# CUDA Accelerated Tree Construction Algorithms
This plugin adds GPU accelerated tree construction algorithms to XGBoost.
## Usage
Specify the 'updater' parameter as one of the following algorithms. 

### Algorithms
| updater | Description |
| --- | --- |
grow_gpu | The standard XGBoost tree construction algorithm. Performs exact search for splits. Slower and uses considerably more memory than 'grow_gpu_hist' |
grow_gpu_hist | Equivalent to the XGBoost fast histogram algorithm. Faster and uses considerably less memory. Splits may be less accurate. |

### Supported parameters 
| parameter | grow_gpu | grow_gpu_hist |
| --- | --- | --- |
subsample | ✔ | ✔ |
colsample_bytree | ✔ | ✔|
colsample_bylevel | ✔ | ✔ |
max_bin | ✖ | ✔ |
gpu_id | ✔ | ✔ | 

All algorithms currently use only a single GPU. The device ordinal can be selected using the 'gpu_id' parameter, which defaults to 0.

This plugin currently works with the CLI version and python version.

Python example:
```python
param['gpu_id'] = 1
param['max_bin'] = 16
param['updater'] = 'grow_gpu_hist'
```
## Benchmarks
To run benchmarks on synthetic data for binary classification:
```bash
$ python benchmark/benchmark.py
```

Training time time on 1000000 rows x 50 columns with 500 boosting iterations on i7-6700K CPU @ 4.00GHz and Pascal Titan X.

| Updater | Time (s) |
| --- | --- |
| grow_gpu_hist | 11.09 |
| grow_fast_histmaker (histogram XGBoost - CPU) | 41.75 |
| grow_gpu | 193.90 |
| grow_colmaker (standard XGBoost - CPU) | 720.12 |


[See here](http://dmlc.ml/2016/12/14/GPU-accelerated-xgboost.html) for additional performance benchmarks of the 'grow_gpu' updater.

## Test
To run tests:
```bash
$ python -m nose test/python/
```
## Dependencies
A CUDA capable GPU with at least compute capability >= 3.5 (the algorithm depends on shuffle and vote instructions introduced in Kepler).

Building the plug-in requires CUDA Toolkit 7.5 or later.


## Build

### Using cmake
To use the plugin xgboost must be built by specifying the option PLUGIN_UPDATER_GPU=ON. CMake will prepare a build system depending on which platform you are on.

On Linux, from the xgboost directory:
```bash
$ mkdir build
$ cd build
$ cmake .. -DPLUGIN_UPDATER_GPU=ON
$ make
```
If 'make' fails try invoking make again. There can sometimes be problems with the order items are built.

On Windows you may also need to specify your generator as 64 bit, so the cmake command becomes:
```bash
$ cmake .. -G"Visual Studio 12 2013 Win64" -DPLUGIN_UPDATER_GPU=ON
```
You may also  be able to use a later version of visual studio depending on whether the CUDA toolkit supports it.
cmake will generate an xgboost.sln solution file in the build directory. Build this solution in release mode. This is also a good time to check it is being built as x64. If not make sure the cmake generator is set correctly.

### Using make
Now, it also supports the usual 'make' flow to build gpu-enabled tree construction plugins. It's currently only tested on Linux. From the xgboost directory
```bash
# make sure CUDA SDK bin directory is in the 'PATH' env variable
$ make PLUGIN_UPDATER_GPU=ON
```

### For Developers!

Now, some of the code-base inside gpu plugins have googletest unit-tests inside 'tests/'.
They can be enabled run along with other unit-tests inside '<xgboostRoot>/tests/cpp' using:
```bash
# make sure CUDA SDK bin directory is in the 'PATH' env variable
# below 2 commands need only be executed once
$ source ./dmlc-core/scripts/travis/travis_setup_env.sh
$ make -f dmlc-core/scripts/packages.mk gtest
$ make PLUGIN_UPDATER_GPU=ON GTEST_PATH=${CACHE_PREFIX} test
```

## Changelog
##### 2017/5/31
* Faster version of the grow_gpu plugin
* Added support for building gpu plugin through 'make' flow too

##### 2017/5/5
* Histogram performance improvements
* Fix gcc build issues 

##### 2017/4/25
* Add fast histogram algorithm
* Fix Linux build
* Add 'gpu_id' parameter

## References
[Mitchell, Rory, and Eibe Frank. Accelerating the XGBoost algorithm using GPU computing. No. e2911v1. PeerJ Preprints, 2017.](https://peerj.com/preprints/2911/)

## Author
Rory Mitchell 

Please report bugs to the xgboost/issues page. You can tag me with @RAMitchell.

Otherwise I can be contacted at r.a.mitchell.nz at gmail.