Package org.apache.hadoop.io

Class ReadaheadPool

java.lang.Object

org.apache.hadoop.io.ReadaheadPool

@Private
@Evolving
public class ReadaheadPool
extends Object

Manages a pool of threads which can issue readahead requests on file descriptors.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	ReadaheadPool.ReadaheadRequest	An outstanding readahead request that has been submitted to the pool.

Method Summary

Modifier and Type	Method	Description
static ReadaheadPool	getInstance()
ReadaheadPool.ReadaheadRequest	readaheadStream(String identifier, FileDescriptor fd, long curPos, long readaheadLength, long maxOffsetToRead, ReadaheadPool.ReadaheadRequest lastReadahead)	Issue a request to readahead on the given file descriptor.
static void	resetInstance()
ReadaheadPool.ReadaheadRequest	submitReadahead(String identifier, FileDescriptor fd, long off, long len)	Submit a request to readahead on the given file descriptor.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

getInstance

public static ReadaheadPool getInstance()

Returns:

Return the singleton instance for the current process.

resetInstance

@VisibleForTesting
public static void resetInstance()

readaheadStream

public ReadaheadPool.ReadaheadRequest readaheadStream(String identifier,
 FileDescriptor fd,
 long curPos,
 long readaheadLength,
 long maxOffsetToRead,
 ReadaheadPool.ReadaheadRequest lastReadahead)

Issue a request to readahead on the given file descriptor.

Parameters:

identifier - a textual identifier that will be used in error messages (e.g. the file name)

fd - the file descriptor to read ahead

curPos - the current offset at which reads are being issued

readaheadLength - the configured length to read ahead

maxOffsetToRead - the maximum offset that will be readahead (useful if, for example, only some segment of the file is requested by the user). Pass Long.MAX_VALUE to allow readahead to the end of the file.

lastReadahead - the result returned by the previous invocation of this function on this file descriptor, or null if this is the first call

Returns:

an object representing this outstanding request, or null if no readahead was performed

submitReadahead

public ReadaheadPool.ReadaheadRequest submitReadahead(String identifier,
 FileDescriptor fd,
 long off,
 long len)

Submit a request to readahead on the given file descriptor.

Parameters:

identifier - a textual identifier used in error messages, etc.

fd - the file descriptor to readahead

off - the offset at which to start the readahead

len - the number of bytes to read

Returns:

an object representing this pending request