mark Laan, Eric Polley, and Alan Hubbard. Super learner. Statistical applications in
genetics and molecular biology, 6:Article25, 02 2007. doi: 10.2202/1544-6115.1309.
F. Mohr, M. Wever, and E. Hüllermeier. Ml-plan: Automated machine learning via hierarchical planning. Machine Learning, 107(8):1495–1515, Sep 2018. ISSN 1573-0565. doi:
10.1007/s10994-018-5735-z. URL https://doi.org/10.1007/s10994-018-5735-z.
7

E. LeDell and S. Poirier

R.S. Olson, R.J. Urbanowicz, P.C. Andrews, N.A. Lavender, L.C. Kidd, and J.H. Moore.
Applications of Evolutionary Computation: 19th European Conference, EvoApplications
2016, Porto, Portugal, March 30 – April 1, 2016, Proceedings, Part I, chapter Automating Biomedical Data Science Through Tree-Based Pipeline Optimization, pages 123–137. Springer International Publishing, 2016. ISBN 978-3-319-31204-0. doi: 10.1007/
978-3-319-31204-0 9. URL http://dx.doi.org/10.1007/978-3-319-31204-0_9.
F. Pedregosa, G. Varoquaux, A. Gramfort, V. Michel, B. Thirion, O. Grisel, M. Blondel, P. Prettenhofer, R. Weiss, V. Dubourg, J. Vanderplas, A. Passos, D. Cournapeau,
M. Brucher, M. Perrot, and E. Duchesnay. Scikit-learn: Machine learning in Python.
Journal of Machine Learning Research, 12:2825–2830, 2011a.
F. Pedregosa, G. Varoquaux, A. Gramfort, V. Michel, B. Thirion, O. Grisel, M. Blondel,
P. Prettenhofer, R. Weiss, V. Dubourg, et al. Scikit-learn: Machine learning in python.
Journal of machine learning research, 12(Oct):2825–2830, 2011b.
Benjamin Recht, Christopher Re, Stephen Wright, and Feng Niu.
Hogwild: A
lock-free approach to parallelizing stochastic gradient descent.
In J. ShaweTaylor, R. S. Zemel, P. L. Bartlett, F. Pereira, and K. Q. Weinberger, editors, Advances in Neural Information Processing Systems 24, pages 693–
701. Curran Associates, Inc., 2011.
URL http://papers.nips.cc/paper/
4390-hogwild-a-lock-free-approach-to-parallelizing-stochastic-gradient-descent.
pdf.
Salesforce.com.
TransmogrifAI, 2018.
TransmogrifAI.

URL https://github.com/salesforce/

J. Thomas, S. Coors, and B. Bischl. Automatic gradient boosting. In International Workshop on Automatic Machine Learning at ICML, 2018.
C. Thornton, F. Hutter, H. H. Hoos, and K. Leyton-Brown. Auto-WEKA: Combined
selection and hyperparameter optimization of classification algorithms. In Proc. of KDD2013, pages 847–855, 2013.
Anh Truong, Austin Walters, Jeremy Goodsitt, Keegan Hines, C. Bayan Bruss, and Reza
Farivar. Towards automated machine learning: Evaluation and comparison of automl
approaches and tools. 2019 IEEE 31st International Conference on Tools with Artificial
Intelligence (ICTAI), Nov 2019. doi: 10.1109/ictai.2019.00209. URL https://arxiv.
org/abs/1908.05557.
C. Yang, Y. Akimoto, D.W. Kim, and M. Udell. OBOE: collaborative filtering for automl
initialization. CoRR, abs/1808.03233, 2018. URL http://arxiv.org/abs/1808.03233.

8

H2O AutoML

Appendix A. H2O Distributed Machine Learning Platform
The H2O machine learning platform was designed with very large training sets in mind, so
each of the algorithms available in H2O are scalable to larger-than-memory datasets andare trained in a fully parallelized fashion across multiple nodes and cores.
A.1 Architecture
The “H2O Cluster” is the Java process running H2O across one or many computers. The
first task you do when working with H2O is to initialize the cluster, either locally, oron a remote machine. Once the cluster is running, data can be imported from disk into
distributed data frames called “H2O Frames”, and model training is also performed inmemory inside the H2O cluster. The distributed data frame architecture as well as thein-memory computing aspect are similar to Apache Spark, though H2O’s implementation(2013) pre-dates Spark (2014). This is why H2O implements its own distributed computing
infrastructure versus relying upon a popular framework like Spark. At the time H2O was
first introduced, MapReduce and Hadoop were very popular and so H2O is designed towork seamlessly with “big data” platforms such as Hadoop and (later) Spark.
H2O’s core is written in highly optimized Java code, using primitive types (no Java
objects) which gives FORTRAN-like speed. Inside H2O, a lock-free distributed key/value
store (DKV) is used to read and write data (frames, models, objects) asynchronously acrossall nodes and machines. The algorithms are implemented on top of H2O’s distributed
Map/Reduce framework and utilize the Java Fork/Join framework for multi-threading. The
data is read in parallel and is distributed across the cluster and stored in memory in acolumnar format in a compressed way. H2O’s data parser has built-in intelligence to guessthe schema of the incoming dataset and supports data ingest from multiple sources invarious formats.22
A.2 Scalability & Parallelism
To scale training to datasets that cannot fit inside the memory (RAM) of a single machine,
you simply add compute nodes to your H2O cluster. The training set will be distributed
across multiple machines, row-wise (a full row is contained in a single node, so different
rows will be stored on different nodes in the cluster). The Java implementations of the
distributed machine learning algorithms inside H2O are highly optimized and the training
speeds are further accelerated by parallelized training. To reduce communication overhead
between nodes, data is compressed and uncompressed on the fly.
Within the algorithm implementations, there are many optimizations made to speed up
the algorithms. Examples of such optimizations include pre-computing histograms (usedextensively in tree-based methods), so they are available on-demand when needed. H2O
GBM and Random Forest utilize group-splits for categorical columns, meaning that one-hot
encoding (large memory cost) or label encoding (loss of categorical nature) is not necessary. Cutting edge optimization techniques such as the alternating direction method ofmultipliers (ADMM) (Boyd et al., 2011), an algorithm that solves convex optimization
problems by breaking them into smaller pieces, are used extensively, providing both speed
22. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/getting-data-into-h2o.html

9E. LeDell and S. Poirier

and scalability. Optimization techniques are also selected dynamically based on data sizeand shape for further speed-up. For example, the H2O GLM uses a iteratively reweighted
least squares method (IRLSM) with a Gram Matrix approach, which is efficient for tall andnarrow datasets and when running lambda search via a sparse solution. For wider and dense
datasets (thousands of predictors and up), the limited-memory Broyden-Fletcher-GoldfarbShanno (L-BFGS) solver scales better, so in that case, it will be used automatically.23 H2ODeep Learning24 includes many optimizations for speeding up training such as the HOGWILD! (Recht et al., 2011) approach to parelleizing stochastic gradient descent. Whenthe associated optimization problem is sparse, meaning most gradient updates only modify small parts of the decision variable, then HOGWILD! achieves a nearly optimal rateof convergence. Here we have listed a few notable examples, but there are many otheroptimizations included in H2O algorithms, designed to promote speed and scalability.
Random search is an embarrassingly parallel task, which offers additional opportunity forspeed-up. The current stable version of H2O AutoML (H2O 3.30.0.3) parallelizes training
within a single model and partially for cross-validation across all cores of the H2O cluster,
however the random search is executed serially, training a single model at any given time.
H2O grid/random searches have a parallel argument which allows the user to specify
how many models will be trained at once on the H2O cluster. Automatic parallelizationof model training, in which we dynamically decide how many models to train in parallel,
is a work in progress and is planned for a future release of H2O AutoML. The goal is tomaximize the number of models that can be trained at once, given training set size andcompute resources, without overloading the system.

Appendix B. H2O AutoML API

Figure 1: Basic, complete code examples of the H2O AutoML Python and R APIs.

Appendix C. H2O AutoML Customization
Though H2O AutoML is designed to be a fully automatic hyperparamter-free algorithm,
discrete pieces of the algorithm can be modified, turned on/off or re-ordered. Other thancontrolling the length of the AutoML run, the most significant way you modify the algorithm
is by turning on or off or re-ordering the constituent algorithms.
23. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/glm.html#irlsm-and-l-bfgs
24. http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/deep-learning.html

10

H2O AutoML

Families of algorithms, including the Stacked Ensembles, can be switched off using theexclude algos argument. This is useful if you have constraints of preferences about thealgorithms that you’d like to train. Diversity among the set of base models generally
increases the performance of the Stacked Ensemble, though in some cases, it’s practical or
beneficial to turn some algorithms off. The H2O AutoML function can also be used as autility to tune a single algorithm (e.g. "XGBoost") by specifying a single algorithm (andoptionally "StackedEnsemble") with the include algos argument. The modeling planargument offers the ability to re-order discrete steps of the algorithm, such as the gridsearches or pre-specified models. The max runtime allowed per model can also be specified
via the max runtime secs per model argument.
The leaderboard will be scored using cross-validated metrics by default, but the user
can pass in a test frame to the leaderboard frame argument, which will then be used
exclusively to score and rank models for the leaderboard. If the user prefers using a holdout
frame to train the metalearner of the Stacked Ensemble instead of using cross-validation
predictions (for example, if cross-validation is turned off), that frame can be passed to a
blending frame argument. Row weights and cross-validation fold row assignment can be
specified via the weights column and fold column arguments.
The nfolds parameter allows the user to modify how many folds to use for crossvalidation. Cross-validation is used to train Stacked Ensembles, as well as to rank models
on the leaderboard. Fewer folds allow more models to train in the total AutoML runtime,
however more folds generally lead to strong Stacked Ensemble models, so there’s a trade-off
to be considered. On larger datasets (anything over a million rows, for example), especiallyin time-limited scenarios, it can be advantageous to turn off cross-validation and train
the Stacked Ensemble metalearner with the blending frame instead of a cross-validation
predictions frame.
There are arguments related to early stopping of the individual algorithms, grid searches
and AutoML run itself. The model performance metric which should be used for early
stopping (e.g. "AUC", "logloss", "RMSE") is specified via stopping metric, and thesort metric specifies which metric the models on the leaderboard should be sorted by.
The stopping tolerance and stopping rounds parameters allow fine tuning of the early
stopping sensitivity.
An experimental argument called exploitation ratio was introduced in H2O v3.30.0.1,
which allows the user to specify the budget ratio (between 0 and 1) dedicated to the exploitation (vs exploration) phase. By default, the exploitation phase is disabled, however,if activated, the exploitation phase will fine-tune the best XGBoost GBM and the best
H2O GBM found during exploration (random search) and add the tuned models to theleaderboard. The tuned models will also be included in the set of base models for Stacked
Ensembles. Additional fine-tuning may be added to other model types in the future.
Monotonic constraints which control the nature of the relationship between an input
variable and the prediction can be passed through to supporting algorithms (XGBoost andH2O GBMs) via the monotonic constraints argument.25 This may be useful or necessary
in certain industrial or scientific applications. Partial dependence plots, included in H2O,
can be used to observe and validate the monotonicity.
25. https://gist.github.com/ledell/91beb929dcdb04a964f5f580faa48a93

11

E. LeDell and S. Poirier

There are three arguments related to controlling for class imbalance which can beswitched on manually, however we plan to address class imbalance in a more automatic
fashion in future version. The remainder of the the arguments to the H2O AutoML function are related to keeping or deleting certain data artifacts, exporting models, and settinga random seed.

Appendix D. H2O AutoML Output
D.1 Leaderboard
Each AutoML run generates a leaderboard. The models are ranked by a default metricbased on the problem type (the second column of the leaderboard). In binary classification
problems, that metric is AUC, and in multiclass classification problems, the metric is mean
per-class error. In regression problems, the default sort metric is deviance. The user canchange the default leaderboard ranking metric via the sort metric argument.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

model id
StackedEnsemble AllModels AutoML 20200519 023639
StackedEnsemble BestOfFamily AutoML 20200519 023639
GBM 5 AutoML 20200519 023639
GBM 2 AutoML 20200519 023639
GBM 1 AutoML 20200519 023639
GBM 3 AutoML 20200519 023639
GBM grid 1 AutoML 20200519 023639 model 1
XGBoost grid 1 AutoML 20200519 023639 model 4
GBM 4 AutoML 20200519 023639
XGBoost grid 1 AutoML 20200519 023639 model 3
GBM grid 1 AutoML 20200519 023639 model 2
XGBoost 3 AutoML 20200519 023639
DRF 1 AutoML 20200519 023639
XRT 1 AutoML 20200519 023639
XGBoost grid 1 AutoML 20200519 023639 model 1
XGBoost 2 AutoML 20200519 023639
XGBoost 1 AutoML 20200519 023639
DeepLearning grid 2 AutoML 20200519 023639 model 1
XGBoost grid 1 AutoML 20200519 023639 model 2
DeepLearning 1 AutoML 20200519 023639
DeepLearning grid 1 AutoML 20200519 023639 model 1
GLM 1 AutoML 20200519 023639

auc
0.788
0.787
0.782
0.778
0.777
0.775
0.773
0.773
0.772
0.772
0.770
0.770
0.765
0.760
0.759
0.755
0.746
0.732
0.730
0.704
0.690
0.683

logloss
0.553
0.554
0.558
0.563
0.563
0.565
0.568
0.568
0.569
0.570
0.569
0.573
0.580
0.585
0.585
0.606
0.620
0.614
0.814
0.631
0.670
0.639

aucpr
0.806
0.804
0.802
0.796
0.799
0.795
0.791
0.789
0.793
0.791
0.789
0.785
0.782
0.777
0.774
0.774
0.765
0.738
0.748
0.708
0.699
0.681

mpce
0.311
0.318
0.320
0.334
0.356
0.328
0.323
0.322
0.337
0.342
0.370
0.338
0.336
0.339
0.359
0.361
0.360
0.375
0.397
0.395
0.401
0.397

rmse
0.433
0.433
0.436
0.438
0.438
0.439
0.440
0.440
0.441
0.441
0.441
0.442
0.445
0.448
0.447
0.455
0.460
0.459
0.497
0.468
0.480
0.473

mse
0.187
0.188
0.190
0.191
0.192
0.192
0.194
0.194
0.194
0.194
0.194
0.195
0.198
0.200
0.200
0.207
0.211
0.211
0.247
0.219
0.230
0.223

Table 1: Example of a binary classification Leaderboard. Taken from the H2O AutoML
User Guide code examples section. http://docs.h2o.ai/h2o/latest-stable/h2odocs/automl.html#code-examples

D.2 Log
When using Python or R clients, you can also access meta information through the eventlog (event log), an H2OFrame with selected AutoML backend events generated during
training. This information can also be viewed during the AutoML run by setting verbose ="info" in the AutoML function. There is another object generated called training info, a
simple dictionary/list exposing data that could be useful for post-analysis (various timings).
12

H2O AutoML

Appendix E. Benchmark Results
E.1 Ablation Study: CV-Stacking vs Blending
We compared different settings of H2O AutoML, specifically related to stacking, as thetraining set size increases. We used a version of the the Airlines dataset26 which contains
flight statistics for all flights in the U.S. from 1987 - 2013, totaling slightly more than 150
million rows. The goal is to predict departure delay, encoded as a binary response. There
are a number of different types of features in this dataset (categorical, binary, numeric),
which contain pre-flight information that may be useful in trying to predict whether a flightwill be delayed.
We evaluated the difference between using 5-fold cross-validation (nfolds = 5) and a
10% blending frame (with nfolds = 0) to train the metalearner of the Stacked Ensembles.
We prepared training sets of increasing sizes: 10k, 100k, 1M, 10M, 100M rows. The sametest set of 100k rows was used to evaluate all the AutoML runs. The code and results fromthese benchmarks are available on Github.27 .

Figure 2: H2O AutoML scaling from 10,000 to 100M rows on the Airlines binary classification dataset on a single machine. Stacking with 5-fold cross-validated predictions
versus stacking with a 10% blending frame partitioned from the training set.
As training set size increases, the added burden of doing k -fold cross-validation for
purposes of metalearning, becomes less effective. At a certain point, the computational
cost of doing cross-validation out-weighs the benefits of training the metalearner on moredata and a Stacked Ensemble trained with a simple 10% holdout frame can out-performthe ensembles trained on the cross-validated predicted values from the base learners.
26. https://www.transtats.bts.gov/Fields.asp?Table ID=236
27. http://github.com/h2oai/h2o-automl-paper

13E. LeDell and S. Poirier

Tool
Auto-WEKA
auto-sklearn
TPOT
H2O AutoML

Back-end

Optimization

Meta-learning

Post-processing

WEKA
scikit-learn
scikit-learn
H2O

Bayesian
Bayesian
Genetic Programming
Random Search

warm-start-

ensemble selection
stacked ensembles

Table 2: List of the AutoML tools in the OpenML AutoML Benchmark (2019).
With smaller data, or when your data-size-to-compute-resources ratio is high, H2O
AutoML will typically produce a better Stacked Ensemble model using cross-validation,
however, for larger datasets, especially in time-constrained scenarios (< 1 hour), it’s recommended to reduce the number of cross-validation folds (e.g. nfolds = 3) or skip crossvalidation completely and instead use a blending frame to train the Stacked Ensemble. The
blending frame argument allows the user to pass a holdout frame for purposes of training
the metalearner in the Stacked Ensemble. A dynamic strategy, based on data to compute
ratio, for choosing the number of folds or using a blending frame is planned for a future
release of H2O AutoML.

Appendix F. OpenML AutoML Benchmark
A high level summary of the tools in the 2019 OpenML AutoML Benchmark, the first andonly published edition of the benchmark thus far, is shown in Table 2.
The first prominent AutoML tool was Auto-WEKA, which used Bayesian optimization
to select and tune the algorithms in a machine learning pipeline based on WEKA (Hallet al., 2009). It was followed by auto-sklearn, which uses the same basic technique on top of
scikit-learn (Pedregosa et al., 2011b) and added meta-learning to warm-start the search withthe best pipelines on similar datasets, as well as ensemble construction. TPOT optimizes
scikit-learn pipelines via genetic programming, starting with simple ones and evolving themover generations. As noted in the main text, we removed Auto-WEKA from the benchmark
since it was not competitive with the other tools, so the results shown in Figures 3 and 4
include auto-sklearn, TPOT, H2O AutoML and AutoGluon-Tabular.

14

H2O AutoML

Figure 3: Updated OpenML AutoML Benchmark Results on binary and multiclass classification datasets (1 hour). AUC shown is 10-fold cross-validated AUC (the betterscores are on the top). We added 5 datasets that were part of the “validation” inthe benchmark, for a total of 44.

15

E. LeDell and S. Poirier

Figure 4: Updated OpenML AutoML Benchmark Results on binary and multiclass classification datasets (1 hour). The bold points are the 10-fold cross-validated values
and the scores for each fold are also shown (the better scores are on the right).
Both plots show the relative scores, as compared to H2O AutoML. We added 5
datasets that were part of the “validation” in the benchmark, for a total of 44.

16