Ant-xgboost

* Python module
* Weighted samples instances
* Initial version of pairwise rank

XGBoost Python package drops Python 2.x (4379, 4381)
Python 2.x is reaching its end-of-life at the end of this year. [Many scientific Python packages are now moving to drop Python 2.x](https://python3statement.org/).

XGBoost4J-Spark now requires Spark 2.4.x (4377)
* Spark 2.3 is reaching its end-of-life soon. See discussion at 4389.
* **Consistent handling of missing values** (4309, 4349, 4411): Many users had reported issue with inconsistent predictions between XGBoost4J-Spark and the Python XGBoost package. The issue was caused by Spark mis-handling non-zero missing values (NaN, -1, 999 etc). We now alert the user whenever Spark doesn't handle missing values correctly (4309, 4349). See [the tutorial for dealing with missing values in XGBoost4J-Spark](https://xgboost.readthedocs.io/en/release_0.90/jvm/xgboost4j_spark_tutorial.htmldealing-with-missing-values). This fix also depends on the availability of Spark 2.4.x.

Roadmap: better performance scaling for multi-core CPUs (4310)
* Poor performance scaling of the `hist` algorithm for multi-core CPUs has been under investigation (3810). 4310 optimizes quantile sketches and other pre-processing tasks. Special thanks to SmirnovEgorRu.

Roadmap: Harden distributed training (4250)
* Make distributed training in XGBoost more robust by hardening [Rabit](https://github.com/dmlc/rabit), which implements [the AllReduce primitive](https://en.wikipedia.org/wiki/Reduce_%28parallel_pattern%29). In particular, improve test coverage on mechanisms for fault tolerance and recovery. Special thanks to chenqin.

New feature: Multi-class metric functions for GPUs (4368)
* Metrics for multi-class classification have been ported to GPU: `merror`, `mlogloss`. Special thanks to trivialfis.
* With supported metrics, XGBoost will select the correct devices based on your system and `n_gpus` parameter.

New feature: Scikit-learn-like random forest API (4148, 4255, 4258)
* XGBoost Python package now offers `XGBRFClassifier` and `XGBRFRegressor` API to train random forests. See [the tutorial](https://xgboost.readthedocs.io/en/release_0.90/tutorials/rf.html). Special thanks to canonizer

New feature: use external memory in GPU predictor (4284, 4396, 4438, 4457)
* It is now possible to make predictions on GPU when the input is read from external memory. This is useful when you want to make predictions with big dataset that does not fit into the GPU memory. Special thanks to rongou, canonizer, sriramch.

python
dtest = xgboost.DMatrix('test_data.libsvmdtest.cache')
bst.set_param('predictor', 'gpu_predictor')
bst.predict(dtest)


* Coming soon: GPU training (`gpu_hist`) with external memory

New feature: XGBoost can now handle comments in LIBSVM files (4430)
* Special thanks to trivialfis and hcho3

New feature: Embed XGBoost in your C/C++ applications using CMake (4323, 4333, 4453)
* It is now easier than ever to embed XGBoost in your C/C++ applications. In your CMakeLists.txt, add `xgboost::xgboost` as a linked library:

cmake
find_package(xgboost REQUIRED)
add_executable(api-demo c-api-demo.c)
target_link_libraries(api-demo xgboost::xgboost)


[XGBoost C API documentation is available.](https://xgboost.readthedocs.io/en/release_0.90/dev) Special thanks to trivialfis

Performance improvements
* Use feature interaction constraints to narrow split search space (4341, 4428)
* Additional optimizations for `gpu_hist` (4248, 4283)
* Reduce OpenMP thread launches in `gpu_hist` (4343)
* Additional optimizations for multi-node multi-GPU random forests. (4238)
* Allocate unique prediction buffer for each input matrix, to avoid re-sizing GPU array (4275)
* Remove various synchronisations from CUDA API calls (4205)
* XGBoost4J-Spark
- Allow the user to control whether to cache partitioned training data, to potentially reduce execution time (4268)

Bug-fixes
* Fix node reuse in `hist` (4404)
* Fix GPU histogram allocation (4347)
* Fix matrix attributes not sliced (4311)
* Revise AUC and AUCPR metrics now work with weighted ranking task (4216, 4436)
* Fix timer invocation for InitDataOnce() in `gpu_hist` (4206)
* Fix R-devel errors (4251)
* Make gradient update in GPU linear updater thread-safe (4259)
* Prevent out-of-range access in column matrix (4231)
* Don't store DMatrix handle in Python object until it's initialized, to improve exception safety (4317)
* XGBoost4J-Spark
- Fix non-deterministic order within a zipped partition on prediction (4388)
- Remove race condition on tracker shutdown (4224)
- Allow set the parameter `maxLeaves`. (4226)
- Allow partial evaluation of dataframe before prediction (4407)
- Automatically set `maximize_evaluation_metrics` if not explicitly given (4446)

API changes
* Deprecate `reg:linear` in favor of `reg:squarederror`. (4267, 4427)
* Add attribute getter and setter to the Booster object in XGBoost4J (4336)

Maintenance: Refactor C++ code for legibility and maintainability
* Fix clang-tidy warnings. (4149)
* Remove deprecated C APIs. (4266)
* Use Monitor class to time functions in `hist`. (4273)
* Retire DVec class in favour of c++20 style span for device memory. (4293)
* Improve HostDeviceVector exception safety (4301)

Maintenance: testing, continuous integration, build system
* **Major refactor of CMakeLists.txt** (4323, 4333, 4453): adopt modern CMake and export XGBoost as a target
* **Major improvement in Jenkins CI pipeline** (4234)
- Migrate all Linux tests to Jenkins (4401)
- Builds and tests are now de-coupled, to test an artifact against multiple versions of CUDA, JDK, and other dependencies (4401)
- Add Windows GPU to Jenkins CI pipeline (4463, 4469)
* Support CUDA 10.1 (4223, 4232, 4265, 4468)
* Python wheels are now built with CUDA 9.0, so that JIT is not required on Volta architecture (4459)
* Integrate with NVTX CUDA profiler (4205)
* Add a test for cpu predictor using external memory (4308)
* Refactor tests to get rid of duplication (4358)
* Remove test dependency on `craigcitro/r-travis`, since it's deprecated (4353)
* Add files from local R build to `.gitignore` (4346)
* Make XGBoost4J compatible with Java 9+ by revising NativeLibLoader (4351)
* Jenkins build for CUDA 10.0 (4281)
* Remove remaining `silent` and `debug_verbose` in Python tests (4299)
* Use all cores to build XGBoost4J lib on linux (4304)
* Upgrade Jenkins Linux build environment to GCC 5.3.1, CMake 3.6.0 (4306)
* Make CMakeLists.txt compatible with CMake 3.3 (4420)
* Add OpenMP option in CMakeLists.txt (4339)
* Get rid of a few trivial compiler warnings (4312)
* Add external Docker build cache, to speed up builds on Jenkins CI (4331, 4334, 4458)
* Fix Windows tests (4403)
* Fix a broken python test (4395)
* Use a fixed seed to split data in XGBoost4J-Spark tests, for reproducibility (4417)
* Add additional Python tests to test training under constraints (4426)
* Enable building with shared NCCL. (4447)

Usability Improvements, Documentation
* Document limitation of one-split-at-a-time Greedy tree learning heuristic (4233)
* Update build doc: PyPI wheel now support multi-GPU (4219)
* Fix docs for `num_parallel_tree` (4221)
* Fix document about `colsample_by*` parameter (4340)
* Make the train and test input with same colnames. (4329)
* Update R contribute link. (4236)
* Fix travis R tests (4277)
* Log version number in crash log in XGBoost4J-Spark (4271, 4303)
* Allow supression of Rabit output in Booster::train in XGBoost4J (4262)
* Add tutorial on handling missing values in XGBoost4J-Spark (4425)
* Fix typos (4345, 4393, 4432, 4435)
* Added language classifier in setup.py (4327)
* Added Travis CI badge (4344)
* Add BentoML to use case section (4400)
* Remove subtly sexist remark (4418)
* Add R vignette about parsing JSON dumps (4439)

Acknowledgement
**Contributors**: Nan Zhu (CodingCat), Adam Pocock (Craigacp), Daniel Hen (Daniel8hen), Jiaxiang Li (JiaxiangBU), Rory Mitchell (RAMitchell), Egor Smirnov (SmirnovEgorRu), Andy Adinets (canonizer), Jonas (elcombato), Harry Braviner (harrybraviner), Philip Hyunsu Cho (hcho3), Tong He (hetong007), James Lamb (jameslamb), Jean-Francois Zinque (jeffzi), Yang Yang (jokerkeny), Mayank Suman (mayanksuman), jess (monkeywithacupcake), Hajime Morrita (omo), Ravi Kalia (project-delphi), ras44, Rong Ou (rongou), Shaochen Shi (shishaochen), Xu Xiao (sperlingxx), sriramch, Jiaming Yuan (trivialfis), Christopher Suchanek (wsuchy), Bozhao (yubozhao)

**Reviewers**: Nan Zhu (CodingCat), Adam Pocock (Craigacp), Daniel Hen (Daniel8hen), Jiaxiang Li (JiaxiangBU), Laurae (Laurae2), Rory Mitchell (RAMitchell), Egor Smirnov (SmirnovEgorRu), alois-bissuel, Andy Adinets (canonizer), Chen Qin (chenqin), Harry Braviner (harrybraviner), Philip Hyunsu Cho (hcho3), Tong He (hetong007), jakirkham, James Lamb (jameslamb), Julien Schueller (jschueller), Mayank Suman (mayanksuman), Hajime Morrita (omo), Rong Ou (rongou), Sara Robinson (sararob), Shaochen Shi (shishaochen), Xu Xiao (sperlingxx), sriramch, Sean Owen (srowen), Sergei Lebedev (superbobry), Yuan (Terry) Tang (terrytangyuan), Theodore Vasiloudis (thvasilo), Matthew Tovbin (tovbinm), Jiaming Yuan (trivialfis), Xin Yin (xydrolase)

This release is packed with many new features and bug fixes.

Roadmap: better performance scaling for multi-core CPUs (3957)
* Poor performance scaling of the `hist` algorithm for multi-core CPUs has been under investigation (3810). 3957 marks an important step toward better performance scaling, by using software pre-fetching and replacing STL vectors with C-style arrays. Special thanks to Laurae2 and SmirnovEgorRu.
* See 3810 for latest progress on this roadmap.

New feature: Distributed Fast Histogram Algorithm (`hist`) (4011, 4102, 4140, 4128)
* It is now possible to run the `hist` algorithm in distributed setting. Special thanks to CodingCat. The benefits include:
1. Faster local computation via feature binning
2. Support for monotonic constraints and feature interaction constraints
3. Simpler codebase than `approx`, allowing for future improvement
* Depth-wise tree growing is now performed in a separate code path, so that cross-node syncronization is performed only once per level.

New feature: Multi-Node, Multi-GPU training (4095)
* Distributed training is now able to utilize clusters equipped with NVIDIA GPUs. In particular, the rabit AllReduce layer will communicate GPU device information. Special thanks to mt-jones, RAMitchell, rongou, trivialfis, canonizer, and jeffdk.
* Resource management systems will be able to assign a rank for each GPU in the cluster.
* In Dask, users will be able to construct a collection of XGBoost processes over an inhomogeneous device cluster (i.e. workers with different number and/or kinds of GPUs).

New feature: Multiple validation datasets in XGBoost4J-Spark (3904, 3910)
* You can now track the performance of the model during training with multiple evaluation datasets. By specifying `eval_sets` or call `setEvalSets` over a `XGBoostClassifier` or `XGBoostRegressor`, you can pass in multiple evaluation datasets typed as a `Map` from `String` to `DataFrame`. Special thanks to CodingCat.
* See the usage of multiple validation datasets [here](https://github.com/dmlc/xgboost/blob/0c1d5f1120c0a159f2567b267f0ec4ffadee00d0/jvm-packages/xgboost4j-example/src/main/scala/ml/dmlc/xgboost4j/scala/example/spark/SparkTraining.scalaL66-L78)

New feature: Additional metric functions for GPUs (3952)
* Element-wise metrics have been ported to GPU: `rmse`, `mae`, `logloss`, `poisson-nloglik`, `gamma-deviance`, `gamma-nloglik`, `error`, `tweedie-nloglik`. Special thanks to trivialfis and RAMitchell.
* With supported metrics, XGBoost will select the correct devices based on your system and `n_gpus` parameter.

New feature: Column sampling at individual nodes (splits) (3971)
* Columns (features) can now be sampled at individual tree nodes, in addition to per-tree and per-level sampling. To enable per-node sampling, set `colsample_bynode` parameter, which represents the fraction of columns sampled at each node. This parameter is set to 1.0 by default (i.e. no sampling per node). Special thanks to canonizer.
* The `colsample_bynode` parameter works cumulatively with other `colsample_by*` parameters: for example, `{'colsample_bynode':0.5, 'colsample_bytree':0.5}` with 100 columns will give 25 features to choose from at each split.

Major API change: consistent logging level via `verbosity` (3982, 4002, 4138)
* XGBoost now allows fine-grained control over logging. You can set `verbosity` to 0 (silent), 1 (warning), 2 (info), and 3 (debug). This is useful for controlling the amount of logging outputs. Special thanks to trivialfis.
* Parameters `silent` and `debug_verbose` are now deprecated.
* Note: Sometimes XGBoost tries to change configurations based on heuristics, which is displayed as warning message. If there's unexpected behaviour, please try to increase value of verbosity.

Major bug fix: external memory (4040, 4193)
* Clarify object ownership in multi-threaded prefetcher, to avoid memory error.
* Correctly merge two column batches (which uses [CSC layout](https://en.wikipedia.org/wiki/Sparse_matrixCompressed_sparse_column_(CSC_or_CCS))).
* Add unit tests for external memory.
* Special thanks to trivialfis and hcho3.

Major bug fix: early stopping fixed in XGBoost4J and XGBoost4J-Spark (3928, 4176)
* Early stopping in XGBoost4J and XGBoost4J-Spark is now consistent with its counterpart in the Python package. Training stops if the current iteration is `earlyStoppingSteps` away from the best iteration. If there are multiple evaluation sets, only the last one is used to determinate early stop.
* See the updated documentation [here](https://xgboost.readthedocs.io/en/release_0.82/jvm/xgboost4j_spark_tutorial.htmlearly-stopping)
* Special thanks to CodingCat, yanboliang, and mingyang.

Major bug fix: infrequent features should not crash distributed training (4045)
* For infrequently occuring features, some partitions may not get any instance. This scenario used to crash distributed training due to mal-formed ranges. The problem has now been fixed.
* In practice, one-hot-encoded categorical variables tend to produce rare features, particularly when the cardinality is high.
* Special thanks to CodingCat.

Performance improvements
* Faster, more space-efficient radix sorting in `gpu_hist` (3895)
* Subtraction trick in histogram calculation in `gpu_hist` (3945)
* More performant re-partition in XGBoost4J-Spark (4049)

Bug-fixes
* Fix semantics of `gpu_id` when running multiple XGBoost processes on a multi-GPU machine (3851)
* Fix page storage path for external memory on Windows (3869)
* Fix configuration setup so that DART utilizes GPU (4024)
* Eliminate NAN values from SHAP prediction (3943)
* Prevent empty quantile sketches in `hist` (4155)
* Enable running objectives with 0 GPU (3878)
* Parameters are no longer dependent on system locale (3891, 3907)
* Use consistent data type in the GPU coordinate descent code (3917)
* Remove undefined behavior in the CLI config parser on the ARM platform (3976)
* Initialize counters in GPU AllReduce (3987)
* Prevent deadlocks in GPU AllReduce (4113)
* Load correct values from sliced NumPy arrays (4147, 4165)
* Fix incorrect GPU device selection (4161)
* Make feature binning logic in `hist` aware of query groups when running a ranking task (4115). For ranking task, query groups are weighted, not individual instances.
* Generate correct C++ exception type for `LOG(FATAL)` macro (4159)
* Python package
- Python package should run on system without `PATH` environment variable (3845)
- Fix `coef_` and `intercept_` signature to be compatible with `sklearn.RFECV` (3873)
- Use UTF-8 encoding in Python package README, to support non-English locale (3867)
- Add AUC-PR to list of metrics to maximize for early stopping (3936)
- Allow loading pickles without `self.booster` attribute, for backward compatibility (3938, 3944)
- White-list DART for feature importances (4073)
- Update usage of [h2oai/datatable](https://github.com/h2oai/datatable) (4123)
* XGBoost4J-Spark
- Address scalability issue in prediction (4033)
- Enforce the use of per-group weights for ranking task (4118)
- Fix vector size of `rawPredictionCol` in `XGBoostClassificationModel` (3932)
- More robust error handling in Spark tracker (4046, 4108)
- Fix return type of `setEvalSets` (4105)
- Return correct value of `getMaxLeaves` (4114)

API changes
* Add experimental parameter `single_precision_histogram` to use single-precision histograms for the `gpu_hist` algorithm (3965)
* Python package
- Add option to select type of feature importances in the scikit-learn inferface (3876)
- Add `trees_to_df()` method to dump decision trees as Pandas data frame (4153)
- Add options to control node shapes in the GraphViz plotting function (3859)
- Add `xgb_model` option to `XGBClassifier`, to load previously saved model (4092)
- Passing lists into `DMatrix` is now deprecated (3970)
* XGBoost4J
- Support multiple feature importance features (3801)

Maintenance: Refactor C++ code for legibility and maintainability
* Refactor `hist` algorithm code and add unit tests (3836)
* Minor refactoring of split evaluator in `gpu_hist` (3889)
* Removed unused leaf vector field in the tree model (3989)
* Simplify the tree representation by combining `TreeModel` and `RegTree` classes (3995)
* Simplify and harden tree expansion code (4008, 4015)
* De-duplicate parameter classes in the linear model algorithms (4013)
* Robust handling of ranges with C++20 span in `gpu_exact` and `gpu_coord_descent` (4020, 4029)
* Simplify tree training code (3825). Also use Span class for robust handling of ranges.

Maintenance: testing, continuous integration, build system
* Disallow `std::regex` since it's not supported by GCC 4.8.x (3870)
* Add multi-GPU tests for coordinate descent algorithm for linear models (3893, 3974)
* Enforce naming style in Python lint (3896)
* Refactor Python tests (3897, 3901): Use pytest exclusively, display full trace upon failure
* Address `DeprecationWarning` when using Python collections (3909)
* Use correct group for maven site plugin (3937)
* Jenkins CI is now using on-demand EC2 instances exclusively, due to unreliability of Spot instances (3948)
* Better GPU performance logging (3945)
* Fix GPU tests on machines with only 1 GPU (4053)
* Eliminate CRAN check warnings and notes (3988)
* Add unit tests for tree serialization (3989)
* Add unit tests for tree fitting functions in `hist` (4155)
* Add a unit test for `gpu_exact` algorithm (4020)
* Correct JVM CMake GPU flag (4071)
* Fix failing Travis CI on Mac (4086)
* Speed up Jenkins by not compiling CMake (4099)
* Analyze C++ and CUDA code using clang-tidy, as part of Jenkins CI pipeline (4034)
* Fix broken R test: Install Homebrew GCC (4142)
* Check for empty datasets in GPU unit tests (4151)
* Fix Windows compilation (4139)
* Comply with latest convention of cpplint (4157)
* Fix a unit test in `gpu_hist` (4158)
* Speed up data generation in Python tests (4164)

Usability Improvements
* Add link to [InfoWorld 2019 Technology of the Year Award](https://www.infoworld.com/article/3336072/application-development/infoworlds-2019-technology-of-the-year-award-winners.html) (4116)
* Remove outdated AWS YARN tutorial (3885)
* Document current limitation in number of features (3886)
* Remove unnecessary warning when `gblinear` is selected (3888)
* Document limitation of CSV parser: header not supported (3934)
* Log training parameters in XGBoost4J-Spark (4091)
* Clarify early stopping behavior in the scikit-learn interface (3967)
* Clarify behavior of `max_depth` parameter (4078)
* Revise Python docstrings for ranking task (4121). In particular, weights must be per-group in learning-to-rank setting.
* Document parameter `num_parallel_tree` (4022)
* Add Jenkins status badge (4090)
* Warn users against using internal functions of `Booster` object (4066)
* Reformat `benchmark_tree.py` to comply with Python style convention (4126)
* Clarify a comment in `objectiveTrait` (4174)
* Fix typos and broken links in documentation (3890, 3872, 3902, 3919, 3975, 4027, 4156, 4167)

Acknowledgement
**Contributors** (in no particular order): Jiaming Yuan (trivialfis), Hyunsu Cho (hcho3), Nan Zhu (CodingCat), Rory Mitchell (RAMitchell), Yanbo Liang (yanboliang), Andy Adinets (canonizer), Tong He (hetong007), Yuan Tang (terrytangyuan)

**First-time Contributors** (in no particular order): Jelle Zijlstra (JelleZijlstra), Jiacheng Xu (jiachengxu), ajing, Kashif Rasul (kashif), theycallhimavi, Joey Gao (pjgao), Prabakaran Kumaresshan (nixphix), Huafeng Wang (huafengw), lyxthe, Sam Wilkinson (scwilkinson), Tatsuhito Kato (stabacov), Shayak Banerjee (shayakbanerjee), Kodi Arfer (Kodiologist), KyleLi1985, Egor Smirnov (SmirnovEgorRu), tmitanitky, Pasha Stetsenko (st-pasha), Kenichi Nagahara (keni-chi), Abhai Kollara Dilip (abhaikollara), Patrick Ford (pford221), hshujuan, Matthew Jones (mt-jones), Thejaswi Rao (teju85), Adam November (anovember)

**First-time Reviewers** (in no particular order): Mingyang Hu (mingyang), Theodore Vasiloudis (thvasilo), Jakub Troszok (troszok), Rong Ou (rongou), Denisevi4, Matthew Jones (mt-jones), Jeff Kaplan (jeffdk)

New feature: feature interaction constraints
* Users are now able to control which features (independent variables) are allowed to interact by specifying feature interaction constraints (3466).
* [Tutorial](https://xgboost.readthedocs.io/en/release_0.81/tutorials/feature_interaction_constraint.html) is available, as well as [R](https://github.com/dmlc/xgboost/blob/9254c58e4dfff6a59dc0829a2ceb02e45ed17cd0/R-package/demo/interaction_constraints.R) and [Python](https://github.com/dmlc/xgboost/blob/9254c58e4dfff6a59dc0829a2ceb02e45ed17cd0/tests/python/test_interaction_constraints.py) examples.

New feature: learning to rank using scikit-learn interface
* Learning to rank task is now available for the scikit-learn interface of the Python package (3560, 3848). It is now possible to integrate the XGBoost ranking model into the scikit-learn learning pipeline.
* Examples of using `XGBRanker` class is found at [demo/rank/rank_sklearn.py](https://github.com/dmlc/xgboost/blob/24a268a2e3cb17302db3d72da8f04016b7d352d9/demo/rank/rank_sklearn.py).

New feature: R interface for SHAP interactions
* SHAP (SHapley Additive exPlanations) is a unified approach to explain the output of any machine learning model. Previously, this feature was only available from the Python package; now it is available from the R package as well (3636).

New feature: GPU predictor now use multiple GPUs to predict
* GPU predictor is now able to utilize multiple GPUs at once to accelerate prediction (3738)

New feature: Scale distributed XGBoost to large-scale clusters
* Fix OS file descriptor limit assertion error on large cluster (3835, dmlc/rabit73) by replacing `select()` based AllReduce/Broadcast with `poll()` based implementation.
* Mitigate tracker "thundering herd" issue on large cluster. Add exponential backoff retry when workers connect to tracker.
* With this change, we were able to scale to 1.5k executors on a 12 billion row dataset after some tweaks here and there.

New feature: Additional objective functions for GPUs
* New objective functions ported to GPU: `hinge`, `multi:softmax`, `multi:softprob`, `count:poisson`, `reg:gamma`, `"reg:tweedie`.
* With supported objectives, XGBoost will select the correct devices based on your system and `n_gpus` parameter.

Major bug fix: learning to rank with XGBoost4J-Spark
* Previously, `repartitionForData` would shuffle data and lose ordering necessary for ranking task.
* To fix this issue, data points within each RDD partition is explicitly group by their group (query session) IDs (3654). Also handle empty RDD partition carefully (3750).

Major bug fix: early stopping fixed in XGBoost4J-Spark
* Earlier implementation of early stopping had incorrect semantics and didn't let users to specify direction for optimizing (maximize / minimize)
* A parameter `maximize_evaluation_metrics` is defined so as to tell whether a metric should be maximized or minimized as part of early stopping criteria (3808). Also early stopping now has correct semantics.

API changes
* Column sampling by level (`colsample_bylevel`) is now functional for `hist` algorithm (3635, 3862)
* GPU tag `gpu:` for regression objectives are now deprecated. XGBoost will select the correct devices automatically (3643)
* Add `disable_default_eval_metric` parameter to disable default metric (3606)
* Experimental AVX support for gradient computation is removed (3752)
* XGBoost4J-Spark
- Add `rank:ndcg` and `rank:map` to supported objectives (3697)
* Python package
- Add `callbacks` argument to `fit()` function of sciki-learn API (3682)
- Add `XGBRanker` to scikit-learn interface (3560, 3848)
- Add `validate_features` argument to `predict()` function of scikit-learn API (3653)
- Allow scikit-learn grid search over parameters specified as keyword arguments (3791)
- Add `coef_` and `intercept_` as properties of scikit-learn wrapper (3855). Some scikit-learn functions expect these properties.

Performance improvements
* Address very high GPU memory usage for large data (3635)
* Fix performance regression within `EvaluateSplits()` of `gpu_hist` algorithm. (3680)

Bug-fixes
* Fix a problem in GPU quantile sketch with tiny instance weights. (3628)
* Fix copy constructor for `HostDeviceVectorImpl` to prevent dangling pointers (3657)
* Fix a bug in partitioned file loading (3673)
* Fixed an uninitialized pointer in `gpu_hist` (3703)
* Reshared data among GPUs when number of GPUs is changed (3721)
* Add back `max_delta_step` to split evaluation (3668)
* Do not round up integer thresholds for integer features in JSON dump (3717)
* Use `dmlc::TemporaryDirectory` to handle temporaries in cross-platform way (3783)
* Fix accuracy problem with `gpu_hist` when `min_child_weight` and `lambda` are set to 0 (3793)
* Make sure that `tree_method` parameter is recognized and not silently ignored (3849)
* XGBoost4J-Spark
- Make sure `thresholds` are considered when executing `predict()` method (3577)
- Avoid losing precision when computing probabilities by converting to `Double` early (3576)
- `getTreeLimit()` should return `Int` (3602)
- Fix checkpoint serialization on HDFS (3614)
- Throw `ControlThrowable` instead of `InterruptedException` so that it is properly re-thrown (3632)
- Remove extraneous output to stdout (3665)
- Allow specification of task type for custom objectives and evaluations (3646)
- Fix distributed updater check (3739)
- Fix issue when spark job execution thread cannot return before we execute `first()` (3758)
* Python package
- Fix accessing `DMatrix.handle` before it is set (3599)
- `XGBClassifier.predict()` should return margin scores when `output_margin` is set to true (3651)
- Early stopping callback should maximize metric of form `NDCGn-` (3685)
- Preserve feature names when slicing `DMatrix` (3766)
* R package
- Replace `nround` with `nrounds` to match actual parameter (3592)
- Amend `xgb.createFolds` to handle classes of a single element (3630)
- Fix buggy random generator and make `colsample_bytree` functional (3781)

Maintenance: testing, continuous integration, build system
* Add sanitizers tests to Travis CI (3557)
* Add NumPy, Matplotlib, Graphviz as requirements for doc build (3669)
* Comply with CRAN submission policy (3660, 3728)
* Remove copy-paste error in JVM test suite (3692)
* Disable flaky tests in `R-package/tests/testthat/test_update.R` (3723)
* Make Python tests compatible with scikit-learn 0.20 release (3731)
* Separate out restricted and unrestricted tasks, so that pull requests don't build downloadable artifacts (3736)
* Add multi-GPU unit test environment (3741)
* Allow plug-ins to be built by CMake (3752)
* Test wheel compatibility on CPU containers for pull requests (3762)
* Fix broken doc build due to Matplotlib 3.0 release (3764)
* Produce `xgboost.so` for XGBoost-R on Mac OSX, so that `make install` works (3767)
* Retry Jenkins CI tests up to 3 times to improve reliability (3769, 3769, 3775, 3776, 3777)
* Add basic unit tests for `gpu_hist` algorithm (3785)
* Fix Python environment for distributed unit tests (3806)
* Test wheels on CUDA 10.0 container for compatibility (3838)
* Fix JVM doc build (3853)

Maintenance: Refactor C++ code for legibility and maintainability
* Merge generic device helper functions into `GPUSet` class (3626)
* Re-factor column sampling logic into `ColumnSampler` class (3635, 3637)
* Replace `std::vector` with `HostDeviceVector` in `MetaInfo` and `SparsePage` (3446)
* Simplify `DMatrix` class (3395)
* De-duplicate CPU/GPU code using `Transform` class (3643, 3751)
* Remove obsoleted `QuantileHistMaker` class (3761)
* Remove obsoleted `NoConstraint` class (3792)

Other Features
* C++20-compliant Span class for safe pointer indexing (3548, 3588)
* Add helper functions to manipulate multiple GPU devices (3693)
* XGBoost4J-Spark
- Allow specifying host ip from the `xgboost-tracker.properties file` (3833). This comes in handy when `hosts` files doesn't correctly define localhost.

Usability Improvements
* Add reference to GitHub repository in `pom.xml` of JVM packages (3589)
* Add R demo of multi-class classification (3695)
* Document JSON dump functionality (3600, 3603)
* Document CUDA requirement and lack of external memory for GPU algorithms (3624)
* Document LambdaMART objectives, both pairwise and listwise (3672)
* Document `aucpr` evaluation metric (3687)
* Document gblinear parameters: `feature_selector` and `top_k` (3780)
* Add instructions for using MinGW-built XGBoost with Python. (3774)
* Removed nonexistent parameter `use_buffer` from documentation (3610)
* Update Python API doc to include all classes and members (3619, 3682)
* Fix typos and broken links in documentation (3618, 3640, 3676, 3713, 3759, 3784, 3843, 3852)
* Binary classification demo should produce LIBSVM with 0-based indexing (3652)
* Process data once for Python and CLI examples of learning to rank (3666)
* Include full text of Apache 2.0 license in the repository (3698)
* Save predictor parameters in model file (3856)
* JVM packages
- Let users specify feature names when calling `getModelDump` and `getFeatureScore` (3733)
- Warn the user about the lack of over-the-wire encryption (3667)
- Fix errors in examples (3719)
- Document choice of trackers (3831)
- Document that vanilla Apache Spark is required (3854)
* Python package
- Document that custom objective can't contain colon (:) (3601)
- Show a better error message for failed library loading (3690)
- Document that feature importance is unavailable for non-tree learners (3765)
- Document behavior of `get_fscore()` for zero-importance features (3763)
- Recommend pickling as the way to save `XGBClassifier` / `XGBRegressor` / `XGBRanker` (3829)
* R package
- Enlarge variable importance plot to make it more visible (3820)

BREAKING CHANGES
* External memory page files have changed, breaking backwards compatibility for temporary storage used during external memory training. This only affects external memory users upgrading their xgboost version - we recommend clearing all `*.page` files before resuming training. Model serialization is unaffected.

Known issues
* Quantile sketcher fails to produce any quantile for some edge cases (2943)
* The `hist` algorithm leaks memory when used with learning rate decay callback (3579)
* Using custom evaluation funciton together with early stopping causes assertion failure in XGBoost4J-Spark (3595)
* Early stopping doesn't work with `gblinear` learner (3789)
* Label and weight vectors are not reshared upon the change in number of GPUs (3794). To get around this issue, delete the `DMatrix` object and re-load.
* The `DMatrix` Python objects are initialized with incorrect values when given array slices (3841)
* The `gpu_id` parameter is broken and not yet properly supported (3850)

Acknowledgement
**Contributors** (in no particular order): Hyunsu Cho (hcho3), Jiaming Yuan (trivialfis), Nan Zhu (CodingCat), Rory Mitchell (RAMitchell), Andy Adinets (canonizer), Vadim Khotilovich (khotilov), Sergei Lebedev (superbobry)

**First-time Contributors** (in no particular order): Matthew Tovbin (tovbinm), Jakob Richter (jakob-r), Grace Lam (grace-lam), Grant W Schneider (grantschneider), Andrew Thia (BlueTea88), Sergei Chipiga (schipiga), Joseph Bradley (jkbradley), Chen Qin (chenqin), Jerry Lin (linjer), Dmitriy Rybalko (rdtft), Michael Mui (mmui), Takahiro Kojima (515hikaru), Bruce Zhao (BruceZhaoR), Wei Tian (weitian), Saumya Bhatnagar (Sam1301), Juzer Shakir (JuzerShakir), Zhao Hang (cleghom), Jonathan Friedman (jontonsoup), Bruno Tremblay (meztez), Boris Filippov (frenzykryger), Shiki-H, mrgutkun, gorogm, htgeis, jakehoare, zengxy, KOLANICH

**First-time Reviewers** (in no particular order): Nikita Titov (StrikerRUS), Xiangrui Meng (mengxr), Nirmal Borah (Nirmal-Neel)

This version is only applicable for the Python package. The content is identical to that of v0.72.