/**
* Merge zero or more spill files together, choosing the fastest merging strategy based on the
* number of spills and the IO compression codec.
*
* @return the partition lengths in the merged file.
*/
private long[] mergeSpills(SpillInfo[] spills) throws IOException {
final File outputFile = shuffleBlockResolver.getDataFile(shuffleId, mapId);
final boolean compressionEnabled = sparkConf.getBoolean("spark.shuffle.compress", true);
final CompressionCodec compressionCodec = CompressionCodec$.MODULE$.createCodec(sparkConf);
final boolean fastMergeEnabled =
sparkConf.getBoolean("spark.shuffle.unsafe.fastMergeEnabled", true);
final boolean fastMergeIsSupported =
!compressionEnabled || compressionCodec instanceof LZFCompressionCodec;
try {
if (spills.length == 0) {
new FileOutputStream(outputFile).close(); // Create an empty file
return new long[partitioner.numPartitions()];
} else if (spills.length == 1) {
// Here, we don't need to perform any metrics updates because the bytes written to this
// output file would have already been counted as shuffle bytes written.
Files.move(spills[0].file, outputFile);
return spills[0].partitionLengths;
} else {
final long[] partitionLengths;
// There are multiple spills to merge, so none of these spill files' lengths were counted
// towards our shuffle write count or shuffle write time. If we use the slow merge path,
// then the final output file's size won't necessarily be equal to the sum of the spill
// files' sizes. To guard against this case, we look at the output file's actual size when
// computing shuffle bytes written.
//
// We allow the individual merge methods to report their own IO times since different merge
// strategies use different IO techniques. We count IO during merge towards the shuffle
// shuffle write time, which appears to be consistent with the "not bypassing merge-sort"
// branch in ExternalSorter.
if (fastMergeEnabled && fastMergeIsSupported) {
// Compression is disabled or we are using an IO compression codec that supports
// decompression of concatenated compressed streams, so we can perform a fast spill merge
// that doesn't need to interpret the spilled bytes.
if (transferToEnabled) {
logger.debug("Using transferTo-based fast merge");
partitionLengths = mergeSpillsWithTransferTo(spills, outputFile);
} else {
logger.debug("Using fileStream-based fast merge");
partitionLengths = mergeSpillsWithFileStream(spills, outputFile, null);
}
} else {
logger.debug("Using slow merge");
partitionLengths = mergeSpillsWithFileStream(spills, outputFile, compressionCodec);
}
// When closing an UnsafeShuffleExternalSorter that has already spilled once but also has
// in-memory records, we write out the in-memory records to a file but do not count that
// final write as bytes spilled (instead, it's accounted as shuffle write). The merge needs
// to be counted as shuffle write, but this will lead to double-counting of the final
// SpillInfo's bytes.
writeMetrics.decShuffleBytesWritten(spills[spills.length - 1].file.length());
writeMetrics.incShuffleBytesWritten(outputFile.length());
return partitionLengths;
}
} catch (IOException e) {
if (outputFile.exists() && !outputFile.delete()) {
logger.error("Unable to delete output file {}", outputFile.getPath());
}
throw e;
}
}
