MapReduce Application中mapper的数目和分片的数目是一样的,但是分片数目和什么有关呢?
FileInputFormat.getSplits返回文件的分片数目,这部分将介绍其运行流程,后面将粘贴其源码并给出注释
public InputSplit[] getSplits(JobConf job, int numSplits)
throws IOException {
//获取输入文件列表files,其中会遍历输入目录的子目录,并过滤掉部分文件,如文件_SUCCESS
FileStatus[] files = listStatus(job);
// Save the number of input files for metrics/loadgen
job.setLong(NUM_INPUT_FILES, files.length);
long totalSize = 0; // compute total size
for (FileStatus file: files) { // check we have valid files
if (file.isDirectory()) {
throw new IOException("Not a file: "+ file.getPath());
}
totalSize += file.getLen();
}
/*
* numSplits为设置的map数目
* 期待的分片大小
*/
long goalSize = totalSize / (numSplits == 0 ? 1 : numSplits);
/*
* FileInputFormat.SPLIT_MINSIZE为参数值:mapreduce.input.fileinputformat.split.minsize,默认为0
* minSplitSize默认为1
*/
long minSize = Math.max(job.getLong(org.apache.hadoop.mapreduce.lib.input.
FileInputFormat.SPLIT_MINSIZE, 1), minSplitSize);
// generate splits
ArrayList<FileSplit> splits = new ArrayList<FileSplit>(numSplits);
NetworkTopology clusterMap = new NetworkTopology();
for (FileStatus file: files) {
Path path = file.getPath();
long length = file.getLen();
if (length != 0) {
FileSystem fs = path.getFileSystem(job);
BlockLocation[] blkLocations;
if (file instanceof LocatedFileStatus) {
blkLocations = ((LocatedFileStatus) file).getBlockLocations();
} else {
blkLocations = fs.getFileBlockLocations(file, 0, length);
}
if (isSplitable(fs, path)) {
long blockSize = file.getBlockSize();
/*
* 计算分片的大小,每一个文件都要计算一次
*computeSplitSize的计算公式为 Math.max(minSize, Math.min(goalSize, blockSize));
*/
long splitSize = computeSplitSize(goalSize, minSize, blockSize);
long bytesRemaining = length;
while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
String[] splitHosts = getSplitHosts(blkLocations,
length-bytesRemaining, splitSize, clusterMap);
splits.add(makeSplit(path, length-bytesRemaining, splitSize,
splitHosts));
bytesRemaining -= splitSize;
}
if (bytesRemaining != 0) {
String[] splitHosts = getSplitHosts(blkLocations, length
- bytesRemaining, bytesRemaining, clusterMap);
splits.add(makeSplit(path, length - bytesRemaining, bytesRemaining,
splitHosts));
}
} else {
String[] splitHosts = getSplitHosts(blkLocations,0,length,clusterMap);
splits.add(makeSplit(path, 0, length, splitHosts));
}
} else {
//Create empty hosts array for zero length files
splits.add(makeSplit(path, 0, length, new String[0]));
}
}
LOG.debug("Total # of splits: " + splits.size());
return splits.toArray(new FileSplit[splits.size()]);
}
看源码还是很有用的。很多时候,博客或者书介绍的不是很中肯,或者会有错误,看源码就不会出现这些问题。