Which algorithm takes the crown: Light GBM vs XGBOOST?

Introduction

If you are an active member of the Machine Learning community, you must be aware of Boosting Machines and their capabilities. The development of Boosting Machines started from AdaBoost to today’s favorite XGBOOST. XGBOOST has become a de-facto algorithm for winning competitions at Analytics Vidhya and Kaggle, simply because it is extremely powerful. But given lots and lots of data, even XGBOOST takes a long time to train.

Enter…. Light GBM.

Many of you might not be familiar with the Light Gradient Boosting, but you will be after reading this article. The most natural question that will come to your mind is – Why another boosting machine algorithm? Is it superior to XGBOOST?

Well, you very well must have guessed the answer otherwise why would a topic deserve its own article :p

P.S. This article assumes knowledge about Light GBM and XGBoost. If you don’t know them, you should first look at these articles.

These algorithms are a type of ensemble technique. To learn more about ensemble learning and other such techniques in a comprehensive manner, you can enrol in this free course: Ensemble Learning and Ensemble Learning Techniques

What is Light GBM?

Light GBM is a fast, distributed, high-performance gradient boosting framework based on decision tree algorithm, used for ranking, classification and many other machine learning tasks.

Since it is based on decision tree algorithms, it splits the tree leaf wise with the best fit whereas other boosting algorithms split the tree depth wise or level wise rather than leaf-wise. So when growing on the same leaf in Light GBM, the leaf-wise algorithm can reduce more loss than the level-wise algorithm and hence results in much better accuracy which can rarely be achieved by any of the existing boosting algorithms. Also, it is surprisingly very fast, hence the word ‘Light’.

Before is a diagrammatic representation by the makers of the Light GBM to explain the difference clearly.

Level-wise tree growth in XGBOOST.

Leaf wise tree growth in Light GBM.

Leaf wise splits lead to increase in complexity and may lead to overfitting and it can be overcome by specifying another parameter max-depth which specifies the depth to which splitting will occur.

Below, we will see the steps to install Light GBM and run a model using it. We will be comparing the results with XGBOOST results to prove that you should take Light GBM in a ‘LIGHT MANNER’.

Let us look at some of the advantages of Light GBM.

Advantages of Light GBM

1. Faster training speed and higher efficiency: Light GBM use histogram based algorithm i.e it buckets continuous feature values into discrete bins which fasten the training procedure.
2. Lower memory usage: Replaces continuous values to discrete bins which result in lower memory usage.
3. Better accuracy than any other boosting algorithm: It produces much more complex trees by following leaf wise split approach rather than a level-wise approach which is the main factor in achieving higher accuracy. However, it can sometimes lead to overfitting which can be avoided by setting the max_depth parameter.
4. Compatibility with Large Datasets: It is capable of performing equally good with large datasets with a significant reduction in training time as compared to XGBOOST.
5. Parallel learning supported.

I guess you must have got excited about the advantages of Light GBM. Let us now proceed to install the library into our system.

Installing Light GBM

1. For Windows

   Using Visual Studio (Or MSBuild)
   -Install git for windows, cmake and MS Build (Not need the MSbuild if you already install Visual Studio).
   -Run following command:

   git clone --recursive https://github.com/Microsoft/LightGBM cd LightGBM mkdir build cd build cmake -DCMAKE_GENERATOR_PLATFORM=x64 .. cmake --build . --target ALL_BUILD --config Release

   The exe and dll will be in LightGBM/Release folder.

    

   Using MinGW64
   -Install git for windows, cmake and MinGW64.
   -Run following command:

   git clone --recursive https://github.com/Microsoft/LightGBM cd LightGBM mkdir build cd build cmake -G "MinGW Makefiles" .. mingw32-make.exe -j4

   The exe and dll will be in LightGBM/ folder.

2. For Linux

   Light GBM uses cmake to build. Run following:

   git clone --recursive https://github.com/Microsoft/LightGBM cd LightGBM mkdir build cd build cmake .. make -j4

    

3. For OSX

LightGBM depends on OpenMP for compiling, which isn’t supported by Apple Clang.Please use gcc/g++ instead.
-Run following:

brew install cmake
brew install gcc --without-multilib
git clone --recursive https://github.com/Microsoft/LightGBM
cd LightGBM
mkdir build 
cd build
cmake ..
make -j4

Now before we dive head first into building our first Light GBM model, let us look into some of the parameters of Light GBM to have an understanding of its underlying procedures.

Important Parameters of light GBM

• task : default value = train ; options = train , prediction ; Specifies the task we wish to perform which is either train or prediction.

• application: default=regression, type=enum, options= options :
  • regression : perform regression task
  • binary : Binary classification
  • multiclass: Multiclass Classification
  • lambdarank : lambdarank application
• data: type=string; training data , LightGBM will train from this data
• num_iterations: number of boosting iterations to be performed ; default=100; type=int
• num_leaves : number of leaves in one tree ; default = 31 ; type =int
• device : default= cpu ; options = gpu,cpu. Device on which we want to train our model. Choose GPU for faster training.

• max_depth: Specify the max depth to which tree will grow. This parameter is used to deal with overfitting.
• min_data_in_leaf: Min number of data in one leaf.
• feature_fraction: default=1 ; specifies the fraction of features to be taken for each iteration
• bagging_fraction: default=1 ; specifies the fraction of data to be used for each iteration and is generally used to speed up the training and avoid overfitting.
• min_gain_to_split: default=.1 ; min gain to perform splitting
• max_bin : max number of bins to bucket the feature values.
• min_data_in_bin : min number of data in one bin
• num_threads: default=OpenMP_default, type=int ;Number of threads for Light GBM.

• label : type=string ; specify the label column
• categorical_feature : type=string ; specify the categorical features we want to use for training our model
• num_class: default=1 ; type=int ; used only for multi-class classification

Also, go through this article explaining parameter tuning in XGBOOST in detail.

LightGBM vs XGBoost

So now let’s compare LightGBM with XGBoost ensemble learning techniques by applying both the algorithms to a dataset and then comparing the performance.

Here we are using dataset that contains the information about individuals from various countries. Our target is to predict whether a person makes <=50k or >50k annually on basis of the other information available. Dataset consists of 32561 observations and 14 features describing individuals.

Here is the link to the dataset: http://archive.ics.uci.edu/ml/datasets/Adult.

Go through the dataset to have a proper intuition about predictor variables and so that you could understand the code below properly.

Before we get to the code for this dataset, did you know that you can now code your own model in this very window? That’s right! Here’s a live coding window to play around with the code and see the results in real-time:

#importing standard libraries 
import numpy as np 
import pandas as pd 
from pandas import Series, DataFrame 

#import lightgbm and xgboost 
import lightgbm as lgb 
import xgboost as xgb 

#loading our training dataset 'adult.csv' with name 'data' using pandas 
data=pd.read_csv('adult.csv',header=None) 

#Assigning names to the columns 
data.columns=['age','workclass','fnlwgt','education','education-num','marital_Status','occupation','relationship','race','sex','capital_gain','capital_loss','hours_per_week','native_country','Income'] 

#glimpse of the dataset 
data.head() 

# Label Encoding our target variable 
from sklearn.preprocessing import LabelEncoder,OneHotEncoder
l=LabelEncoder() 
l.fit(data.Income) 

l.classes_ 
data.Income=Series(l.transform(data.Income))  #label encoding our target variable 
data.Income.value_counts() 

 

#One Hot Encoding of the Categorical features 
one_hot_workclass=pd.get_dummies(data.workclass) 
one_hot_education=pd.get_dummies(data.education) 
one_hot_marital_Status=pd.get_dummies(data.marital_Status) 
one_hot_occupation=pd.get_dummies(data.occupation)
one_hot_relationship=pd.get_dummies(data.relationship) 
one_hot_race=pd.get_dummies(data.race) 
one_hot_sex=pd.get_dummies(data.sex) 
one_hot_native_country=pd.get_dummies(data.native_country) 

#removing categorical features 
data.drop(['workclass','education','marital_Status','occupation','relationship','race','sex','native_country'],axis=1,inplace=True) 

 

#Merging one hot encoded features with our dataset 'data' 
data=pd.concat([data,one_hot_workclass,one_hot_education,one_hot_marital_Status,one_hot_occupation,one_hot_relationship,one_hot_race,one_hot_sex,one_hot_native_country],axis=1) 

#removing dulpicate columns 
 _, i = np.unique(data.columns, return_index=True) 
data=data.iloc[:, i] 

#Here our target variable is 'Income' with values as 1 or 0.  
#Separating our data into features dataset x and our target dataset y 
x=data.drop('Income',axis=1) 
y=data.Income 

 

#Imputing missing values in our target variable 
y.fillna(y.mode()[0],inplace=True) 

#Now splitting our dataset into test and train 
from sklearn.model_selection import train_test_split 
x_train,x_test,y_train,y_test=train_test_split(x,y,test_size=.3)

#Applying xgboost

#The data is stored in a DMatrix object 
#label is used to define our outcome variable
dtrain=xgb.DMatrix(x_train,label=y_train)
dtest=xgb.DMatrix(x_test)

#setting parameters for xgboost
parameters={'max_depth':7, 'eta':1, 'silent':1,'objective':'binary:logistic','eval_metric':'auc','learning_rate':.05}

#training our model 
num_round=50
from datetime import datetime 
start = datetime.now() 
xg=xgb.train(parameters,dtrain,num_round) 
stop = datetime.now()

#Execution time of the model 
execution_time_xgb = stop-start 
execution_time_xgb

#datetime.timedelta( , , ) representation => (days , seconds , microseconds) 
#now predicting our model on test set 
ypred=xg.predict(dtest) 
ypred

#Converting probabilities into 1 or 0  
for i in range(0,9769): 
    if ypred[i]>=.5:       # setting threshold to .5 
       ypred[i]=1 
    else: 
       ypred[i]=0  

#calculating accuracy of our model 
from sklearn.metrics import accuracy_score 
accuracy_xgb = accuracy_score(y_test,ypred) 
accuracy_xgb

# Light GBM

train_data=lgb.Dataset(x_train,label=y_train)

#setting parameters for lightgbm
param = {'num_leaves':150, 'objective':'binary','max_depth':7,'learning_rate':.05,'max_bin':200}
param['metric'] = ['auc', 'binary_logloss']

#Here we have set max_depth in xgb and LightGBM to 7 to have a fair comparison between the two.

#training our model using light gbm
num_round=50
start=datetime.now()
lgbm=lgb.train(param,train_data,num_round)
stop=datetime.now()

#Execution time of the model
execution_time_lgbm = stop-start
execution_time_lgbm

#predicting on test set
ypred2=lgbm.predict(x_test)
ypred2[0:5]  # showing first 5 predictions

#converting probabilities into 0 or 1
for i in range(0,9769):
    if ypred2[i]>=.5:       # setting threshold to .5
       ypred2[i]=1
    else:  
       ypred2[i]=0

#calculating accuracy
accuracy_lgbm = accuracy_score(ypred2,y_test)
accuracy_lgbm
y_test.value_counts()

from sklearn.metrics import roc_auc_score

#calculating roc_auc_score for xgboost
auc_xgb =  roc_auc_score(y_test,ypred)
auc_xgb

#calculating roc_auc_score for light gbm. 
auc_lgbm = roc_auc_score(y_test,ypred2)
auc_lgbm comparison_dict = {'accuracy score':(accuracy_lgbm,accuracy_xgb),'auc score':(auc_lgbm,auc_xgb),'execution time':(execution_time_lgbm,execution_time_xgb)}

#Creating a dataframe ‘comparison_df’ for comparing the performance of Lightgbm and xgb. 
comparison_df = DataFrame(comparison_dict) 
comparison_df.index= ['LightGBM','xgboost'] 
comparison_df

Performance comparison

There has been only a slight increase in accuracy and auc score by applying Light GBM over XGBOOST but there is a significant difference in the execution time for the training procedure. Light GBM is almost 7 times faster than XGBOOST and is a much better approach when dealing with large datasets.

This turns out to be a huge advantage when you are working on large datasets in limited time competitions.

Tuning Parameters of Light GBM

Light GBM uses leaf wise splitting over depth-wise splitting which enables it to converge much faster but also leads to overfitting. So here is a quick guide to tune the parameters in Light GBM.

For best fit

• num_leaves : This parameter is used to set the number of leaves to be formed in a tree. Theoretically relation between num_leaves and max_depth is num_leaves= 2^(max_depth). However, this is not a good estimate in case of Light GBM since splitting takes place leaf wise rather than depth wise. Hence num_leaves set must be smaller than 2^(max_depth) otherwise it may lead to overfitting. Light GBM does not have a direct relation between num_leaves and max_depth and hence the two must not be linked with each other.
• min_data_in_leaf : It is also one of the important parameters in dealing with overfitting. Setting its value smaller may cause overfitting and hence must be set accordingly. Its value should be hundreds to thousands of large datasets.
• max_depth: It specifies the maximum depth or level up to which tree can grow.

For faster speed

• bagging_fraction : Is used to perform bagging for faster results
• feature_fraction : Set fraction of the features to be used at each iteration
• max_bin : Smaller value of max_bin can save much time as it buckets the feature values in discrete bins which is computationally inexpensive.

For better accuracy

• Use bigger training data
• num_leaves : Setting it to high value produces deeper trees with increased accuracy but lead to overfitting. Hence its higher value is not preferred.
• max_bin : Setting it to high values has similar effect as caused by increasing value of num_leaves and also slower our training procedure.

Frequently Asked Questions

End Notes

In this blog, I’ve tried to give an intuitive idea of Light GBM. One of the disadvantages of using this algorithm currently is its narrow user base – but that is changing fast. This algorithm apart from being more accurate and time-saving than XGBOOST has been limited in usage due to less documentation available.

However, this algorithm has shown far better results and has outperformed existing boosting algorithms. I’ll strongly recommend you to implement Light GBM over the other boosting algorithms and see the difference yourself.

It might be still early days to crown LightGBM – but it has clearly challenged XGBoost. A word of caution – like all other ML algorithms,  make sure you properly tune the parameters before training the model!

Do let us know your thoughts and opinions in the comment section below.