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Java SDK for Microsoft Azure Data Lake Store
package com.microsoft.azure.datalake.store;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.Collection;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
import java.util.concurrent.CountDownLatch;
/**
* Internal-use only; do not use.
*
* Manage the read-ahead buffers and queues.
*
* The class has a pool of 16 byte-buffers it manages ('buffers' variable). It has a freeList, which contains the
* index in the buffers array of buffers that are available to take. It also contains the in-progress and completed
* lists.
*
* When a read-ahead comes in, the manager looks to see if there is an available buffer in the free-list. If there is,
* it creates a ReadBuffer object and assigns the byte-buffer to it. The ReadBuffer object tracks a read-ahead through
* it's life-cycle. At the end if the life-cycle, the buffer is returned back (by adding it's index to free-list), and
* the ReadBuffer object is garbage collected.
*
* The ReadBuffer is initially put in the readAheadQueue (i.e., the waiting queue). When a worker thread picks it up,
* the ReadBuffer is moved to the inProgressList. When the worker threads gets done with the read, the ReadBuffer is
* moved to the completedReadList. the buffer sits there until the space is needed by another read-ahead - it is only
* evicted if the space is needed. Upon eviction, the byte[] buffer is returned to the free pool and the ReadBuffer
* object is GC'ed. If a read request comes in for an offset that is still in the readAheadQueue (i.e., nothing has
* been done on it), then the object is removed from the queue, and the call returns 0, indicating to the calling thread
* to do the read itself (since there is no sense in waiting in the queue for some unbounded future time).
*
* Overhead of the ReadBufferManager: there are a total of 16 buffers (so 16 * 4MB = 64MB total memory overhead). Also,
* there are 8 worker threads (that are either reading data so blocked waiting for IO, or are blocked on readAheadQueue,
* waiting for read-ahead requests to arrive).
*
* Side benefit: since the prefetched buffers sit around in the completedReadList until evicted, there may be a slight
* benefit of cache hits, although the main benefit is expected to be from the read-ahead, not so much from the cache
* effect.
*/
class ReadBufferManager {
private static final Logger log = LoggerFactory.getLogger("com.microsoft.azure.datalake.store.ReadBufferManager");
private static final int numBuffers = 16;
private final static int blocksize = 4 * 1024 * 1024;
private final static int numThreads = 8;
private final static int thresholdAgeMilliseconds = 3000; // have to see if 3 seconds is a good threshold
private Thread[] threads = new Thread[numThreads];
private byte[][] buffers; // array of byte[] buffers, to hold the data that is read
private Stack freeList = new Stack(); // indices in buffers[] array that are available
private Queue readAheadQueue = new LinkedList(); // queue of requests that are not picked up by any worker thread yet
private LinkedList inProgressList = new LinkedList(); // requests being processed by worker threads
private LinkedList completedReadList = new LinkedList(); // buffers available for reading
private static final ReadBufferManager bufferManager; // singleton, initialized in static initialization block
static {
bufferManager = new ReadBufferManager();
bufferManager.init();
}
static ReadBufferManager getBufferManager() { return bufferManager; }
private void init() {
buffers = new byte[numBuffers][];
for (int i = 0; i < numBuffers; i++) {
buffers[i] = new byte[blocksize]; // same buffers are reused. The byte array never goes back to GC
freeList.add(i);
}
for (int i = 0; i < numThreads; i++) {
Thread t = new Thread(new ReadBufferWorker(i));
t.setDaemon(true);
threads[i] = t;
t.setName("ADLS-prefetch-" + i);
t.start();
}
ReadBufferWorker.unleashWorkers.countDown();
}
// hide instance constructor
private ReadBufferManager() {
}
/*
*
* ADLFileInputStream-facing methods
*
*/
/**
* {@link ADLFileInputStream} calls this method to queue read-aheads
* @param file The {@link ADLFileInputStream} for which to do the read-ahead
* @param requestedOffset The offset in the file which shoukd be read
* @param requestedLength The length to read
*/
void queueReadAhead(ADLFileInputStream file, long requestedOffset, int requestedLength) {
if (log.isTraceEnabled())
log.trace("Start Queueing readAhead for " + file.getFilename() + " offset " + requestedOffset + " length " + requestedLength);
ReadBuffer buffer;
synchronized (this) {
if (isAlreadyQueued(file, requestedOffset)) return; // already queued, do not queue again
if (freeList.size() == 0 && !tryEvict()) return; // no buffers available, cannot queue anything
buffer = new ReadBuffer();
buffer.file = file;
buffer.offset = requestedOffset;
buffer.length = 0;
buffer.requestedLength = requestedLength;
buffer.status = ReadBufferStatus.NOT_AVAILABLE;
buffer.latch = new CountDownLatch(1);
Integer bufferIndex = freeList.pop(); // will return a value, since we have checked size > 0 already
buffer.buffer = buffers[bufferIndex];
buffer.bufferindex = bufferIndex;
readAheadQueue.add(buffer);
notifyAll();
}
if (log.isTraceEnabled()) {
log.trace("Done q-ing readAhead for file " + file.getFilename() + " offset " + requestedOffset + " buffer idx " + buffer.bufferindex);
}
}
/**
* {@link ADLFileInputStream} calls this method read any bytes already available in a buffer (thereby saving a
* remote read). This returns the bytes if the data already exists in buffer. If there is a buffer that is reading
* the requested offset, then this method blocks until that read completes. If the data is queued in a read-ahead
* but not picked up by a worker thread yet, then it cancels that read-ahead and reports cache miss. This is because
* depending on worker thread availability, the read-ahead may take a while - the calling thread can do it's own
* read to get the data faster (copmared to the read waiting in queue for an indeterminate amount of time).
*
* @param file the file to read bytes for
* @param position the offset in the file to do a read for
* @param length the length to read
* @param buffer the buffer to read data into. Note that the buffer will be written into from offset 0.
* @return the number of bytes read
*/
int getBlock(ADLFileInputStream file, long position, int length, byte[] buffer) {
// not synchronized, so have to be careful with locking
if (log.isTraceEnabled())
log.trace("getBlock for file " + file.getFilename() + " position " + position + " thread " + Thread.currentThread().getName());
{ // block scope, to scope the usage of readbuf. The two synchronized blocks should not share any data, to
// ensure there are no race conditions.
ReadBuffer readBuf;
synchronized (this) {
clearFromReadAheadQueue(file, position);
readBuf = getFromList(inProgressList, file, position);
}
if (readBuf != null) { // if in in-progress queue, then block for it
try {
if (log.isTraceEnabled())
log.trace("got a relevant read buffer for file " + file.getFilename() + " offset " + readBuf.offset + " buffer idx " + readBuf.bufferindex);
readBuf.latch.await(); // blocking wait on the caller stream's thread
// Note on correctness: readBuf gets out of inProgressList only in 1 place: after worker thread
// is done processing it (in doneReading). There, the latch is set after removing the buffer from
// inProgressList. So this latch is safe to be outside the synchronized block.
// Putting it in synchronized would result in a deadlock, since this thread would be holding the lock
// while waiting, so no one will be able to change any state. If this becomes more complex in the future,
// then the latch cane be removed and replaced with wait/notify whenever inProgressList is touched.
} catch (InterruptedException ex) {
Thread.currentThread().interrupt();
}
if (log.isTraceEnabled())
log.trace("latch done for file " + file.getFilename() + " buffer idx " + readBuf.bufferindex + " length " + readBuf.length);
}
}
int bytesRead = 0;
synchronized (this) {
bytesRead = getBlockFromCompletedQueue(file, position, length, buffer);
}
if (bytesRead > 0) {
if (log.isTraceEnabled())
log.trace("Done read from Cache for " + file.getFilename() + " position " + position + " length " + bytesRead);
return bytesRead;
}
// otherwise, just say we got nothing - calling thread can do it's own read
return 0;
}
/*
*
* Internal methods
*
*/
/**
* If any buffer in the completedlist can be reclaimed then reclaim it and return the buffer to free list.
* The objective is to find just one buffer - there is no advantage to evicting more than one.
* @return whether the eviction succeeeded - i.e., were we able to free up one buffer
*/
private synchronized boolean tryEvict() {
ReadBuffer nodeToEvict = null;
if (completedReadList.size() <= 0) return false; // there are no evict-able buffers
// first, try buffers where all bytes have been consumed (approximated as first and last bytes consumed)
for (ReadBuffer buf : completedReadList) {
if (buf.firstByteConsumed && buf.lastByteConsumed) {
nodeToEvict = buf;
break;
}
}
if (nodeToEvict != null) return evict(nodeToEvict);
// next, try buffers where any bytes have been consumed (may be a bad idea? have to experiment and see)
for (ReadBuffer buf : completedReadList) {
if (buf.anyByteConsumed) {
nodeToEvict = buf;
break;
}
}
if (nodeToEvict != null) return evict(nodeToEvict);
// next, try any old nodes that have not been consumed
long earliestBirthday = Long.MAX_VALUE;
for (ReadBuffer buf : completedReadList) {
if (buf.birthday < earliestBirthday) {
nodeToEvict = buf;
earliestBirthday = buf.birthday;
}
}
if ((currentTimeMillis() - earliestBirthday > thresholdAgeMilliseconds) && (nodeToEvict != null) )
{ return evict(nodeToEvict); }
// nothing can be evicted
return false;
}
private boolean evict(ReadBuffer buf) {
freeList.push(buf.bufferindex);
completedReadList.remove(buf);
if (log.isTraceEnabled())
log.trace("Evicting buffer idx " + buf.bufferindex + "; was used for file " + buf.file.getFilename() +
" offset " + buf.offset + " length " + buf.length);
return true;
}
private boolean isAlreadyQueued(ADLFileInputStream file, long requestedOffset) {
// returns true if any part of the buffer is already queued
return (isInList(readAheadQueue, file, requestedOffset) ||
isInList(inProgressList, file, requestedOffset) ||
isInList(completedReadList, file, requestedOffset) );
}
private boolean isInList(Collection list, ADLFileInputStream file, long requestedOffset ) {
return (getFromList(list, file, requestedOffset) != null);
}
private ReadBuffer getFromList(Collection list, ADLFileInputStream file, long requestedOffset ) {
for (ReadBuffer buffer : list) {
if (buffer.file == file) {
if (buffer.status == ReadBufferStatus.AVAILABLE
&& requestedOffset >= buffer.offset
&& requestedOffset < buffer.offset + buffer.length
) {
return buffer;
} else if (requestedOffset >= buffer.offset
&& requestedOffset < buffer.offset + buffer.requestedLength
) {
return buffer;
}
}
}
return null;
}
private void clearFromReadAheadQueue(ADLFileInputStream file, long requestedOffset) {
ReadBuffer buffer = getFromList(readAheadQueue, file, requestedOffset);
if (buffer != null) {
readAheadQueue.remove(buffer);
notifyAll(); // lock is held in calling method
freeList.push(buffer.bufferindex);
}
}
private int getBlockFromCompletedQueue(ADLFileInputStream file, long position, int length, byte[] buffer) {
ReadBuffer buf = getFromList(completedReadList, file, position);
if (buf == null || position >= buf.offset + buf.length) return 0;
int cursor = (int) (position - buf.offset);
int availableLengthInBuffer = buf.length - cursor;
int lengthToCopy = Math.min(length, availableLengthInBuffer);
System.arraycopy(buf.buffer, cursor, buffer, 0, lengthToCopy);
if (cursor == 0) buf.firstByteConsumed = true;
if (cursor + lengthToCopy == buf.length) buf.lastByteConsumed = true;
buf.anyByteConsumed = true;
return lengthToCopy;
}
/*
*
* ReadBufferWorker-thread-facing methods
*
*/
/**
* ReadBufferWorker thread calls this to get the next buffer that it should work on.
* @return {@link ReadBuffer}
* @throws InterruptedException if thread is interrupted
*/
ReadBuffer getNextBlockToRead() throws InterruptedException {
ReadBuffer buffer = null;
synchronized (this) {
//buffer = readAheadQueue.take(); // blocking method
while (readAheadQueue.size() == 0) wait();
buffer = readAheadQueue.remove();
notifyAll();
if (buffer == null) return null; // should never happen
buffer.status = ReadBufferStatus.READING_IN_PROGRESS;
inProgressList.add(buffer);
}
if (log.isTraceEnabled())
log.trace("ReadBufferWorker picked file " + buffer.file.getFilename() + " for offset " + buffer.offset);
return buffer;
}
/**
*
* ReadBufferWorker thread calls this method to post completion
*
* @param buffer the buffer whose read was completed
* @param result the {@link ReadBufferStatus} after the read operation in the worker thread
* @param bytesActuallyRead the number of bytes that the worker thread was actually able to read
*/
void doneReading(ReadBuffer buffer, ReadBufferStatus result, int bytesActuallyRead) {
if (log.isTraceEnabled())
log.trace("ReadBufferWorker completed file " + buffer.file.getFilename() + " for offset " + buffer.offset + " bytes " + bytesActuallyRead);
synchronized (this) {
inProgressList.remove(buffer);
if (result == ReadBufferStatus.AVAILABLE && bytesActuallyRead > 0) {
buffer.status = ReadBufferStatus.AVAILABLE;
buffer.birthday = currentTimeMillis();
buffer.length = bytesActuallyRead;
completedReadList.add(buffer);
} else {
freeList.push(buffer.bufferindex);
// buffer should go out of scope after the end of the calling method in ReadBufferWorker, and eligible for GC
}
}
//outside the synchronized, since anyone receiving a wake-up from the latch must see safe-published results
buffer.latch.countDown(); // wake up waiting threads (if any)
}
/**
* Similar to System.currentTimeMillis, except implemented with System.nanoTime().
* System.currentTimeMillis can go backwards when system clock is changed (e.g., with NTP time synchronization),
* making it unsuitable for measuring time intervals. nanotime is strictly monotonically increasing,
* so it is much more suitable to measuring intervals.
*
* @return current time in milliseconds
*/
private long currentTimeMillis() {
return System.nanoTime() / 1000/ 1000;
}
}
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