聊聊flink的NetworkBufferPool

序

本文主要研究一下flink的NetworkBufferPool

BufferPoolFactory

flink-release-1.7.2/flink-runtime/src/main/java/org/apache/flink/runtime/io/network/buffer/BufferPoolFactory.java

public interface BufferPoolFactory {

    /**
     * Tries to create a buffer pool, which is guaranteed to provide at least the number of required
     * buffers.
     *
     * <p>The buffer pool is of dynamic size with at least <tt>numRequiredBuffers</tt> buffers.
     *
     * @param numRequiredBuffers
     *         minimum number of network buffers in this pool
     * @param maxUsedBuffers
     *         maximum number of network buffers this pool offers
     */
    BufferPool createBufferPool(int numRequiredBuffers, int maxUsedBuffers) throws IOException;

    /**
     * Tries to create a buffer pool with an optional owner, which is guaranteed to provide at least the
     * number of required buffers.
     *
     * <p>The buffer pool is of dynamic size with at least <tt>numRequiredBuffers</tt> buffers.
     *
     * @param numRequiredBuffers
     *         minimum number of network buffers in this pool
     * @param maxUsedBuffers
     *         maximum number of network buffers this pool offers
     *     @param owner
     *         the optional owner of this buffer pool to release memory when needed
     */
    BufferPool createBufferPool(int numRequiredBuffers, int maxUsedBuffers, Optional<BufferPoolOwner> owner) throws IOException;

    /**
     * Destroy callback for updating factory book keeping.
     */
    void destroyBufferPool(BufferPool bufferPool) throws IOException;

}

• BufferPoolFactory定义了createBufferPool、destroyBufferPool方法；其中createBufferPool支持numRequiredBuffers、maxUsedBuffers、owner参数

NetworkBufferPool

flink-release-1.7.2/flink-runtime/src/main/java/org/apache/flink/runtime/io/network/buffer/NetworkBufferPool.java

public class NetworkBufferPool implements BufferPoolFactory {
    //......

    private static final Logger LOG = LoggerFactory.getLogger(NetworkBufferPool.class);

    private final int totalNumberOfMemorySegments;

    private final int memorySegmentSize;

    private final ArrayBlockingQueue<MemorySegment> availableMemorySegments;

    private volatile boolean isDestroyed;

    // ---- Managed buffer pools ----------------------------------------------

    private final Object factoryLock = new Object();

    private final Set<LocalBufferPool> allBufferPools = new HashSet<>();

    private int numTotalRequiredBuffers;

    /**
     * Allocates all {@link MemorySegment} instances managed by this pool.
     */
    public NetworkBufferPool(int numberOfSegmentsToAllocate, int segmentSize) {

        this.totalNumberOfMemorySegments = numberOfSegmentsToAllocate;
        this.memorySegmentSize = segmentSize;

        final long sizeInLong = (long) segmentSize;

        try {
            this.availableMemorySegments = new ArrayBlockingQueue<>(numberOfSegmentsToAllocate);
        }
        catch (OutOfMemoryError err) {
            throw new OutOfMemoryError("Could not allocate buffer queue of length "
                    + numberOfSegmentsToAllocate + " - " + err.getMessage());
        }

        try {
            for (int i = 0; i < numberOfSegmentsToAllocate; i++) {
                availableMemorySegments.add(MemorySegmentFactory.allocateUnpooledOffHeapMemory(segmentSize, null));
            }
        }
        catch (OutOfMemoryError err) {
            int allocated = availableMemorySegments.size();

            // free some memory
            availableMemorySegments.clear();

            long requiredMb = (sizeInLong * numberOfSegmentsToAllocate) >> 20;
            long allocatedMb = (sizeInLong * allocated) >> 20;
            long missingMb = requiredMb - allocatedMb;

            throw new OutOfMemoryError("Could not allocate enough memory segments for NetworkBufferPool " +
                    "(required (Mb): " + requiredMb +
                    ", allocated (Mb): " + allocatedMb +
                    ", missing (Mb): " + missingMb + "). Cause: " + err.getMessage());
        }

        long allocatedMb = (sizeInLong * availableMemorySegments.size()) >> 20;

        LOG.info("Allocated {} MB for network buffer pool (number of memory segments: {}, bytes per segment: {}).",
                allocatedMb, availableMemorySegments.size(), segmentSize);
    }

    @Override
    public BufferPool createBufferPool(int numRequiredBuffers, int maxUsedBuffers) throws IOException {
        return createBufferPool(numRequiredBuffers, maxUsedBuffers, Optional.empty());
    }

    @Override
    public BufferPool createBufferPool(int numRequiredBuffers, int maxUsedBuffers, Optional<BufferPoolOwner> owner) throws IOException {
        // It is necessary to use a separate lock from the one used for buffer
        // requests to ensure deadlock freedom for failure cases.
        synchronized (factoryLock) {
            if (isDestroyed) {
                throw new IllegalStateException("Network buffer pool has already been destroyed.");
            }

            // Ensure that the number of required buffers can be satisfied.
            // With dynamic memory management this should become obsolete.
            if (numTotalRequiredBuffers + numRequiredBuffers > totalNumberOfMemorySegments) {
                throw new IOException(String.format("Insufficient number of network buffers: " +
                                "required %d, but only %d available. The total number of network " +
                                "buffers is currently set to %d of %d bytes each. You can increase this " +
                                "number by setting the configuration keys '%s', '%s', and '%s'.",
                        numRequiredBuffers,
                        totalNumberOfMemorySegments - numTotalRequiredBuffers,
                        totalNumberOfMemorySegments,
                        memorySegmentSize,
                        TaskManagerOptions.NETWORK_BUFFERS_MEMORY_FRACTION.key(),
                        TaskManagerOptions.NETWORK_BUFFERS_MEMORY_MIN.key(),
                        TaskManagerOptions.NETWORK_BUFFERS_MEMORY_MAX.key()));
            }

            this.numTotalRequiredBuffers += numRequiredBuffers;

            // We are good to go, create a new buffer pool and redistribute
            // non-fixed size buffers.
            LocalBufferPool localBufferPool =
                new LocalBufferPool(this, numRequiredBuffers, maxUsedBuffers, owner);

            allBufferPools.add(localBufferPool);

            try {
                redistributeBuffers();
            } catch (IOException e) {
                try {
                    destroyBufferPool(localBufferPool);
                } catch (IOException inner) {
                    e.addSuppressed(inner);
                }
                ExceptionUtils.rethrowIOException(e);
            }

            return localBufferPool;
        }
    }

    @Override
    public void destroyBufferPool(BufferPool bufferPool) throws IOException {
        if (!(bufferPool instanceof LocalBufferPool)) {
            throw new IllegalArgumentException("bufferPool is no LocalBufferPool");
        }

        synchronized (factoryLock) {
            if (allBufferPools.remove(bufferPool)) {
                numTotalRequiredBuffers -= bufferPool.getNumberOfRequiredMemorySegments();

                redistributeBuffers();
            }
        }
    }

    //......
}

• NetworkBufferPool实现了BufferPoolFactory接口，它的构造器接收numberOfSegmentsToAllocate、segmentSize两个参数；构造器里头根据numberOfSegmentsToAllocate创建了availableMemorySegments这个ArrayBlockingQueue，然后通过MemorySegmentFactory.allocateUnpooledOffHeapMemory挨个创建MemorySegment添加到availableMemorySegments
• createBufferPool方法创建的是LocalBufferPool(传递了自身NetworkBufferPool实例进去)，然后添加到allBufferPools这个set中，同时增加numTotalRequiredBuffers；destroyBufferPool方法则从allBufferPools移除该bufferPool，同时减少numTotalRequiredBuffers
• createBufferPool方法及destroyBufferPool方法会调用到redistributeBuffers方法，通过调用LocalBufferPool的setNumBuffers方法来调整buffer pool的大小

LocalBufferPool

flink-release-1.7.2/flink-runtime/src/main/java/org/apache/flink/runtime/io/network/buffer/LocalBufferPool.java

class LocalBufferPool implements BufferPool {
    private static final Logger LOG = LoggerFactory.getLogger(LocalBufferPool.class);

    /** Global network buffer pool to get buffers from. */
    private final NetworkBufferPool networkBufferPool;

    /** The minimum number of required segments for this pool. */
    private final int numberOfRequiredMemorySegments;

    /**
     * The currently available memory segments. These are segments, which have been requested from
     * the network buffer pool and are currently not handed out as Buffer instances.
     *
     * <p><strong>BEWARE:</strong> Take special care with the interactions between this lock and
     * locks acquired before entering this class vs. locks being acquired during calls to external
     * code inside this class, e.g. with
     * {@link org.apache.flink.runtime.io.network.partition.consumer.RemoteInputChannel#bufferQueue}
     * via the {@link #registeredListeners} callback.
     */
    private final ArrayDeque<MemorySegment> availableMemorySegments = new ArrayDeque<MemorySegment>();

    /**
     * Buffer availability listeners, which need to be notified when a Buffer becomes available.
     * Listeners can only be registered at a time/state where no Buffer instance was available.
     */
    private final ArrayDeque<BufferListener> registeredListeners = new ArrayDeque<>();

    /** Maximum number of network buffers to allocate. */
    private final int maxNumberOfMemorySegments;

    /** The current size of this pool. */
    private int currentPoolSize;

    /**
     * Number of all memory segments, which have been requested from the network buffer pool and are
     * somehow referenced through this pool (e.g. wrapped in Buffer instances or as available segments).
     */
    private int numberOfRequestedMemorySegments;

    private boolean isDestroyed;

    private final Optional<BufferPoolOwner> owner;

    /**
     * Local buffer pool based on the given <tt>networkBufferPool</tt> with a minimal number of
     * network buffers being available.
     *
     * @param networkBufferPool
     *         global network buffer pool to get buffers from
     * @param numberOfRequiredMemorySegments
     *         minimum number of network buffers
     */
    LocalBufferPool(NetworkBufferPool networkBufferPool, int numberOfRequiredMemorySegments) {
        this(networkBufferPool, numberOfRequiredMemorySegments, Integer.MAX_VALUE, Optional.empty());
    }

    /**
     * Local buffer pool based on the given <tt>networkBufferPool</tt> with a minimal and maximal
     * number of network buffers being available.
     *
     * @param networkBufferPool
     *         global network buffer pool to get buffers from
     * @param numberOfRequiredMemorySegments
     *         minimum number of network buffers
     * @param maxNumberOfMemorySegments
     *         maximum number of network buffers to allocate
     */
    LocalBufferPool(NetworkBufferPool networkBufferPool, int numberOfRequiredMemorySegments,
            int maxNumberOfMemorySegments) {
        this(networkBufferPool, numberOfRequiredMemorySegments, maxNumberOfMemorySegments, Optional.empty());
    }

    /**
     * Local buffer pool based on the given <tt>networkBufferPool</tt> and <tt>bufferPoolOwner</tt>
     * with a minimal and maximal number of network buffers being available.
     *
     * @param networkBufferPool
     *         global network buffer pool to get buffers from
     * @param numberOfRequiredMemorySegments
     *         minimum number of network buffers
     * @param maxNumberOfMemorySegments
     *         maximum number of network buffers to allocate
     *     @param owner
     *         the optional owner of this buffer pool to release memory when needed
     */
    LocalBufferPool(
        NetworkBufferPool networkBufferPool,
        int numberOfRequiredMemorySegments,
        int maxNumberOfMemorySegments,
        Optional<BufferPoolOwner> owner) {
        checkArgument(maxNumberOfMemorySegments >= numberOfRequiredMemorySegments,
            "Maximum number of memory segments (%s) should not be smaller than minimum (%s).",
            maxNumberOfMemorySegments, numberOfRequiredMemorySegments);

        checkArgument(maxNumberOfMemorySegments > 0,
            "Maximum number of memory segments (%s) should be larger than 0.",
            maxNumberOfMemorySegments);

        LOG.debug("Using a local buffer pool with {}-{} buffers",
            numberOfRequiredMemorySegments, maxNumberOfMemorySegments);

        this.networkBufferPool = networkBufferPool;
        this.numberOfRequiredMemorySegments = numberOfRequiredMemorySegments;
        this.currentPoolSize = numberOfRequiredMemorySegments;
        this.maxNumberOfMemorySegments = maxNumberOfMemorySegments;
        this.owner = owner;
    }

    //......

    @Override
    public void recycle(MemorySegment segment) {
        BufferListener listener;
        NotificationResult notificationResult = NotificationResult.BUFFER_NOT_USED;
        while (!notificationResult.isBufferUsed()) {
            synchronized (availableMemorySegments) {
                if (isDestroyed || numberOfRequestedMemorySegments > currentPoolSize) {
                    returnMemorySegment(segment);
                    return;
                } else {
                    listener = registeredListeners.poll();
                    if (listener == null) {
                        availableMemorySegments.add(segment);
                        availableMemorySegments.notify();
                        return;
                    }
                }
            }
            notificationResult = fireBufferAvailableNotification(listener, segment);
        }
    }

    private MemorySegment requestMemorySegment(boolean isBlocking) throws InterruptedException, IOException {
        synchronized (availableMemorySegments) {
            returnExcessMemorySegments();

            boolean askToRecycle = owner.isPresent();

            // fill availableMemorySegments with at least one element, wait if required
            while (availableMemorySegments.isEmpty()) {
                if (isDestroyed) {
                    throw new IllegalStateException("Buffer pool is destroyed.");
                }

                if (numberOfRequestedMemorySegments < currentPoolSize) {
                    final MemorySegment segment = networkBufferPool.requestMemorySegment();

                    if (segment != null) {
                        numberOfRequestedMemorySegments++;
                        return segment;
                    }
                }

                if (askToRecycle) {
                    owner.get().releaseMemory(1);
                }

                if (isBlocking) {
                    availableMemorySegments.wait(2000);
                }
                else {
                    return null;
                }
            }

            return availableMemorySegments.poll();
        }
    }

    //......
}

• LocalBufferPool的构造器要求传入NetworkBufferPool，而其内部的requestMemorySegment方法，在availableMemorySegments为空且numberOfRequestedMemorySegments < currentPoolSize时，会调用networkBufferPool.requestMemorySegment()来申请MemorySegment；而recycle方法在numberOfRequestedMemorySegments > currentPoolSize时会归还MemorySegment到networkBufferPool，否则在BufferListener为null的时候会归还到availableMemorySegments

小结

• BufferPoolFactory定义了createBufferPool、destroyBufferPool方法；其中createBufferPool支持numRequiredBuffers、maxUsedBuffers、owner参数；NetworkBufferPool实现了BufferPoolFactory接口，它的构造器接收numberOfSegmentsToAllocate、segmentSize两个参数；构造器里头根据numberOfSegmentsToAllocate创建了availableMemorySegments这个ArrayBlockingQueue，然后通过MemorySegmentFactory.allocateUnpooledOffHeapMemory挨个创建MemorySegment添加到availableMemorySegments
• NetworkBufferPool的createBufferPool方法创建的是LocalBufferPool(传递了自身NetworkBufferPool实例进去)，然后添加到allBufferPools这个set中，同时增加numTotalRequiredBuffers；destroyBufferPool方法则从allBufferPools移除该bufferPool，同时减少numTotalRequiredBuffers；createBufferPool方法及destroyBufferPool方法会调用到redistributeBuffers方法，通过调用LocalBufferPool的setNumBuffers方法来调整buffer pool的大小
• LocalBufferPool的构造器要求传入NetworkBufferPool，而其内部的requestMemorySegment方法，在availableMemorySegments为空且numberOfRequestedMemorySegments < currentPoolSize时，会调用networkBufferPool.requestMemorySegment()来申请MemorySegment；而recycle方法在numberOfRequestedMemorySegments > currentPoolSize时会归还MemorySegment到networkBufferPool，否则在BufferListener为null的时候会归还到availableMemorySegments

doc

• NetworkBufferPool