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Deeplearning4j 实战（2）：Deeplearning4j 手写体数字识别Spark实现
在前两天的博客中，我们用Deeplearning4j做了Mnist数据集的分类。算是第一个深度学习的应用。像Mnist数据集这样图片尺寸不大，而且是黑白的开源图片集在本地完成训练是可以的，毕竟我们用了Lenet这样相对简单的网络结构，而且本地的机器配置也有8G左右的内存。但实际生产中，图片的数量要多得多，尺寸也大得多，用的网络也会是AlexNet、GoogLenet这样更多层数的网络，所以往往我们需要用集群来解决计算资源的问题。由于Deeplearning4j本身基于Spark实现了神经网络的分布式训练，所以我们就以此作为我们的解决方案。
我们还是以Mnist数据集为例来做Deeplearning4j的第一个Spark版本的应用。首先需要在上一篇博客的基础上，在pom里面加入新的依赖：
<dependency>
<groupId>org.nd4j</groupId>
<artifactId>nd4j-kryo_${scala.binary.version}</artifactId>
<version>${nd4j.version}</version>
</dependency>这个是为了将Nd4j的序列化形式从Java默认的形式转到kryo的格式，以此提高序列化的效率。如果在代码中不为该类注册kryo的序列化格式，那么训练的时候会抛异常。接着代码分为2个部分，一个部分是将Mnist数据集在本地以JavaRDD<DataSet>的形式存到磁盘并最终推到HDFS上作为Spark job的输入数据源。另一个部分则是模型的训练和保存。
第一部分的逻辑大致如下：本地建立Spark任务-->获取所有Mnist图片的路径-->读取图片并提取特征，打上标注，以DataSet的形式作为一张图片的wrapper-->将所有图片构成的JavaRDD<DataSet>存储下来。
这里原始的Mnist数据集是以图片形式存在，不再是二进制格式的数据。这个例子这样处理，也是方便日后用同样的方式读取一般的图片。Mnist的图片如下：
SparkConf conf = new SparkConf()
.setMaster("local[*]") //local mode
.set("spark.kryo.registrator", "org.nd4j.Nd4jRegistrator")
.setAppName("Mnist Java Spark (Java)");
JavaSparkContext jsc = new JavaSparkContext(conf);
final List<String> lstLabelNames = Arrays.asList("零","一","二","三","四","五","六","七","八","九"); //Chinese Label
final ImageLoader imageLoader = new ImageLoader(28, 28, 1); //Load Image
final DataNormalization scaler = new ImagePreProcessingScaler(0, 1); //Normalize
String srcPath = args[0];
FileSystem hdfs = FileSystem.get(URI.create(srcPath),jsc.hadoopConfiguration()); //hdfs read local file system
FileStatus[] fileList = hdfs.listStatus(new Path(srcPath));
List<String> lstFilePath = new ArrayList<>();
for( FileStatus fileStatus : fileList){
lstFilePath.add(srcPath + "/" + fileStatus.getPath().getName());
}
JavaRDD<String> javaRDDImagePath = jsc.parallelize(lstFilePath);
JavaRDD<DataSet> javaRDDImageTrain = javaRDDImagePath.map(new Function<String, DataSet>() {
@Override
public DataSet call(String imagePath) throws Exception {
FileSystem fs = FileSystem.get(new Configuration());
DataInputStream in = fs.open(new Path(imagePath));
INDArray features = imageLoader.asRowVector(in); //features tensor
String[] tokens = imagePath.split("\\/");
String label = tokens[tokens.length-1].split("\\.")[0];
int intLabel = Integer.parseInt(label);
INDArray labels = Nd4j.zeros(10); //labels tensor
labels.putScalar(0, intLabel, 1.0);
DataSet trainData = http://www.mamicode.com/new DataSet(features, labels); //DataSet, wrapper of features and labels>这里有几点需要解释。
1.用hdfs.filesystem来获取文件。用Java原生态的File来操作也是完全可以的。只不过，这样读取文件的方式，同时适用于读取本地和HDFS上的文件。
2.ImageLoader类。这个类是用来读取图片文件的。类似的还有一个类，叫NativeImageLoader。不同的在于，NativeImageLoader是调用了OpenCV的相关方法来对图片做处理的，效率更高，因此推荐使用NativeImageLoader
然后，讲下模型训练任务的逻辑。读取HDFS上的以DataSet形式存储的Mnist文件-->定义参数中心服务-->定义神经网络结构（Lenet）--> 训练网络-->保存训练好的模型。首先看前两步的操作：
SparkConf conf = new SparkConf()
.set("spark.kryo.registrator", "org.nd4j.Nd4jRegistrator") //register kryo for nd4j
.setAppName("Mnist Java Spark (Java)");
final String imageFilePath = args[0];
final int numEpochs = Integer.parseInt(args[1]);
final String modelPath = args[2];
final int numBatch = Integer.parseInt(args[3]);
//
JavaSparkContext jsc = new JavaSparkContext(conf);
//
JavaRDD<DataSet> javaRDDImageTrain = jsc.objectFile(imageFilePath); //load image data from hdfs
ParameterAveragingTrainingMaster trainMaster = new ParameterAveragingTrainingMaster.Builder(numBatch) //weight average service
.workerPrefetchNumBatches(0)
.saveUpdater(true)
.averagingFrequency(5)
.batchSizePerWorker(numBatch)
这里我们获取传入的一些参数，如文件的hdfs路径，最后保存model的路径，mini-batch的大小（一般32，62，128这样的值为好，可以自行尝试），总的训练的轮次epoch。
这里需要解释的是ParameterAveragingTrainingMaster这个类。这个类的作用是用于将spark worker节点上各自计算的权重收回到driver节点上进行加权平均，并将最新的权重广播到worker节点上。也即为：将各个工作节点的参数的均值作为全局参数值。这种分布式机器学习中，数据并行化的一种操作。下面是定义神经网络结构和训练网络：
int nChannels = 1;
int outputNum = 10;
int iterations = 1;
int seed = 123;
MultiLayerConfiguration.Builder builder = new NeuralNetConfiguration.Builder() //define lenent
.seed(seed)
.iterations(iterations)
.regularization(true).l2(0.0005)
.learningRate(0.1)
.learningRateScoreBasedDecayRate(0.5)
.optimizationAlgo(OptimizationAlgorithm.STOCHASTIC_GRADIENT_DESCENT)
.updater(Updater.ADAM)
.list()
.layer(0, new ConvolutionLayer.Builder(5, 5)
.nIn(nChannels)
.stride(1, 1)
.nOut(20)
.weightInit(WeightInit.XAVIER)
.activation("relu")
.build())
.layer(1, new SubsamplingLayer.Builder(SubsamplingLayer.PoolingType.MAX)
.kernelSize(2, 2)
.build())
.layer(2, new ConvolutionLayer.Builder(5, 5)
.nIn(20)
.nOut(50)
.stride(2,2)
.weightInit(WeightInit.XAVIER)
.activation("relu")
.build())
.layer(3, new SubsamplingLayer.Builder(SubsamplingLayer.PoolingType.MAX)
.kernelSize(2, 2)
.build())
.layer(4, new DenseLayer.Builder().activation("relu")
.weightInit(WeightInit.XAVIER)
.nOut(500).build())
.layer(5, new OutputLayer.Builder(LossFunctions.LossFunction.NEGATIVELOGLIKELIHOOD)
.nOut(outputNum)
.weightInit(WeightInit.XAVIER)
.activation("softmax")
.build())
.backprop(true).pretrain(false);
new ConvolutionLayerSetup(builder,28,28,1);
MultiLayerConfiguration netconf = builder.build();
MultiLayerNetwork net = new MultiLayerNetwork(netconf);
net.setListeners(new ScoreIterationListener(1));
net.init();
SparkDl4jMultiLayer sparkNetwork = new SparkDl4jMultiLayer(jsc, net, trainMaster);
//train the network on Spark
for( int i = 0; i < numEpochs; ++i ){
sparkNetwork.fit(javaRDDImageTrain);
System.out.println("----- Epoch " + i + " complete -----");
Evaluation evalActual = sparkNetwork.evaluate(javaRDDImageTrain);
System.out.println(evalActual.stats());
}
这部分没有什么特别的地方，和单机的形式差不太多。值得说明的就是，我们在每一轮次的训练后，直接预测全部的训练数据来做评估，并没有做交叉验证。当然，做交叉验证也是完全可以的。
最后一部分是保存模型到hdfs上： //save model
FileSystem hdfs = FileSystem.get(jsc.hadoopConfiguration());
Path hdfsPath = new Path(modelPath);
FSDataOutputStream outputStream = hdfs.create(hdfsPath);
MultiLayerNetwork trainedNet = sparkNetwork.getNetwork();
ModelSerializer.writeModel(trainedNet, outputStream, true);到此coding的部分就结束了，我们构建了在Spark进行分布式深度神经网络的训练并保存了模型。Spark的提交命令如下：spark-submit --master yarn-cluster --executor-memory 5g --num-executors 16 --driver-memory 8g --conf "spark.executor.extraJavaOptions=-Dorg.bytedeco.javacpp.maxbytes=2921225472" --conf spark.yarn.executor.memoryOverhead=5000
需要说明的是--conf后面的内容，因为Nd4j在计算的时候，实际需要两部分的内存：on-heap memory和off-heap memory。前者就是jvm为开辟对象所需内存，后者是C++的内存。Nd4j为了效率，在底层是通过JavaCPP调用C++进行计算的。如果不显示地申请C++的内存，那默认会从on-heap中分出10%给off-heap，但这样可能会不够。所以我们显示地申请off-heap内存。
然后，我们看下训练的结果：
----- Epoch 149 complete -----
Examples labeled as 0 classified by model as 0: 4011 times
Examples labeled as 0 classified by model as 1: 2 times
Examples labeled as 0 classified by model as 2: 14 times
Examples labeled as 0 classified by model as 4: 9 times
Examples labeled as 0 classified by model as 5: 11 times
Examples labeled as 0 classified by model as 6: 28 times
Examples labeled as 0 classified by model as 7: 6 times
Examples labeled as 0 classified by model as 8: 40 times
Examples labeled as 0 classified by model as 9: 11 times
Examples labeled as 1 classified by model as 0: 1 times
Examples labeled as 1 classified by model as 1: 4598 times
Examples labeled as 1 classified by model as 2: 20 times
Examples labeled as 1 classified by model as 3: 7 times
Examples labeled as 1 classified by model as 4: 12 times
Examples labeled as 1 classified by model as 5: 3 times
Examples labeled as 1 classified by model as 6: 8 times
Examples labeled as 1 classified by model as 7: 10 times
Examples labeled as 1 classified by model as 8: 20 times
Examples labeled as 1 classified by model as 9: 5 times
Examples labeled as 2 classified by model as 0: 13 times
Examples labeled as 2 classified by model as 1: 20 times
Examples labeled as 2 classified by model as 2: 3910 times
Examples labeled as 2 classified by model as 3: 63 times
Examples labeled as 2 classified by model as 4: 22 times
Examples labeled as 2 classified by model as 5: 5 times
Examples labeled as 2 classified by model as 6: 4 times
Examples labeled as 2 classified by model as 7: 70 times
Examples labeled as 2 classified by model as 8: 54 times
Examples labeled as 2 classified by model as 9: 16 times
Examples labeled as 3 classified by model as 0: 2 times
Examples labeled as 3 classified by model as 1: 10 times
Examples labeled as 3 classified by model as 2: 55 times
Examples labeled as 3 classified by model as 3: 4104 times
Examples labeled as 3 classified by model as 4: 5 times
Examples labeled as 3 classified by model as 5: 53 times
Examples labeled as 3 classified by model as 6: 2 times
Examples labeled as 3 classified by model as 7: 42 times
Examples labeled as 3 classified by model as 8: 56 times
Examples labeled as 3 classified by model as 9: 22 times
Examples labeled as 4 classified by model as 0: 5 times
Examples labeled as 4 classified by model as 1: 6 times
Examples labeled as 4 classified by model as 2: 5 times
Examples labeled as 4 classified by model as 4: 3960 times
Examples labeled as 4 classified by model as 5: 3 times
Examples labeled as 4 classified by model as 6: 22 times
Examples labeled as 4 classified by model as 7: 9 times
Examples labeled as 4 classified by model as 8: 16 times
Examples labeled as 4 classified by model as 9: 46 times
Examples labeled as 5 classified by model as 0: 5 times
Examples labeled as 5 classified by model as 1: 7 times
Examples labeled as 5 classified by model as 2: 5 times
Examples labeled as 5 classified by model as 3: 40 times
Examples labeled as 5 classified by model as 4: 8 times
Examples labeled as 5 classified by model as 5: 3626 times
Examples labeled as 5 classified by model as 6: 27 times
Examples labeled as 5 classified by model as 7: 5 times
Examples labeled as 5 classified by model as 8: 66 times
Examples labeled as 5 classified by model as 9: 6 times
Examples labeled as 6 classified by model as 0: 9 times
Examples labeled as 6 classified by model as 1: 6 times
Examples labeled as 6 classified by model as 2: 5 times
Examples labeled as 6 classified by model as 3: 2 times
Examples labeled as 6 classified by model as 4: 47 times
Examples labeled as 6 classified by model as 5: 34 times
Examples labeled as 6 classified by model as 6: 3990 times
Examples labeled as 6 classified by model as 8: 43 times
Examples labeled as 6 classified by model as 9: 1 times
Examples labeled as 7 classified by model as 0: 6 times
Examples labeled as 7 classified by model as 1: 15 times
Examples labeled as 7 classified by model as 2: 57 times
Examples labeled as 7 classified by model as 3: 45 times
Examples labeled as 7 classified by model as 4: 22 times
Examples labeled as 7 classified by model as 5: 4 times
Examples labeled as 7 classified by model as 7: 4168 times
Examples labeled as 7 classified by model as 8: 21 times
Examples labeled as 7 classified by model as 9: 63 times
Examples labeled as 8 classified by model as 0: 15 times
Examples labeled as 8 classified by model as 1: 11 times
Examples labeled as 8 classified by model as 2: 23 times
Examples labeled as 8 classified by model as 3: 17 times
Examples labeled as 8 classified by model as 4: 19 times
Examples labeled as 8 classified by model as 5: 27 times
Examples labeled as 8 classified by model as 6: 35 times
Examples labeled as 8 classified by model as 7: 15 times
Examples labeled as 8 classified by model as 8: 3848 times
Examples labeled as 8 classified by model as 9: 53 times
Examples labeled as 9 classified by model as 0: 21 times
Examples labeled as 9 classified by model as 1: 3 times
Examples labeled as 9 classified by model as 2: 8 times
Examples labeled as 9 classified by model as 3: 26 times
Examples labeled as 9 classified by model as 4: 109 times
Examples labeled as 9 classified by model as 5: 23 times
Examples labeled as 9 classified by model as 6: 6 times
Examples labeled as 9 classified by model as 7: 62 times
Examples labeled as 9 classified by model as 8: 42 times
Examples labeled as 9 classified by model as 9: 3888 times
==========================Scores========================================
Accuracy: 0.9548
Precision: 0.9546
Recall: 0.9547
F1 Score: 0.9547
========================================================================
在150轮的训练过后，模型的准确率达到了95.48%。误判的情况也列在上面了。
到此，在Spark上进行Mnist数据集的训练和评估就完成了。总结一下就是，先将数据以RDD的形式保存到HDFS上，然后建模读取RDD并训练模型。其实，将图片存在HDFS上也是一种方案，但是HDFS的一个block可能需要占用32M，64M这样的空间。因此图片这样的小文件，是很占用集群的存储空间的。并且，当图片数量很多的时候，我们会为了读取图片频繁地和HDFS建立和释放网络链接，这样同样消耗HDFS的资源。因此我们选择先在本地存储RDD的形式来处理。其实分布式的机器学习有很多策略，比如数据的并行化和模型的并行化，这里只是一笔掠过，待自己研究清楚了再写点东西。最后就是模型的调参。这里面我们也没有提，其实是极其重要的。因为目前，还没有非常权威的，或者定义的调参方案，因为训练过程每个人是不同的，所以只能结合自己的训练情况来调。一般当loss不下降的时候，调小学习率，batch-size也试着调小来看看效果，分布式的学习率较单机的要大些，这些原则去调。
Deeplearning4j 实战（2）：Deeplearning4j 手写体数字识别Spark实现
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