You can also find the Java native IO mlock and munlock call to memory mapped data in org.apache.hadoop.io.nativeio.NativeIO.java

https://github.com/apache/hadoop-common/blob/trunk/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/io/nativeio/NativeIO.java

and the corresponding C native code
https://github.com/apache/hadoop-common/blob/trunk/hadoop-common-project/hadoop-common/src/main/native/src/org/apache/hadoop/io/nativeio/NativeIO.c

/**
* Used to manipulate the operating system cache.
*/
@VisibleForTesting
public static class CacheManipulator {
public void mlock(String identifier, ByteBuffer buffer,
long len) throws IOException {
POSIX.mlock(buffer, len);
}

public long getMemlockLimit() {
return NativeIO.getMemlockLimit();
}

public long getOperatingSystemPageSize() {
return NativeIO.getOperatingSystemPageSize();
}

public void posixFadviseIfPossible(String identifier,
FileDescriptor fd, long offset, long len, int flags)
throws NativeIOException {
NativeIO.POSIX.posixFadviseIfPossible(identifier, fd, offset,
len, flags);
}

public boolean verifyCanMlock() {
return NativeIO.isAvailable();
}
}

NameNode has to keep track of the memory cache replicas at different DataNodes with this feature. The cache blocks infos are exchanged between them via heartbeat protocol. As a result, additional metadata e.g.. cachedHosts array is introduced in BlockLocation as seen below.

public class BlockLocation {
  private String[] hosts; // Datanode hostnames
  <strong>private String[] cachedHosts; // Datanode hostnames with a cached replica</strong>
  private String[] names; // Datanode IP:xferPort for accessing the block
  private String[] topologyPaths; // Full path name in network topology
  private long offset;  // Offset of the block in the file
  private long length;
  private boolean corrupt;

Stay tuned for more infos…