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/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.fs.azurebfs.services;
import org.apache.hadoop.fs.azurebfs.contracts.services.ReadBufferStatus;
import com.facebook.presto.hadoop.$internal.org.slf4j.Logger;
import com.facebook.presto.hadoop.$internal.org.slf4j.LoggerFactory;
import java.util.Collection;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
import java.util.concurrent.CountDownLatch;
/**
* The Read Buffer Manager for Rest AbfsClient.
*/
final class ReadBufferManager {
private static final Logger LOGGER = LoggerFactory.getLogger(ReadBufferManager.class);
private static final int NUM_BUFFERS = 16;
private static final int BLOCK_SIZE = 4 * 1024 * 1024;
private static final int NUM_THREADS = 8;
private static final int THRESHOLD_AGE_MILLISECONDS = 3000; // have to see if 3 seconds is a good threshold
private Thread[] threads = new Thread[NUM_THREADS];
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 BUFFER_MANAGER; // singleton, initialized in static initialization block
static {
BUFFER_MANAGER = new ReadBufferManager();
BUFFER_MANAGER.init();
}
static ReadBufferManager getBufferManager() {
return BUFFER_MANAGER;
}
private void init() {
buffers = new byte[NUM_BUFFERS][];
for (int i = 0; i < NUM_BUFFERS; i++) {
buffers[i] = new byte[BLOCK_SIZE]; // same buffers are reused. The byte array never goes back to GC
freeList.add(i);
}
for (int i = 0; i < NUM_THREADS; i++) {
Thread t = new Thread(new ReadBufferWorker(i));
t.setDaemon(true);
threads[i] = t;
t.setName("ABFS-prefetch-" + i);
t.start();
}
ReadBufferWorker.UNLEASH_WORKERS.countDown();
}
// hide instance constructor
private ReadBufferManager() {
}
/*
*
* AbfsInputStream-facing methods
*
*/
/**
* {@link AbfsInputStream} calls this method to queue read-aheads.
*
* @param stream The {@link AbfsInputStream} 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(final AbfsInputStream stream, final long requestedOffset, final int requestedLength) {
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("Start Queueing readAhead for {} offset {} length {}",
stream.getPath(), requestedOffset, requestedLength);
}
ReadBuffer buffer;
synchronized (this) {
if (isAlreadyQueued(stream, requestedOffset)) {
return; // already queued, do not queue again
}
if (freeList.isEmpty() && !tryEvict()) {
return; // no buffers available, cannot queue anything
}
buffer = new ReadBuffer();
buffer.setStream(stream);
buffer.setOffset(requestedOffset);
buffer.setLength(0);
buffer.setRequestedLength(requestedLength);
buffer.setStatus(ReadBufferStatus.NOT_AVAILABLE);
buffer.setLatch(new CountDownLatch(1));
Integer bufferIndex = freeList.pop(); // will return a value, since we have checked size > 0 already
buffer.setBuffer(buffers[bufferIndex]);
buffer.setBufferindex(bufferIndex);
readAheadQueue.add(buffer);
notifyAll();
}
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("Done q-ing readAhead for file {} offset {} buffer idx {}",
stream.getPath(), requestedOffset, buffer.getBufferindex());
}
}
/**
* {@link AbfsInputStream} 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 stream 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(final AbfsInputStream stream, final long position, final int length, final byte[] buffer) {
// not synchronized, so have to be careful with locking
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("getBlock for file {} position {} thread {}",
stream.getPath(), position, Thread.currentThread().getName());
}
waitForProcess(stream, position);
int bytesRead = 0;
synchronized (this) {
bytesRead = getBlockFromCompletedQueue(stream, position, length, buffer);
}
if (bytesRead > 0) {
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("Done read from Cache for {} position {} length {}",
stream.getPath(), position, bytesRead);
}
return bytesRead;
}
// otherwise, just say we got nothing - calling thread can do its own read
return 0;
}
/*
*
* Internal methods
*
*/
private void waitForProcess(final AbfsInputStream stream, final long position) {
ReadBuffer readBuf;
synchronized (this) {
clearFromReadAheadQueue(stream, position);
readBuf = getFromList(inProgressList, stream, position);
}
if (readBuf != null) { // if in in-progress queue, then block for it
try {
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("got a relevant read buffer for file {} offset {} buffer idx {}",
stream.getPath(), readBuf.getOffset(), readBuf.getBufferindex());
}
readBuf.getLatch().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 (LOGGER.isTraceEnabled()) {
LOGGER.trace("latch done for file {} buffer idx {} length {}",
stream.getPath(), readBuf.getBufferindex(), readBuf.getLength());
}
}
}
/**
* 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.isFirstByteConsumed() && buf.isLastByteConsumed()) {
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.isAnyByteConsumed()) {
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.getTimeStamp() < earliestBirthday) {
nodeToEvict = buf;
earliestBirthday = buf.getTimeStamp();
}
}
if ((currentTimeMillis() - earliestBirthday > THRESHOLD_AGE_MILLISECONDS) && (nodeToEvict != null)) {
return evict(nodeToEvict);
}
// nothing can be evicted
return false;
}
private boolean evict(final ReadBuffer buf) {
freeList.push(buf.getBufferindex());
completedReadList.remove(buf);
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("Evicting buffer idx {}; was used for file {} offset {} length {}",
buf.getBufferindex(), buf.getStream().getPath(), buf.getOffset(), buf.getLength());
}
return true;
}
private boolean isAlreadyQueued(final AbfsInputStream stream, final long requestedOffset) {
// returns true if any part of the buffer is already queued
return (isInList(readAheadQueue, stream, requestedOffset)
|| isInList(inProgressList, stream, requestedOffset)
|| isInList(completedReadList, stream, requestedOffset));
}
private boolean isInList(final Collection list, final AbfsInputStream stream, final long requestedOffset) {
return (getFromList(list, stream, requestedOffset) != null);
}
private ReadBuffer getFromList(final Collection list, final AbfsInputStream stream, final long requestedOffset) {
for (ReadBuffer buffer : list) {
if (buffer.getStream() == stream) {
if (buffer.getStatus() == ReadBufferStatus.AVAILABLE
&& requestedOffset >= buffer.getOffset()
&& requestedOffset < buffer.getOffset() + buffer.getLength()) {
return buffer;
} else if (requestedOffset >= buffer.getOffset()
&& requestedOffset < buffer.getOffset() + buffer.getRequestedLength()) {
return buffer;
}
}
}
return null;
}
private void clearFromReadAheadQueue(final AbfsInputStream stream, final long requestedOffset) {
ReadBuffer buffer = getFromList(readAheadQueue, stream, requestedOffset);
if (buffer != null) {
readAheadQueue.remove(buffer);
notifyAll(); // lock is held in calling method
freeList.push(buffer.getBufferindex());
}
}
private int getBlockFromCompletedQueue(final AbfsInputStream stream, final long position, final int length,
final byte[] buffer) {
ReadBuffer buf = getFromList(completedReadList, stream, position);
if (buf == null || position >= buf.getOffset() + buf.getLength()) {
return 0;
}
int cursor = (int) (position - buf.getOffset());
int availableLengthInBuffer = buf.getLength() - cursor;
int lengthToCopy = Math.min(length, availableLengthInBuffer);
System.arraycopy(buf.getBuffer(), cursor, buffer, 0, lengthToCopy);
if (cursor == 0) {
buf.setFirstByteConsumed(true);
}
if (cursor + lengthToCopy == buf.getLength()) {
buf.setLastByteConsumed(true);
}
buf.setAnyByteConsumed(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.setStatus(ReadBufferStatus.READING_IN_PROGRESS);
inProgressList.add(buffer);
}
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("ReadBufferWorker picked file {} for offset {}",
buffer.getStream().getPath(), buffer.getOffset());
}
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(final ReadBuffer buffer, final ReadBufferStatus result, final int bytesActuallyRead) {
if (LOGGER.isTraceEnabled()) {
LOGGER.trace("ReadBufferWorker completed file {} for offset {} bytes {}",
buffer.getStream().getPath(), buffer.getOffset(), bytesActuallyRead);
}
synchronized (this) {
inProgressList.remove(buffer);
if (result == ReadBufferStatus.AVAILABLE && bytesActuallyRead > 0) {
buffer.setStatus(ReadBufferStatus.AVAILABLE);
buffer.setTimeStamp(currentTimeMillis());
buffer.setLength(bytesActuallyRead);
completedReadList.add(buffer);
} else {
freeList.push(buffer.getBufferindex());
// 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.getLatch().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 per CPU core.
* Note: it is not monotonic across Sockets, and even within a CPU, its only the
* more recent parts which share a clock across all cores.
*
* @return current time in milliseconds
*/
private long currentTimeMillis() {
return System.nanoTime() / 1000 / 1000;
}
}